Month: March 2015

Hexapods

by

Introduction to Hexapods

 

Welcome to the Hexapod Blog! What really is the story behind those creepy crawly critters? They may not be as disgusting and scary as you think! Hexapods have a rich evolutionary history and are one of the most diverse groups of animals on earth. Follow us as we dive into hexapods, and learn all about their biology. Maybe before you squish one under your shoe, you will remember how awesome and beneficial they can be!

 

We have everything you need to know about hexapods right here! First, we provide some history. How long ago did they first appear? What does their family tree look like? We have all heard of dinosaur fossils, but what about hexapod fossils? In fact, hexapods do have their own fossil record which we also discuss. Second, we dive into what makes a hexapod a hexapod. The insect body plan, wings, wing folding, and complete metamorphosis are the main key evolutionary innovations we focus on. For instance, the mealworm beetle, Tenebrio molitor, undergoes complete metamorphosis changing from a mealworm larvae, pupa, and finally an adult beetle. The field cricket, Gryllus pennsylvanicus, is a good example of the insect body plan. They have a clear head, thorax, and abdomen. Of course it would not be a hexapod if it did not have six legs! The Madagascar hissing cockroach, Gromphadorhina portentosa, is one such critter having three sets of legs. Find all this and more in the Hexapod Blog!

Phylogeny

 

There are four different supergroups that help to classify Eukaryotes and each of their lineages. These groups are known as the Chromalveolates, the Excavates, the Archaeplastida, and the Unikonts. Hexapods can be found later down the Unikont lineage (Tree of Life Web Project, 2002).

 

The Unikonts split into various lineages, including Fungi, Amoebozoa, and others. However, Hexapoda is found in the Animal lineage. The lineage of Hexapoda continues to another group deemed Bilateria that includes most groups that are considered to be animals (Tree of Life Web Project, 2002). Scientists have determined that there are two separate lineages that make up Bilateria, which are Deuterostomia and Protostomia. Deuterostomes and Protostomes are defined by their embryonic development. Deuterostomes develop their anus before their mouths, and Protostomes develop their anus after their mouths. Scientists now recognize two lineages within Protostomia that are Lophotrochozoa and Ecdysozoa (Halanych et. al, 1995). Hexapods can be found in the Ecdysozoan or molting insect lineage under the phylum Arthropoda (Tree of Life Web Project, 2002).

 

Hexapoda are traditionally shown to be monophyletic or descended from one evolutionary group. They are one of the most diverse groups, with over 750,000 species described so far (Tree of Life Web Project, 2002). Recent evidence suggests that the closest relatives to Hexapoda are Crustacea (Giribet et. al, 2001). While Crustacea are dominant in aquatic environments, Hexapoda are dominant on the land. The Hexapods split into lineages that include Diplura, Insecta or the true insects, Protura, and Collembola or the springtails (Tree of Life Web Project, 2002). Of each of these groups, Diplura is seen as the most unstable and it’s place in the phylogeny of Hexapoda changes often (Tree of Life Web Project, 2002). A final taxonomy of insects would be ordered as Eukaryotes, Unikonts, Animals, Bilateria, Ecdysozoa, Arthropoda, Hexapoda.

IMG_0040

Phylogenetic tree of Hexapoda. Created using Class Notes and J.E. Blair, 2009. Created by Daniel Pinto

 

Fossil Evidence

 

Hexapods are one of the earliest known lineages of terrestrial based insects, dating all the way back to the Early Devonian period about 412 million years ago or possibly even to the Late Silurian period (Misof et. al, 2014). These early fossils are that of Rhynies (Rhyniognatha hirsti), or springtails (Collembola). Some basal insect fossils could date to 379 million years ago, however the fossil evidence is sparse and therefore scientists are unable to say definitively what lineage these fossils are from. However, fossils of other early insects include those of bristletails (Archaeognatha) that date to about 390 million years ago (Engel et. al, 2004). Most of this evidence suggests that insects were one of the first terrestrial organisms and were major selectors on land based plant life (Engel et. al, 2004). Fossil evidence of key structures have helped scientists to determine if such structures allowed for the massive speciation of the Hexapod lineage (Nicholson et. al, 2014). The fossil record can be a good way to show early existence of particular groups and can help to infer earlier origins if some groups are more diverse or abundant than others (Thomas & Ware, 2011). However, the fossil record is not as complete for certain groups of Hexapods, thus making it not as reliable as other dating methods (Thomas & Ware, 2011).

Molecular Clock

 

Unlike the fossil record, molecular clock evidence can be seen as more reliable in some cases. Using genomes of living Hexapods, estimates for divergence dates of certain groups can be found even in the midst of a sparse fossil record (Thomas & Ware, 2011). In a study on the evolutionary history of insects, molecular clock data led scientists to conclude that Hexapods may have originated earlier than the Silurian period in the Cambrian or Early Ordovician periods (Misof et. al, 2014). These results have been controversial, as there are little to no actual fossils of Hexapods from the Cambrian to the Silurian periods (Misof et. al, 2014). Indeed, some scientists have hypothesized that Hexapod evolution happened drastically earlier than paleontological evidence based on molecular evidence (Thomas & Ware, 2011).

 

Though there are benefits to using molecular clock data, there are also drawbacks as well. Using molecular clock data can help to fill gaps in the fossil record, but data can only be obtained from living or recently extinct species. This is due to the fact that molecular clock data relies on using DNA sequencing to infer genetic changes over time, but genetic material cannot be obtained from ancient species (Thomas & Ware, 2011). Despite these limitations, molecular clock data has supported the idea of Hexapods being a monophyletic group, and even helped to confirm previous hypotheses about the close relation between Hexapoda and Crustacea (Misof et. al, 2014).

Key Evolutionary Innovations of Hexapods

 

Hexapods are one of the most diverse classes in the animal kingdom. In fact, Hexapods alone make up over half of all recorded species. With such diversity and success, you can bet there a few innovations that have warranted their success.

 

  1. Hexapod bauplan: The importance of the bauplan, or body plan, of Hexapods cannot be overlooked when it comes to contributing to their success. Hexapods exhibit metameric, or repeated, segmentation and have one pair of appendages per segment. These segments are organized into three tagmata, or specialized segments: the head, thorax, and abdomen. Their appendages are jointed, which aids in walking, swimming, and feeding, similar to the joints in our bodies. Hexapods also have exoskeletons, meaning their skeletons are on the outside of their bodies, providing them with protection (Nicholson et. al, 2014).

Schneider_Cricket_photo_1

Field Cricket, example of insect bauplan

 

  1. Wings: Hexapods are thought to have begun flying sometime in the Carboniferous Period, about 350 million years ago. While it is not fully understood why wings developed, the hexapod wing and the flight capabilities it provides are extremely advantageous. Hexapod wings are paired, and the pairs are generally referred to as the fore- and the hind-wings. They are heavily veined, and are surrounded by a cuticle for rigidity and protection. Most Hexapods have their fore- and hind-wings coupled, but dragonflies do not; they can move each of their wings completely independently of the others, allowing them to perform amazing flight maneuvers (Kesel, 2000). The benefits of flight are many, and wings have no doubt lent to the overwhelming success of Hexapods and other insects (Yanoviak et. al, 2009).


IMG_0037

Cicada exhibiting folded-over, heavily veined wings.

  1. Metamorphosis: There are two types of metamorphosis, hemimetabolism and holometabolism. Hemimetabolism, known as incomplete metamorphosis, characterizes Hexapods that go through a series of molts, shedding the exoskeleton that has become too small for them. Holometabolism, or complete metamorphosis, is a four stage cycle in which the organism goes from egg to larva to pupa to adult. Hexapods that undergo hemimetabolism typically resemble the adult stage as nymphs, but holometabolous Hexapods do not. A unique advantage to metamorphosis is that it allows them to inhabit different niches throughout the course of their life (Class Notes). For example, a grounded caterpillar will not occupy the same niche when it becomes a winged adult butterfly.  In addition, it has been suggested that holometabolous Hexapods enjoy lower extinction rates than other groups (Nicholson et. al, 2014).

 

  1.  Sensory systems: Hexapods have acute, highly developed senses. They have compound eyes, and each eyeball is comprised of tiny units called ommatidia. Each ommatidium contains its own lens. In addition, most Hexapods also have simple eyes, or ocelli, to further their sense of sight (Mayer, 2006). Many Hexapods, such as wasps and mantids, have two large compound eyes and three smaller ocelli on top of their heads. In addition to their developed eyesight, Hexapods were among the first creatures to sense sounds. Most sound is produced through repeated stimulation of appendages. Some species of moths can even hear ultrasound, helping them avoid predation by bats (Kay, 1969).

A review of key evolutionary innovations of Hexapods, including metamorphosis, wing folding, and body segmentation.

References

Engel, M. S., & Grimaldi, D. A. (January 01, 2004). New light shed on the oldest insect. Nature, 427, 6975, 627-30.

 

Giribet, G., Edgecombe, G. D., & Wheeler, W. C. (January 01, 2001). Arthropod phylogeny based on eight molecular loci and morphology. Nature, 413, 6852, 157-61.

 

Halanych, K. M., Bacheller, J. D., Aguinaldo, A. M., Liva, S. M., Hillis, D. M., & Lake, J. A. (January 01, 1995). Evidence from 18S ribosomal DNA that the lophophorates are protostome animals. Science (New York, N.Y.), 267, 5204, 1641-3.

 

J.E. Blair. Animals (Metazoa). Pg. 223-230 in The Timetree of Life, S.B. Hedges and S. Kumar, Eds. (Oxford University Press, 2009).

 

Kay, Robert E. (1969). “Acoustic signalling and its possible relationship to assembling and navigation in the moth, Heliothis zea”. Journal of Insect Physiology 15 (6): 989–1001.

 

Kesel, A.B. (2000) Aerodynamic characteristics of dragonfly wing sections compared with technical aerofoils. J Exp Biol 203: 3125–3135.

 

Mayer, G. (2006), “Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?”, Arthropod Structure and Development 35 (4): 231–245.

 

Misof, B., Liu, S., Meusemann, K., Peters, R. S., Donath, A., Mayer, C., Frandsen, P. B., … Zhou, X. (November 06, 2014). Phylogenomics resolves the timing and pattern of insect evolution. Science, 346, 6210, 763-767.

 

Nicholson, D.B., Ross, A.J., Mayhew, P.J. Fossil evidence for key innovations in the evolution of insect diversity. Proc. R. Soc. B: 2014; 282(1803).

 

Thomas, J. A., & Ware, J. L. (January 01, 2011). Molecular and Fossil Dating: A Compatible Match?. Entomologica Americana, 117, 1, 1-8.

 

Tree of Life Web Project. 2002. Hexapoda. Insects, springtails, diplurans, and proturans. Version 01 January 2002 (under construction). http://tolweb.org/Hexapoda/2528/2002.01.01 in The Tree of Life Web Project, http://tolweb.org/

 

One Shell of a Phylum

by

One Shell of a Phylum

 

Consisting of more than 85,000 extant species, Mollusca is the second biggest phylum in the animal kingdom, second to only Arthropods. Ten total classes of Molluscs have existed throughout evolutionary history, but only eight of these classes exist today. The word Mollusc was derived from the Modern Latin term mollusca which meant “thin-shelled”. According to Nordsieck (2011) this term originated from the Latin term molluscus, meaning “soft”. These terms were actually initially used to describe many soft-bodied invertebrates that do not fall under this Phylum, including brachiopods, bryozoans and tunicates.

 

One of the coolest things about molluscs is that their range of adaptations is almost limitless. Most Molluscs have a shell (Fig. 1), a rasping tongue known as the radula, and a foot. What is so fascinating about these traits is that even though they are for the most part, very similar, they give rise to an extremely diverse array of functions that have allowed molluscs to thrive! The molluscs’ reach into so many different areas that its makes other phylums kind of jealous. They inhabit freshwater, marine, and terrestrial environments, utilize various food sources, and move around in very different ways.

Speaking of moving around, Molluscs locomotion is extremely diverse. Modes of movement vary greatly from slow crawling, zipping around through the water, or simply staying still. Some other interesting characteristics of Molluscs include: bilateral symmetry, a body with more than two cell layers, tissues and organs (Nordsieck, 2011).They do not have body cavities, but they do posses a gut with a mouth and an anus. Most have an open-circulatory system with a heart and an aorta, and do gas exchange through organs called ctenidial gills. They reproduce sexually, which can be external or internal, and can even be hermaphroditic!

1 - Ondrejech_Andy_Shell_Variance_Photo

Fig 1 – Some of the largest shells found in each class discussed, and an important shared trait.

 

We chose to present the European Squid (Loligo vulgaris) representing the Cephalopods (squids, octopuses, nautiluses, cuttlefish),  the Brown Garden Snail (Cornu aspersum) representing the Gastropods (snails, slugs, sea slugs, limpets), and the Blue Mussel (Mytilus edulis, seen in the photo on the right) representing the Bivalves (mussels, clams, oysters, and scallops). These were selected as they are common enough to be found locally allowing us a chance to observe them first hand and collect media on them. Because they are local molluscs, they are more familiar to the general public and are therefore, great candidates to represent the three classes of molluscs that we will sample from for this blog.
In The Beginning…

 

Fossil evidence shows us that molluscs appeared early in the Cambrian period (about 550 to 580 million years ago) as organisms that crawled along the ocean floor. According to Nordsieck (2011) these fossil records help to explain the division of early molluscs body plans into a soft ventral side used for locomotion (the foot), and an armored dorsal side exposed to the environment. Originally, the dorsal side was protected by a thick tissue layer instead of a shell in order to protect their organs. This was to become the mantle, and can be found in all molluscs. Over time, the mantle developed which, according to Sigwart and Sutton (2007) is a hard horn-like cuticle material that is partly made of the calcium carbonate found in the molluscs’ food sources. This rendered it additionally resistant. Eventually, the tried and true shell developed! While some molluscs developed overlapping shell plates to allow flexibility, others had shell plates that fused together and sacrificed mobility for protection. This one-part shell adaptation proved so successful that molluscs still have it, and it has allowed molluscs to experience a wide range of diversity. For instance, according to the University of Cambridge museum of Zoology (2011) some molluscs use their shells on dry land to protect against desiccation.


Phylogeny

 

The phylogeny in molluscs is still being heavily debated between taxonomists. Regarding the structure of the classes, the depicted phylogenetic tree (Figure 2) regards the Testarian Hypothesis described by Sigwart and Sutton (2007). However in this source and in many others, the exact Divergence of the classes Cephalopoda, Gastropoda, and Bivalvia are unknown. Because of this, the divergence within the tree is based off information found by Kumar and Hedges (2011). As shown below, the phylum mollusca is branching off of the super-group of Lophophores, even though molluscs are trochozoans. According to the University of California-Berkeley, the group trochozoa is determined by the larval body form that the organism exhibits, and lophophores are determined by the presence of a strange tube like feeding appendage. There is a lot of debate with the classification of these groups as well, but currently, according to studies done by Passamaneck and Halanych (2006) along with data from Louisiana State University and Sonoma State University, it is a very polyphyletic group, meaning that they are not all grouped together, and that Trochozoa groups can evolve from Lophophore groups, which is believed to be the case with molluscs.

2 - Ondrejech_Andy_Mollusc_Phylogeny_Photo

Figure 2 – Phylogenetic Tree outlining the classes of Mollusca. The tree itself is tied into the a supergroup of Lophophora, along with two other genuses.  Annelids are the outgroup.

Fossil Record & Molecular Clock Dates of Taxon

 

As mentioned above, fossil evidence, shows that molluscs are believed to have appeared around 550 to 580 million years ago, if not sooner.  Though according to Kumar and Hedges (2011) molecular evidence places the date of divergence from Annelids to be between 560 ato 690 million years ago. Fossil evidence shown below (Fig 3) shows long shell imprints associated with ancient cephalopods. The fossil is dated to be approximately 400 million years old. The shells resembling bivalvia in this figure are actually brachiopoda, and based on the information from Nordsieck (2011), Bivalvia existed, but were heavily out competed until the permian extinction. According to Kumar and Hedges (2011), cephalopoda split off from bivalvia and gastropoda approximately 530 million years ago, while bivalves and gastropods split off from one another 495 million years ago. This long time period between classes can help explain why the separate classes are so diverse and specialized from one another.

3 - Ondrejech_Andy_Ceph_Fossils_Photo

Figure 3 – Devonian Shale with long imprints belonging to that of ancient cephalopods. The shells resembling that of bivalves are really branchiopods that dominated at the time.

Key Evolutionary Innovations

 

Although they are highly derived from one another, molluscs share a few important traits that place them under the same phylum while excluding other soft bodied vertebrates. These traits are called synapomorphies, which include the foot, the shell, the radula and the mantle. But keep in mind, although these are key attributes of all molluscs, they appear very different between classes, and even within families, allowing for greater diversity within the phylum.

 

  1. Foot

We will start with the foot. The foot is the muscular part of the mollusc which is in contact with the substrate. The muscles that are mainly responsible for movement of the bivalve foot are the posterior and anterior pedal retractors. They effect back and forth movement by retracting the foot. According to the University of Cambridge’s University Museum of Zoology, the foot of the Bivalve is used as a mechanism to dig itself into the ground and located in one spot. It  is compressed and blade-like and it is pointed for digging.This is useful since bivalves don’t move very fast, and can be easily carried away by a current, or by another animal. Staying rooted in a single spot is especially helpful if the bivalve is in an ideal location away from predators, or in a nutrient-rich spot.

 

In cephalopods such as the European Squid, the foot derived to be the arm-like tentacles used in hunting (Fig 4). According to Howard (2003), they can use their arms for a wide variety of things, such as movement, capturing prey, or fending off predators. Some molluscs groups had foot divided into left and right halves and separate waves moving on each side.

 

The foot is the organ of locomotion for gastropods such as the Garden Snail. According to Myers and Burch (2001) the movement of the Garden Snail is orchestrated by the contraction of muscular waves starting from the posterior end and moving to the anterior end of the of the foot.

IMG_0080[1]

Figure 4 – This photo depicts the arms and tentacles of a European Squid (Lolgio vulgaris). These tentacles are a highly modified foot used for hunting more than for locomotion.

 

 

  1.    Shell  Usage

The shell mainly serves as a surface for muscle attachment but it also serves other purposes. It acts to protect against predators and also from mechanical damage. According to Nielsen (1995) in freshwater snails, land snails and other species, the shell further serves as protection against the sun and also against drying out. The shell is especially visible in both the Garden Snail and in the Blue Mussel, but is not externally visible in the Cephalopod. In the European Squid, the shell has been reduced to a backbone-like structure known as the “pen” that internally runs along the anterior-posterior axis  along its dorsal side, providing support.

  1.      Radula

The radula, also known as the rasping tongue, consists of an elastic band and contains chitin teeth. It has a bow-shaped jaw used to cut off food particles before it is transported to the gut. According to the Missouri Botanical Gardens (2002) it is suited for different kinds of nutrition based on the different habitats molluscs inhabit. This can be seen in the European Squid, which has a very derived “beak” used for crushing shells before using its tiny hooked radula to tear chunks off of its prey. The radula is absent in Bivalves because they are filter feeders, but present in gastropods which is shaped to scrape algae off of substrate, such as in this video below. (Fig 6).

https://www.youtube.com/watch?v=kEnUw2HNuk4&feature=youtu.be

 

Figure 6 – Gastropod feeding on the side of an aquarium tank. You can see the snail using it’s sharp radula to scrape off the food from the glass.

 

  1.    Mantle Function

 

Another key feature is the mantle. According to Nielsen (1995) it is a thin, skin-like layer that forms the outer wall of the mollusc’s body and encloses the internal organs. It also secretes calcium carbonate to form the shell, and continues secreting it throughout the molluscs’ lifetime to further harden and expand the shell. The mantle of the European squid is the long, slender layer that encases everything posterior to the eyes. The mantle of the mussel simply lines inside of the shell. In the garden snail The mantle in the snail is usually fully or partially hidden inside the gastropod shell.

4.) Where/How Do They Live?

 

Uses of the Foot

 

Bivalves – Burrowing

 

  1. The foot first extends downwards in a probing motion and then expands to form an anchor.
  2. Then the siphons close to prevent any water being ejected.
  3. Next the adductor muscles close the valves rapidly, effectively expelling water from the ventral margin.
  4. This is followed by the contraction of foot retractor muscles, pulling the bivalve downward towards the anchored foot.
  5. Finally, the adductor muscles relax and the ligament opens the valves.

 

Cephalopods – Hunting

 

Gastropods – Movement

Locomotion

Cephalopods are the most mobile of all Molluscs. According to Howard (2003) cephalopods are jet setters. With the exception of the octopus, most spend much of their lives swimming above the bottom. Cephalopod swimming is quite different from that of fish. Cephalopods use jet propulsion, pumping water into their bodies, over their gills and out through a tube called the siphon or funnel. This siphon is a muscular and mobile organ that the animal can use to direct the water jet in almost any direction to steer itself.

Holthuis (1995) makes the point that gastropods (snails, whelks, conchs,) are also quite mobile and crawl along on their large foot. Some bivalves (clams) can even jet surprising distances by pumping water through their siphons with rapid opening and closing movement of their mantles.

 

Range/Life History

 

Gastropods

Gastropods (such as the Garden Snail picture below) range second, only behind insects when it comes to the number of named species. They make up over 80% of all living molluscs and are one of the most highly diversified classes in the phylum (Myers & Burch, 2001). Today there are more than 62,000 living species of gastropods.  They live in different habitats and are extremely diverse in size, body and shell morphology. They are the only molluscs group to have invaded land habitats and thus they occupy the widest ecological niche of all molluscs. They are found in deep seas, freshwater habitats, salt lakes, mountaintops, deserts, rainforests and other habitats.Estimations for the extinct species range from 40,000 to 100,000 and some even believe it to be as many as 150,000 species. As Nordsieck (2011) states, gastropods have rich fossil record which goes back to the late Cambrian period, that is nearly 600 million years ago. These fossils also show both extinctions and diversification of new groups.

Gastropod larvae undergo torsion or a twisting which brings the rear of the body (the mantle cavity, gills, and anus) to a position near the head, which results in the twisting of internal organ systems. In many species, this twisted form is retained by the adult; while in others it is partially lost.

 

The Brown Garden Snail (Cornu aspersum)is a member of Helicidae family. According to Nordsieck (2011), “It is between 25 and 40 millimetres wide and between 25 and 35 millimetres high”. It has originated from western europe, Britain and along the borders of the mediterranean, but today it is one of the most widely spread land snail species in the world. It has been introduced to places like North America, South America, Australia, New Zealand and even parts of Africa.

 

b.) Evolutionary History

Bivalvia diverged from their mobile ancestors in order to live a sessile life. Though present in the Paleozoic, bivalves were outcompeted by brachiopods (which are arthropods that resemble bivalves), crinoids and corals. After the decline of brachiopods during the Permian Extinction, bivalves established their dominance in the marine environment, essentially replacing brachiopods. Eventually, around the Devonian period, bivalves with siphons appeared. With the addition of siphons along with the bipartite shell development, bivalves were able to exhibit extraordinary protection which allows the animal to only need to extend its siphon in order to breathe, to feed, and to reproduce, without having to expose the rest of its body. During the Mesozoic period, burrowing bivalves with siphons underwent some species differentiation that eventually proliferated into other time periods. For example, swimming scallops appeared during the Triassic, reef building Rudist bivalves dominated during the Cretaceous displacing coral and freshwater bivalves appeared in the Devonian.

In Gastropods, the shell is very different from other mollusc shells as it is coiled to form its characteristic spiral. Snails evolved to have developed a dorsal sack, known as the visceral hump, to contain most of the internal organs. This part remains under the mantle and is always within the shell for maximum protection. During embryonic development “torsion” occurs, as the mantle and the visceral hump turn around and coil into the spiral saving space, meaning that gastropod shells are coiled asymmetrically to one side depending on this torsion . Because of the twisting of the digestive tract, the anus in Gastropods is located above their head.They primarily herbivores, relying on their shell as a protection in order to slowly explore environments to intake algae from rocks and other hard substrates with their rasping radula tongue.

Cephalopods are the most derived mollusc group. Even though they reside in the subphylum conchifera, containing only molluscs retaining shells, the shell in cephalopods is highly diminished. They demonstrate a body plan similar to that of slugs and other unshelled gastropods: A reduction of the shell, at the cost of protection but improving movability. However, Nautilus, an extant species of cephalopods, still bears an external shell. As stated by Howard (2003), cephalopods are also the least dependent on a solid substrate to move, and so are able to catch prey unlike the herbivorous and filter-feeding bivalves and gastropods. They have the ability to hunt and developed long arms with suckers, along with sharp muscular chitin beaks in order to catch and process food.

 

5.) Survey of Extant Taxa

Importance

 

One role that molluscs serve in the environment is actually an indirect role; the shell, used as a barrier to the outside environment, can actually serve as a home to many other organisms, according to the Virginia Department of Game and Inland Fisheries. For example, many aquatic insects, plants and algae live on the outside of a live mussel and use it as a food source. Even after the mussel dies, the shell can serve as a nesting site for smaller fish.

According to Morton (2013) mussels are filter feeders, so they are one of the few animals that actually improve the quality of the water. Mussels are also an important food source for many predators, both aquatic and non-aquatic. However, cephalopods have nearly always been one of the biggest and most dominant predators living in oceans. As a dominant predator, squids naturally accumulate heavy metals and toxins when exposed to pollution. This is a process known as bioaccumulation. Because squids are so sensitive to changes in the water quality, they are usually found in areas of cleaner water. Snails also serve an important service to their environment.They are decomposers which feed off the dead tissues of plants as well as detritus. Not to mention, snails serve a key role in the calcium cycle, as they are an important source of calcium for their predators.

Molluscs are found in environments around the world, and are an important source of food for many animals. In many places, they are a  delicacy and are thought to give the eater special properties. This occurs in various forms including; Calamari, steamed clam, oyster, or mussel bakes, and Escargot in France.
Molluscs also produce luxury items important to the fashion and jewelry industry such as mother of pearl and purple dye. Some gastropods however, are serious pests; the common slug, for example, causes much garden damage.

Work Cited

  1. Nielsen, C. (1995). Animal evolution interrelationships of the living phyla. Oxford: Oxford University Press.
  2. Myers, P., & Burch, J. (2001). ADW: Gastropoda: INFORMATION. Retrieved February 12, from http://animaldiversity.org/accounts/Gastropoda/
  3. Missouri Botanical Gardens.(2002). Ocean Animals: Molluscks. Retrieved February 12, from http://www.mbgnet.net/salt/coral/animals/mollusk.htm
  4. Holthuis, B.V. (1995). Evolution between marine and freshwater habitats: a case study of the gastropod suborder Neritopsina. Ph.D. thesis, University of Washington. Retrieved February 12, from http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/gastropoda.php
  5. Nordsieck, R. (2011). The Living World of Molluscs. Retrieved February 12, from http://www.molluscs.at/index.htm.
  6. Learn About Squids! (n.d.). Retrieved February 12, from http://tolweb.org/treehouses/?treehouse_id=4225
  7. Freshwater Mussels. (2015). Retrieved February 12, from http://www.dgif.virginia.gov/wildlife/freshwater-mussels.asp
  8. Morton, B. (2013). bivalve | class of mollusks :: Ecology and habitats | Encyclopedia Britannica. Retrieved February 12, from http://www.britannica.com/EBchecked/topic/67293/bivalve/35737/Ecology-and-habitats#toc35738
  9. Sigwart, J. and Sutton M. (2007). Deep molluscan phylogeny: synthesis of palaeontological and neontological data. Proceedings of the Royal Society B: Biological Sciences. Retrieved March 5 from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2274978/
  10. Passamaneck, Y. and Halanych K, M. (2006).  Lophotrochozoan phylogeny assessed with LSU and SSU data: evidence of lophophorate polyphyly  Molecular Phylogenetics.
  11. Introduction to Lophotrochozoa. (n.d.). Retrieved March 5, from http://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html
  12. Kumar, S. and Hedges, S.B. (2011). Time Tree2: species divergence times. Retrieved February 12, from http://timetree.org/
  13. Howard, C. (2003). THE JET SET: THE ANATOMY OF SWIMMING IN CEPHALOPODS. Retrieved February 12, from http://jrscience.wcp.miamioh.edu/fieldcourses03/PapersMarineEcologyArticles/THEJETSET.THEANATOMYOFSWIA.html

University Museum of Zoology, Cambridge | Burrowing Bivalves. (2011). Retrieved February 12, from http://www.museum.zoo.cam.ac.uk/bivalve.molluscs/lifestyles.of.bivalve.molluscs/burrowing.bivalves/

Angiosperms

by
From flower shops to the produce section at the supermarket angiosperms, and their by-products, can be seen everywhere. Comprised of more than 260,00 species the angiosperm taxon is extremely diverse. The most abundant of the green plant division, many of the most economically and agriculturally important plants are angiosperms. Their diversity has allowed them to colonize multiple different types of habits and survive in various environments across the world. Clovers, Sunflowers, and Zebra Succulent are three exemplary species for angiosperm diversity. Though they are diverse they share several features such as their unique reproduction morphology, which will be discussed in this blog.

Phylogenetic Tree of Life

Phylogeny of Angiosperms and its groups

Phylogeny of Angiosperms and it’s groups. Created by Alyssa Riddle.

There are four supergroups of Eukaryotes and they include the Unikonts, the Chromalveolates, the Excavates, and the Archeaplastida. Archeaplastida are also called Plantae, and is the supergroup that the angiosperms belong to.

Archeaplastida contains three major lineages including Glaucophytes (microalgae), Rhodophyta (red algae), and the lineage that contains angiosperms, the Green Plants (Hedges & Kumar, 2009). The lineage of land plants stem from the Green Plants and are known as the Embryophytes. Sixteen different lineages stem from the Embryophytes, but the group that the angiosperms belong to are the Spermatopsida. Spermatopsida contain groups such as the conifers, seed plants, and flowering plants (Hedges & Kumar, 2009).

Analysis in the last five years has led scientists to agree that Amborella is the base of the angiosperm’s evolutionary tree. Major groups that branch off from Amborella trichopoda are Nymphaeaceae (water lilies and relatives), Austrobaileyales, Magnoliids, Chloranthaceae, Ceratophyllaceae, Monocotyledons (lilies, orchids, grasses), and eudicots (most flowering plants).

The order of taxonomic hierarchy for angiosperms is ranked: Eukaryote, Archeaplastida, Green Plants, Embryophytes, Spermatopsida, Angiosperms. Angiosperms contain at least 260,000 living species which are classified into 453 families and over 904,649 species (Hedges & Kumar, 2009).

See photo gallery below for some examples of these species.

Above is a Photo Gallery exampling some species in order to show the wide range of diversity in Archaeplastida. (Photos by Alyssa Riddle)

 Fossil Evidence and the Molecular Clock 

Angiosperms are a specific group within the Plantae Kingdom.

This timeline represents the estimated divergence of the kingdom Plantae. This diagram displays the diversification of various lineages and their relationships to the Angiosperm clade. The timeline is based upon molecular clock data provided by Hedges, Blair, and Kumar through the Timetree of Life project (2009). Created by Emily Thomas.

Fossil and molecular clock evidence agree that angiosperms are the most recently evolved of the major groups of plants. Both bodies of evidence also agree that the clade diverged from their sister group the gymnosperms, the cone-bearing plants (“Angiosperms,” 2008).

The timing of this divergence is not fully resolved by the fossil record and molecular clock estimates. The lack of a comprehensive fossil record has led to molecular clock evidence as more widely accepted by the scientific community. This evidence suggests that angiosperms arose approximately 175 million years ago (Hedges & Kumar, 2009). The hypothesized phylogenetic and chronological relationships of angiosperms to gymnosperms, as well as the other plant lineages, based on molecular clock evidence, are see in the figure to the right. 

Angiosperm Fossil Evidence

The most definite evidence of angiosperms in the fossil record comes from Cretaceous era fossils are the most definite evidence . The fossil record of angiosperms display a wide variety of structures, shape, and size. The vast morphological diversity has made it difficult to resolve relationships between the major angiosperm clades, but shows early diversification of lineages (Soltis, Soltis, & Edwards, 2005). Fossilization of leaves, pollen, wood, and floral structures have allowed for character based analysis of evolution (Dilcher, 2000). While fossil evidence has provided a basic understanding of angiosperm diversity throughout time, scientists must rely on the combination of preserved specimen’s physical and genetic characteristics to develop a more definite understanding of the angiosperm clade and relationships among it’s lineages.

This timeline represents the estimated time of diversification of the angiosperm clade. Based on molecular clock data (Hedges & Kumar, 2009), the diagram shows the rapid diversification of angiosperms. This diversification occurred in a relatively short geological time frame (approx.. 40 million years). Created by Emily Thomas.

Molecular Clock

While molecular clock evidence is the most widely used for examining phylogenetic relationships, complications arise in using molecular clock evidence for plants because of inconsistent evolution rates among different lineages (Dilcher, 2000).

Molecular clock evidence predates fossilization records for angiosperms by approximately 50 million years (Soltis, et. al, 2005). This unifies the angiosperm clade as a monophyletic group, defined by one evolutionary event, but does not fully resolve relations between other plant lineages. (Hedges & Kumar, 2009). 

 Within the angiosperm clade there are 5 major extant groups (Eudicots, Ceratophyllales, Monocots, Magnoliid, Chloroanthales) and 3 other “primitive” (non-extant) groups (Austrobaileyales, Nymphaelales, and Amborellales) (Hedges & Kumar, 2009).

 The major divergences amongst these groups are represented in the phylogenetic timeline above. Molecular evidence suggests the first divergence within the clade was the Amborellales approximately 174.9 mya. The Nymphaeales diverged  approximately 167.3 mya. The Austrobaileyales  diverged 159.5 mya, the Chloroanthales 150.1 mya, and the Magnoliids 147.8 mya. The most recent divergences were of Monocots  146.6 mya, and the Ceratophyllales 146.3 mya (Hedges & Kumar, 2009). 

 

 Evolutionary Innovations

Over time, specific evolutionary features, have distinguished angiosperm reproduction. The development of non-exposed seeds, housed within a flower structure, defines the group. This evolutionary feature has led to an abundance of morphological variation and widespread distribution of this group. Angiosperm flower structures have evolved in response to ecological pressures rapidly, and this success has led to the group’s survival, nearly universally, across the diverse ecosystems of our planet (Carter 1997).

 Angiosperms produce their gametes in separate organs from their bodies and these are generally housed in a flower. Fertilization takes place in structures to keep the process relatively unexposed to the elements. Flowering plants are the most diverse organism on the planet after insects.

Spider Wasp, under a dissection microscope. This organism is a common pollinator and of the family Pompilidae. Photo by Nick White.

Flowers come in an astounding number of colors, shapes, sizes, arrangements, and smells. All of these are evolutionary innovations which assist in attracting pollinators. Attraction is effected by color, scent, and the production of nectar, which may be secreted in some part of the flower. Pollinator’s relationship with their flowers are a textbook example of coevolution, as some animals evolve specifically to cater to a flowers pollination needs. These animals transport the flowers pollen to a wider geographic range to give them an excellent diversity within the population. (Carter, 1997)

Flower organs help to facilitate the reproductive cycle of angiosperms.
Each flower part has a specific function.

Labelled Flower

A labelled, bisected specimen of the Erigeron glaucus, more commonly known as the Daisy. The reproductive (carpel, stamen, anther, and sepals) and non-reproductive structures (receptacle and pedicel) of the flower are displayed. Photo by Nick White.

Pedicel: The stalk of the flower

Receptacle: The part of the stalk where the various parts of the flower are attached

Sepal: Acts as the base for the flower

Petal: Aids in attracting pollinators

Stamen: The male part of a flower

Anther: The part of the stamen where pollen (male gametophytes) is made

Carpel: Houses female gametophytes

20150305_153229

Example of the most commonly cultivated fruit, the citrus fruit of a Rutaceae, commonly called an orange. Photo by Nick White.

After fertilization, the ovule transforms into a seed, and it is surrounding tissues evolve into a fleshy fruit. The fruit protects the seed and also promotes it’s dispersal to a wide geographic range. Much like flowers, fruit also has a large diversity among species. Some is meant to be dispersed by the wind, but many rely on animals to disperse it. Whether by having hooks to hook on to an animal’s skin or fur or being sweet and nutrient rich to promote being eaten, digested, and fertilized by the animals that carry them off (Carter, 1997).

 

 References

Angiosperms. (2008). In L. Lerner & B. Lerner (Eds.), The Gale Encyclopedia of Science (4th ed., Vol. 1, p. 217). Detroit: Gale.Carter, J. (2014, January 17). Angiosperms. Retrieved March 6, 2015, from http://biology.clc.uc.edu/courses/bio106/angio.htm

 Dilcher, D. (2000). Toward a new synthesis: Major evolutionary trends in the angiosperm fossil record. Proceedings of the National Academy of Sciences, 7030-7036. Retrieved March 6, 2015, from http://www.pnas.org/lens/pnas/97/13/7030#info

Hedges, S., & Kumar, S. (2009). Plants. In The Timetree of Life (pp. 133-137, 162-165). Oxford: Oxford University Press.

Soltis, D., Soltis, P., & Edwards, C. (2005, June 3). Angiosperms: Flowering Plants. Retrieved March 6, 2015, from http://tolweb.org/Angiosperms/20646/2005.06.03

Porifera

by

An Introduction to the Sponges

Many of us may find it easy to appreciate the diversity of animals that inhabit our planet. Whether it be a bird, insect, or mammal, we humans are often drawn towards some sort of fascination of their mere existence. But what preceded all of the animals we see around us? What may come to mind are images of dinosaur, trilobite or coral fossils, but there existed animals much less complex than any of these. Porifera, or sponges, represent some of the most primeval of animals, lacking body symmetry or specialized organs.  Instead, their body consists of specialized, individual cells that serve different functions for these filter-feeding, sedentary organisms (Blair, 2009). Sponges can be found worldwide, from shallow reefs to deep ocean trenches. They inhabit both marine and freshwater environments, and come in a variety of shapes and sizes. If these organisms represent such ‘ancient’ animals, how old are they? The oldest reliable sponge fossils date back 535 million years ago from Northern Iran (Antcliffe et al, 2014). In addition, sponges are thought to have diverged from the animal phylogeny during the Precambrian, which lasted up until 540 million years ago (Antcliffe et al, 2014). Their basal status in Metazoa, or animals, and ancient lineage represent just a portion of the significance of these bizarre organisms.


Porifera Phylogeny

The group of organisms known as sponges (Porifera) is considered the earliest branching group of Metazoans, or animals, with fossils described from the Vendian Period, dating back 650-543 million years ago (Porifera: Systematics, 2006). Phylogeny in this phylum, or group of organisms, is an ongoing debate, with the current consensus viewing sponges as possibly mono- or paraphyletic (Blair, 2009). Monophyly would indicate a recent common ancestor of all sponges, while paraphyly would indicate that the group of organisms we regard as sponges is actually made up of groups that developed separately over a relatively extensive time. The sponge phylum consists of four currently recognized classes; the Hexactinellidae, Demospongiae, Calcarea and Homoscleromorpha. The relationship between these four classes is still unresolved (Wörheide et al, 2012). Gross morphology suggests the clade is monophyletic.  With the advent of molecular systematics, this monophyly was put into question; however, after extensive sampling and inclusion of specimens from all classes, one recent paper suggests monophyly may be the agreed relationship (Wörheide et al, 2012). Regardless, Hexactinellida and Demospongia are both regarded as being monophyletic, representing those sponges which contain silica-based skeletons (Blair, 2009).

One of the main diagnostic features of sponges had previously been their spicules, which constitute the hard support structure of these organisms. This was later proven to be an inaccurate means of identification, as currently existing sponges have been discovered with solid calcium-based skeletons, matching some features observed in the fossil record (Porifera: Systematics, 2006). Phylogenetic analysis of Porifera is conducted using mitochondrial DNA sequences, in conjunction with analyses of morphological features as well (Wörheide et al, 2012). Porifera are not just significant for their roles in ecology, pharmaceuticals, and commercial products but also in developing hypotheses of what the last common ancestor of all animals could have been.

 

phylogeny

Two proposed models for Porifera phylogeny.  Hex= Hexactinellida, Demo= Demospongia, Cal= Calcarea, and Homo= Homoscleromorpha.  In the left tree, Homoscleromorpha and Calcarea are more closely related to the rest of Metazoa than the other two sponge classes.  In the right tree, all classes share a common ancestor. (Adapted from Wörheide et al, 2012, by Dylan Sedmak)

Fossil Record

          Porifera are the first animals on the metazoan phylogeny, having diverged from choanoflagellates 1020 million years ago (mya). The sponge group Hexactinellida diverged from the Demospongia group around 750 mya and it is estimated that the Calcarea group later diverged from the other two groups an estimated 754 mya (Sperling, et al., 2010). Porifera are the most primitive of animals and thus have an early branch on the animal phylogenetic tree, so they’re likely candidates for Precambrian ancestry (Gehling & Rigby, 1996). To understand this, one needs to look at two sponge groups: the Hexactinellids and the Demosponges. These are the two oldest sponge groups, and both have siliceous spicules. Sperling, et al., (2010) suggests that these spicules must have evolved before the common ancestor of Hexactinellids and Demosponges, which means that these spicules were present in the Precambrian, but not fossilized.

Sponges have a fossil record that extends back further than 500 million years. The oldest fossil found for Hexactinellids are siliceous spicules that were found in Northern Iran and date back to approximately 535 mya and the earliest fossil found for Demosponges came out of Siberia and was dated to be 523.5-525.5 mya (Antcliffe, et al., 2014). These fossil remains appear to be dated around the Precambrian-Cambrian boundary, which is associated with great diversification of animals. Finding fossils for the Calcarea sponges is very difficult because they are not as diverse as the other two groups and they lack siliceous spicules which makes it difficult to find preserved specimens (Antcliffe, et al., 2014).

Finding these fossils and correctly identifying them as sponges is a difficult task, as most reported sponge fossils tend to be volcanic shards or inorganic crystals (Gehling & Rigby, 1996). One paper by Antcliffe, et al., (2014) discusses 20 potential candidates for being the oldest Porifera fossil found. This paper also discussed how it can be difficult to define the criteria that determines the oldest fossil because there is no substantial studies done on the formation of cells that make up sponges. There are no studies that study fossilized sponge spicules that look like today’s sponges or vise versa – looking at potential spicules that look nothing like spicules we find today. Antcliffe, et al. (2014) also made the point that these potential fossils may be misinterpreted as individuals rather than part of a larger organism, which further complicates the fossil record.

Finding just the fossilized spicules makes it difficult for scientists to get a clear picture of the shape and form these sponges took. Sponge fossils found in Australia that date back to the Ediacaran period (Precambrian) give some insight into what these first sponges must have looked like. Gehling and Rigby (1996) found that these fossils were formed in hypo-relief on sandstones, with external molds of fossils commonly found. These fossils indicate that the sponges were dome shaped with an osculum at the apex. Since there are no siliceous spicules that have been found before the Precambrian-Cambrian boundary, Gehling and Rigby (1996) note that the siliceous spicules would not be expected to survive longer than the organism’s tissue during fossilization due to being preserved in sandstone substrate.

 

Molecular Clock

        The molecular clock for Porifera suggests that their origin was prior to the Cambrian explosion. This fits with the assumptions made by Sperling, et al. (2010) that based on the siliceous spicule fossils of Hexactinellids and Demosponges, these spicules must have evolved prior to the Cambrian explosion. Though this indicates a gap in the fossil record, molecular clock analyses can still be done to determine divergence time estimates.

          A paper by Antcliffe, et al., (2014) discussed how it is sometimes difficult to get accurate divergence times because fossils are needed to help the molecular clock to be more accurate in its predictions. The dates of the fossils help the clock analyses to better date divergence times. These dates were found using a small number of nuclear housekeeping genes, which are genes that are generally used to date animal phylogenies. This paper found that the housekeeping genes indicated Porifera diverged from the animal lineage around 800 mya in the Precambrian.

Another paper by Sperling, et al. (2010) did two sets of molecular analyses. One set was with multiple sponges from Hexactinellida and Demosponges. The second set was done without Hexactinellida because the Hexactinellid group is the first group that diverged from the metazoan lineage. Their analyses led to the conclusion that the Silicea (both Hexactinellids and Demosponges) originated around 759 mya and also Demosponges greatly diverged within its own clade around 699 mya. For more divergence dates in the Poriferan phylogeny, refer to Figure 2.

phylogeny tree porifera

Figure 2. This timeline represents the estimated divergence times of the Porifera clade. Data represented in this timeline comes from class notes, Sperling, et al. (2010) and Antcliffe et al. (2014). Porifera diverged from the animal (Eumetazoan) lineage approx. 800 mya with Demosponges and Hexactinellids diverging 759 mya and Calcareans diverging 754 mya. Demosponges further diversified into its own clade 699 mya and Calcareans also further diversified 488 mya. Hex. = Hexactinellids, Demo. = Demosponges, Cal. = Calcareans. Phylogeny created by Sarah Petersen.

 

Key evolutionary innovations

Slime

Some sponges are able to produce slime as a defense against debris or other marine organisms. The amount of slime a sponge can make depends on the type of sponge. There are sponges that have absolutely no slime at all, while others only produce a little and others can produce a lot (Ackers, Moss, & Picton, 1992). The slime certain sponges produce is actually toxic. This natural defense comes from metabolic waste produced by the actual sponge or from toxins that the sponge has modified from these original chemicals (Goudie, Norman, & Finn, 2013).

slime

Figure 3. In this image, a student is seen displaying the sponge’s natural slime excretions in a laboratory setting. Picture taken by Natalie Iannelli.

 

Spicules

Spicules are part of the sponge’s “skeleton” and help to give it shape. There are a wide variety of spicules that can be seen in varying sponges. They can help us determine when different sponge species evolved because of their ability to be genetically determined. The environment can also cause spicules to develop in different shapes or sizes and for more than one type of spicule to be present at a time. Spicules are thought to help sponge in a variety of other ways, such as by helping sponge larvae maintain buoyancy, allowing the larvae to reach a spot to settle, enhancing reproductive success, and catching prey (Uriz, Turon, Becerro, & Agell, 2003).

microscope spicules

Figure 4. A microscopic view of a sponge slurry; the spicules can be observed. The view is on a compound microscope at 400X magnification. Picture taken by Christine Koporc and Sarah Petersen.

 

Toxins

Sponges are able to reuse toxins from other organisms around them, though they can also create their own toxins or in collaboration with the microbes that live inside of them. Many sponges have been found to release highly toxic chemicals and these excretions make up some of the most toxic chemicals in nature. Many of these toxins are used to protect themselves against predators or to outcompete other organisms in a crowded area, but they can be used by humans as well. It has been determined that some of these chemicals could be used in anti-cancer, anti-malaria, and pain control applications (Queensland Museum, 2012).

Apoptosis

Cell death, or apoptosis, is when a cell determines that it is no longer needed and it uses an intracellular death program to get rid of the excess cells. This is a common occurrence in organisms and it even takes place in healthy humans. For example, in a normal healthy human, billions of bone marrow and intestine cells die every hour. There are various reasons for this phenomenon, some of which are in order to properly form a structure when an organism is an embryo or to help ensure that the number of cells does not become too large (Alberts, et al., 2002). Apoptosis first developed in the transition between sponges and their ancestor, meaning that sponges were the first organisms to have a trait of this sort (Werner & Muller, 2003).

Water flow

Sponges contain holes in their bodies to maximize efficiency of water flow. The more surface area there is to absorb nutrients it gets from the water, the better off the sponge will be. The sponges have porocytes on the outside which are openings the water flows into. It then flows out through an opening called the osculum. They are able to pump the water because of flagella on the inside of their cell walls (Porifera: Systematics, 2006).

water flow poriferaFigure 5. This diagram illustrates the method sponges use in order to create water flow through their bodies. Image created by Christine Koporc.

 Video 1. This video demonstrates the water flow system in a sponge. A neon green dye was injected into the sponge and the dye can be seen coming out of the sponge on the other side. Video taken by Natalie Iannelli, edited by Christine Koporc.

Tissue Regeneration   

Sponges have the ability to regenerate their tissue. A study of the capacity of sponges to redevelop conducted at the Carmabi Marine Research Institute located in the Caribbean showed that there are three phases as to how this happens. The first phase is where the damaged surface is closed off by a scar like tissue. During phase two, the tissue changed back to the normal appearance of the surface of the sponge. The only difference is that there is a depression in the surface.The third phase is the filling of the depression. The regeneration of the sponge does depend on the species; some sponges regenerate faster than others. The ability of sponges to regenerate is an important evolutionary characteristic to their survival because they are the food source in reefs for many fish species as well as turtles (Hoppe, 1988).

tissue regeneration porifera

Figure 6. The result of a sponge slurry regenerating. The red masses that can be seen are what has formed after a couple days since the sponge was broken down in a blender. Picture taken by Christine Koporc.

Immune System

Studying the immune response of sponges has peaked an interest in the medical community as antibiotic resistance has become more of a problem. Sponges filter a lot of water during their lifetime. That water is not only composed of the food they need to survive, but also numerous amounts of viruses, fungi, and bacteria. On the surface of the sponge there are special receptors called lipopolysaccharide or LPS which is a protein that allows them to detect bacterial endotoxins. The sponge has the capability to detect what kind material it is filtering through physical and chemical means. It also is able to rid itself of these unwanted pathogens on a molecular basis. It has what is called a LPS-interacting protein and a macrophage-expressed protein that are activated depending on what its receptors recognize. It was discovered by a man named Metchnikoff that sponges use phagocytosis to kill off bacteria as well. Phagocytosis is the ingestion of bacteria or other kinds of material by a cell. Using its detection methods and the way it kills bacteria, viruses, and fungi, the sponge is able to eliminate the unwanted organic material to keep it from dying (Wiens et al., 2005)

 

Sources:

Ackers, R. G., Moss, D., & Picton, B. E. (1992). Sponges of the British Isles (“Sponge V”) (p. 7).

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Programmed Cell Death (Apoptosis). Molecular Biology of the Cell.

Antcliffe, J.B., Callow, R.H.T., & Brasier, M.D. (2014). Giving the earliest fossil  record of sponges a squeeze. Biological Reviews, 89, 972-1004.

Blair, J. E. (2009). Animals (Metazoa). In S. B. Hedges & S. Kumar (Eds.), The Timetree of Life (p. 223). Oxford University Press.

Gehling, J.G., & Rigby, J.K. (1996) Long expected sponges from the Neoproterozoic Ediacarda fauna of south Australia. Paleontological Society, 70(2), 185-195.

Goudie, L., Norman, M. D., & Finn, J. (2013). Sponges: A Museum Victoria Field Guide (p. 18).

Hoppe, W. F. (1988). Reproductive patterns in three species of large coral reef sponges. Coral Reefs, 7, 45–50. doi:10.1007/BF00301981

Porifera: Systematics. (2006). Retrieved February 06, 2015, from http://www.ucmp.berkeley.edu/porifera/poriferasy.html

Queensland Museum. (2012). Toxic Sponges & Pharmaceutical Properties. Retrieved October 02, 2015, from http://www.qm.qld.gov.au/Find+out+about/Animals+of+Queensland/Sea+Life/Sponges/Toxic+sponges+and+pharmaceutical+properties#.VNt_EObF-gu

Sperling, E.A., Robinson, J.M., Pisani, D., & Peterson, K.J. (2010) Where’s the glass? Biomarkers, molecular clocks, and microRNAs suggest a 200-myr missing Precambrian fossil record of siliceous sponge spicules. Geobiology, 8, 24-36.

Uriz, M.-J., Turon, X., Becerro, M. A., & Agell, G. (2003). Siliceous spicules and skeleton frameworks in sponges: Origin, diversity, ultrascrutural patterns, and biological functions. Microscopy Research and Technique, 62(4), 279–299.

Wiens, M., Korzhev, M., Krasko, A., Thakur, N. L., Perović-Ottstadt, S., Breter, H. J., … Müller, W. E. G. (2005). Innate immune defense of the sponge Suberites domuncula against bacteria involves a MyD88-dependent signaling pathway: Induction of a perforin-like molecule. Journal of Biological Chemistry, 280(30), 27949–27959. doi:10.1074/jbc.M504049200

Werner, E., & Muller, G. (2003). The Origin of Metazoan Complexity: Porifera as Integrated Animals. Integrative & Comparative Biology, 43(1), 3–10.

Wörheide, M., Erpenbeck, D., Larroux, D., Maldonado, C.,  Voigt, M. , C. B. and D. V. L. (2012). Deep Phylogeny and Evolution of Sponges (Phylum Porifera). Advances in Marine Biology, 61.

Ferns

by

With over a quarter of a million plant species estimated to be living today, 13,000 may seem like a drop in the bucket, but ferns actually show a major evolutionary step for plants as we know them today (Pearson Education, 2015). Ferns contain a variety of different types of plants under the supergroup archaeoplastida. Although ferns do not have an official classification; they are a part of the subkingdom embryophyta, which contains all land plants. 

There are two different classifications of ferns: the monilophytes and the pteridophytes. Monilophytes include true ferns like the leptosporangiates, the largest group of ferns including over nine thousand species worldwide, while the term pteridophytes include both ferns and some other vascular plants. The term “pteridophyte” has fallen out of favor since it is not one monophyletic grouping like the monilophytes (Schuettpelz, 2004).

The main organism that we see, the growing and the adult fern, known as a sporophyte, is diploid. Diploidy in organisms means that they have two sets of chromosomes. Many organisms that we see today like cats, dogs, and people are also diploid organisms.

The History of Ferns

 ferntree
Phylogeny of ferns. The above figure shows the phylogeny of ferns with divergence dates. A phylogeny is like a family tree and shows how different organisms are related (Pryer et al., 2004).

The first fossils with fern characteristics, embryophytes, begin to appear on record during the middle of the Devonian period about 300 million years ago (USDA, 2006). The Devonian period lasted from 416 million years ago (mya) to 360 mya, and is characterized by the radiation and diversification of plant species and ended with a large scale extinction event that scientists believe was due to global cooling and rising sea levels.

These early ferns split off from the green algae around 960 mya. These are some of the earliest land plants known. The “great fern radiation” is when many modern families of ferns first began to appear. This radiation occurred in the late-Cretaceous period (Bhattacharya, 2009).  Today, roughly 13,000 different species make up the class Polypodiopsida (The Plant List, 2010).

During the Carboniferous period, giant ferns dominated the landscape that grew 9 to 12 meters in height. This was before the time of the angiosperms, flowering plants that produced fruits and seeds, so there was little competition for land. There are still some giant ferns that exist today like Angiopteris evecta, or the giant fern, that can grow up to 7 meters tall and is native to Indonesia, New Guinea, coastal northern Australia and the south and west Pacific Islands (Christenhusz , 2011).

What Makes a Fern a Fern?

Ferns are classified by their vascular tissue, spore production, and lack of flowers and seeds. These plants have three major parts: the rhizome, the frond, and the sporangia. Generally, one or more fronds are attached to the rhizomes by a stipe. The rhizomes are a specialized root structure that draws up nutrients from the soil. The frond is the leaf structure of the fern while the stipe is the technical term for the stalk of the fern. The sporangia are the structures that hold the spores necessary for reproduction (University of Waikato, 2010).

Vascular plants can be differentiated from non-vascular plants by the presence of xylem and phloem. The xylem and phloem work like arteries and veins and help transport nutrients throughout the body of the plant. As the first vascular plants, ferns were able to grow taller rather than wider. Non-vascular plants, like mosses, needed to grow close to the ground in moist areas in order to gain all the nutrients needed to support the organism.


Fern Life Cycle. The video link above shows the life cycle of a fern.

Like mosses, ferns reproduce via spores instead of by seeds. These spores are dispersed via wind and water. When the spores land in a suitable habitat they will begin to grow. The spores of the plant are very small and must be viewed under a microscope because they are about 1/10th of a millimeter in size, and because of their almost invisible nature, ferns and other spore producing plants are known as cryptogams. The term cryptogam refers to their hidden reproduction, and literally means “hidden marriage” (Sytsma, 2014).

These spores, as mentioned before, are housed in the sporangium. In many species of ferns, the sporangium is found on the underside of the leaves, or fronds, of the adult fern. These tend to be grouped into sori (singular, sorus) which are visible to the naked eye as little brown spots. In many species of ferns these sori are protected by an indusium which is a thin membrane that protects the underdeveloped spores and sori. When the spores are ready to be dispersed the indusium shrivels away.

IMG_1506
Indusium on spores of a fern. In the image above, the tan parts on top of the darker sporangium, are already starting to shrivel up. This structure is called indusium. Scale: ~ 3 cm.

20150210_142413(1)
Spores from the underside of a fern. Not all ferns have indusium. Individual frond: ~ 12 cm.

The spores are produced in the sporangium via meiosis. Meiosis is a form of cell division that reduces the number of parental chromosomes in half forming four daughter cells. This is the same process that produces eggs and sperm in animal species. The four daughter cells are haploid which means they contain only one set of chromosomes. These haploid spores are then dispersed by wind or water and will germinate if they land in a suitable environment. The spores then grow into a gametophyte which is also a haploid organism.

The gametophyte is a thin, photosynthetic structure that is only one cell layer thick. The gametophyte produces the sex cells: the egg and the sperm. It also contains two different types of sex organs to produce both of the sex cells. The organs are known as the antheridium, which produces sperm, and the archegonium, which produces eggs. The archegonium is a funnel shaped grouping of 4 cells which allows the sperm cell to swim down into it to meet the egg. The sperm have to swim to the eggs, so the gametophytes need to be in  a very moist environment (The Plant List, 2010).

image7(1)
Female gametophyte. The female gametophyte contains archegonium near the crevice at the top of the gametophyte. Scale: ~1 mm. x 1 mm.

Image-1(3)
Detail of archegonium. The archegonium house the female gamete, the eggs. This is what the male gamete, the sperm, travel into for fertilization. Shown at 40x magnification.

Image-1(2)
Male gametophyte. The preserved, male gametophyte has antheridium, shown as the brown spots. These antheridium once contained sperm. Shown at 10x magnification.

Since this is only one small  part of the life cycle, the gametophyte does not have a root system, but instead has rhizoids. These rhizoids are like miniature roots that serve to anchor the gametophyte and gain nutrients. Once the haploid egg and sperm meet they begin to grow into the diploid organism that we are more familiar with.


Movement of antheridia. The video link above shows the movement of the antheridia through a solution. The antheridia are the tumbling sphere shaped organisms moving rapidly across the screen.

A lot of fern characteristics are shared among other land plants. Mosses also reproduce with the use of spores, and different flowering plants have vascular tissue. In total, it is the combination of everything listed that truly makes a fern a fern.

References

Bhattacharya, D., Yoon, H. S., Hedges, S. B., and Hackett, J. D. (2009). “Eukaryotes.” The Time Tree of Life. 116-120. Retrieved from http://timetree.org/pdf/Bhattacharya2009Chap08.pdf

Christenhusz , M. (2011). Angiopteris evecta (giant fern or king fern). Retreived February 29, 2015 from http://www.nhm.ac.uk/nature-online/species-of-the-day/biodiversity/loss-of-habitat/angiopteris-evecta/

Eberle, J. R., and Banks J. A. (1996). “Genetic Interactions Among Sex-Determining Genes in the Fern Ceratopteris Richardii.” Genetics, 142, 973-85. Retrieved from http://www.genetics.org/content/142/3/973.full.pdf

Pearson Education. (2015, January). Estimated Number of Animal and Plant Species on Earth. Retrieved March 3, 2015. Retrieved from http://www.factmonster.com/ipka/A0934288.html

Pryer, K., Schuettpelz, E., Wolf, P., Schneider, H., Smith, A., & Cranfill, R. (2004). Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany, 9(10), 1582-1598.

The Plant List. (2010). “Pteridophytes (Ferns and Fern Allies).” The Plant List — A Working List for All Plant Species, 1. Retrieved on 12 Feb. 2015 fromhttp://www.theplantlist.org/browse/P/

United States Department of Agriculture. (2006, March 5).  Fern Questions and Answers. Retrieved March. 5, 2015 from http://www.usna.usda.gov/Gardens/faqs/fernsfaq2.html

Sytsma, K. (2014, April 14). Vascular Cryptogams. Retrieved March 2, 2015 from http://www.botany.wisc.edu/courses/botany_401/lecture/02bLecture.html

University of Waikato. (2010 September 24) “Fern Life Cycle.” Science Learning Hub RSS. Retrieved from http://sciencelearn.org.nz/Contexts/Ferns/Sci-Media/Animations-and-Interactives/Fern-life-cycle

Echinoderms

by

Introduction: What are Echinoderms?

Echinoderms are a group of marine animals consisting of well known organisms such as the starfish, sea cucumber and the sand dollar. The phylum Echinodermata consists of about 7000 living species and the phylum is divided into five smaller classes. Echinodermata is Greek for “spiny skinned.” This is clearly seen on echinoderms such as the brittle star and the sea urchin. The most well-known echinoderms are the species of five-armed sea stars. However, other sea stars species have been found to have up to 40 arms (National Geographic). Many species of echinoderms also have unique features in their bodies which allow them to regenerate a lost limb, spine, or even intestine if it is lost, for example, to predation (Mashanov, 2014). Some echinoderms can regenerate a whole new body from a severed arm (National Geographic). This process has important consequences for scientist studying regeneration in vertebrates, like humans (Mashanov, 2014). Echinoderms are very important in both the environment and to people as well. Sometimes these effects by the echinoderms can be positive or negative. Without echinoderms, many areas of the ocean would be greatly affected and therefore, echinoderms are an important animal phylum to learn about.

 

In the beginning:

It is estimated that there are up to 13,000 extinct species of echinoderms and that the very first echinoderm was alive in the Lower Cambrian period. This period of time would range from 490-540 million years ago. The oldest fossil available is called Arkarua. This species was small, round and disc-like with five grooves extending from the center (Echinoderm Fossils).  The first echinoderm was thought to be very simple (Knott, 2004). The organism was motile and bilateral in symmetry. Bilateral symmetry means the organism can be cut right down the middle and be split into two equal halves. The echinoderm ancestry later developed radial symmetry as it was thought to be more advantageous to the species. The bilateral symmetry can still be seen in the larvae of echinoderms but once they reach adulthood, they develop radial symmetry. The first picture below shows an echinoderm larvae and the bilateral symmetry is clearly shown. The concept of radial symmetry is clearly illustrated in starfish including the Horned starfish (Protoreaster nodosus), shown below. Species of starfish, like the common starfish, have five radially symmetrical projections projecting from a central disk. These feet have symmetrical outer and inner structures (Zubi, 2013).

Bilateral Symmetry in Starfish Larvae

Martin_starfish larvae_photo_1 

This picture represents the bilateral symmetry of the echinoderm larvae. The red line dissects down the middle and divides the larvae into two equal halves. Throughout development the bilateral symmetry is lost and becomes radial symmetry.

Radial Symmetry in an adult Starfish

DSC_0057

This picture clearly shows the radial symmetry of starfish. Specifically this starfish has pentaradial symmetry.

 

  • Phylogeny

The extant echinoderms are divided into five clades including the Sea Lilies (Crinoidea), Starfish (Asteroidea), Brittle Stars (Ophiuroidea), Sea Urchins (Echinoidea), and Sea Cucumbers (Holothuroidea). Out of these it is clear that they form a monophyletic group, however there is doubt as to their phylogenetic relationship within the tree itself. This debate is based on whether Brittle Stars (Ophiuroidea) and Starfish (Asteroidea) form a sister clade, i.e. they are each others closest relative, or not (Wray, 1999). Today there are only really two well supported hypotheses those are as follows:

1. Asterozoan Hypothesis: In this hypothesis it is believed that Brittle Stars and Starfish form a sister clade, and just like in the Cryptosyringid hypothesis Sea Urchins and Sea Cucumbers form another sister clade and Sea Lilies is the most basal group. This hypothesis is based off of molecular phylogenetic studies which help to show that even though Brittle Stars has a pluteus-type larva which is the larval form of both Sea Urchins and Sea Cucumbers this could just be a result of convergent evolution or that Starfish reverted to an older form of larval form (Telford, 2014).

Asterozoan Hypothesis

 

2. Cryptosyringid Hypothesis: Similar to the previous hypothesis, Sea Lilies is the most basal group, however in this hypothesis Brittle Stars and Starfish do not form a sister clade. This hypothesis has support in the development of the organism so that Brittle Stars are sister to Sea Urchins and Sea Cucumbers. This is because they all share a common larval state during early development which could imply that Brittle Stars are more closely related to the sister group containing Sea Urchins and  Sea Cucumbers than Starfish (Telford, 2014).

Cryptosyringid Hypothesis

 

Now that their placement among themselves is better understood, where do Echinoderms in general fit in with other animals and other organisms? Echinoderms fit in the superphylum deuterostomes of which composes animals who during development the anus forms first unlike the protostomes which have mouth first development. Humans also fall into this superphylum whereas snails and insects develop mouth first. they are within the supergroup unikonts which is also composed of many animals.

 

The above figure represents the phylogenetic tree of the Echinodermata back to the supergroup Unikonts (Keeling, 2009). The associated divergence dates, or estimated time periods a group split from a common ancestor, are included above in millions of years (MYA) (Hedges, 2006).

 

  • Fossil record and molecular clock

The oldest echinoderms found to date are from the Cambrian period. This period was about 540 million years ago. Some fossils have been found that may be an ancient echinoderm, but there is no definite proof at the moment. The ancient phyla of echinoderms was divided into classes based on body geometry, type of plating, body symmetry and the absence or presence of appendages. Three basic body plans emerged during the Cambrian echinoderms (Scripps Institution of Oceanography, 2011).

  •   Ctenocystoids: with or without appendages, tessellate plate type and a lateralized and symmetrical/asymmetrical body plan.
  • Helicoplacoidea: no appendages, imbricate type plates, ellipsoidal shaped body and helical symmetry.
  • Edrioasteroid: no appendages, tessellate and imbricate plate type,  disc shaped body and pentaradial symmetry.

From the middle of the Cambrian period to the mid to late Ordovician period, the class diversification of the echinoderms occurred twice. According to the fossil record, the diversification decreased at the end of the Cambrian period but this may be due to the lack of artifact preservation. No diversification is more significant than the time known as the Great Ordovician Biodiversification Event (GOBE). The class level during this period was as high as 21. From the Cambrian period to the Ordovician period, eleven new classes originated. Since this peak of diversification, the amount of class diversity gradually decreased. Eventually the amount of classes decreased to eight. With the Blastoids, Ophiocistiods and Isorophid edrioasteroids going extinct in the Permian period, there were only five classes that survived the Mesozoic. These five classes are the same classes that are around today, including, Starfish (Asteroidia), Sea Lilies (Crinoidea), Sea Urchins and Sand Dollars (Echinoidia), Sea Cucumbers (Holothuroidea), and Brittle Stars (Ophiuroidea)(Fossil record of Echinoderms).

 

Key evolutionary innovations:

Echinoderms developed many key evolutionary characteristics that define all species within the phylum, making them one of the most unique animal phyla.  Four major synapomorphies are identifiable within all species of the Echinoderms that distinguish all members of the phylum. A synapomorphy are traits or characters recognized specifically with that species.

 

Radial Symmetry: Unlike chordates, like humans or sharks, echinoderms possess a radially symmetrical body plan. In almost all situations involving echinoderms, the species exhibits pentamerous radial symmetry (pentaradial), or five sided radial symmetry.  What this means is that observed head on, an observer will be able to distinguish five separate, interconnected segments that are all similar in shape, appearance, and anatomy (Morris, 2009). The best group of animals to show this radial symmetry are the starfish.

 

Water Vascular System: In Echinoderms, the water vascular system is their key to everyday living.  It provides Echinoderms with many functions, including gas exchange, locomotion, feeding, and respiration.  The system allows sea-water to be facilitated through an external pore located on the upper portion of the organism called a madreporite, which acts as like a filtered water pump to bring in and excrete water. This system also provides Echinoderms their locomotion through specialized tube feet.  Tube feet provide locomotion for most Echinoderms by expanding and retracting from an individual when water is pushed into or syphoned out of these structures, allowing them to move within their environment to hunt for food and locate shelter. These tube feet also provide Echinoderms with their primary sensory perception as they possess numerous nerve endings, giving them a “view” of their surrounding environment (Class Notes, Knott, 2014). One species which takes advantage of tube feet locomotion is the pincushion sea urchin (Lytechinus variegatus). They posses many tube feet which provide them with sensory information about their environment and assist with locomotion. Below is a video of the starfish using its tubed feet to walk along the tank.

 Sea Urchin Tubed Feet

 This video shows how the Sea Urchin uses its tubed feet to attach to the wall of an aquarium. They suction cup onto the glass for attachment and movement. 

Mesodermal Skeleton: Echinoderm’s skeleton is unique to the animal kingdom.  It is made up of many tiny plates or spines called ossicles, which are comprised of calcium carbonate. In a typical animal, this would lead to the organism having a heavy skeleton, but in the case of Echinoderms, they remain light through a sponge like material called stereom.  Instead of having a rigid skeleton, the stereom is porous, being comprised of a network of calcium crystals that give an echinoderm its shape and rigidity without carrying extra mass (Manton, 2014). Below is a photo of an exposed skeleton of the common starfish (Asterias rubens).

20150209_092719

This is a photograph of an exposed skeleton of a starfish, as indicated by the arrow. The network of porous ossicles is evident in this structure.

 

Mutable Collagenous Tissue: Echinoderms possess special type of tissue that in effect can very rapidly change from a rigid state to a free moving, or loose, state using its nervous system. These tissues are key to connecting ossicles together as ligaments made up of primarily collagen.  This allows Echinoderms to achieve a wide variety of body positions with very minimal, to no muscular effort, and then instantly lock into place. This provides a unique feeding advantage as well, as in the case of sea stars where they can envelop a selected prey species in a loose tissue state, and then incapacitate them by quickly changing to a rigid state (Knott, 2004).

 

 

References

 Echinoderms: The Spiny Animals! (2007, June 5). Retrieved from http://www.oceanicresearch.org/education/wonders/echinoderm.html

 Echinoderm Fossils. Recieved from http://museumvictoria.com.au/discoverycentre/infosheets/marine-fossils/echinoderms/

Hedges, S., & Kumar, S. (2006). TimeTree: A public knowledge-base of divergence times among organisms. Retrieved March 6, 2015, from http://timetree.org

Keeling, P., Leander, B., & Simpson, A. (2009, October 1). Eukaryotes. Eukaryota, Organisms with nucleated cells. Retrieved from http://tolweb.org/Eukaryotes/3

 Knott, E. (2004, October 7). Asteroidea, Sea stars and starfishes. Retrieved from http://tolweb.org/Asteroidea/19238/2004.10.07

 Mashanov, V., Zueva, O., & Garcia-Arraras, J. (2014). Transcriptomic changes during regeneration of the central nervous system in an echinoderm. BMC Genomics, 15(1), 1-38. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4229883/

 Manton, S. (2014, August 25). Skeleton of Echinoderms. Retrieved from http://www.britannica.com/EBchecked/topic/547371/skeleton/41987/Skeleton-of-echinoderms

 Morris, V. B., Selvakumaraswamy, P., Whan, R., & Byrne, M. (2009). Development of the five primary podia from the coeloms of a sea star larva: homology with the echinoid echinoderms and other deuterostomes. Retrieved from http://rspb.royalsocietypublishing.org/content/276/1660/1277

 Pawson, D. (2014, July 24). Echinoderm. Retrieved from http://www.britannica.com/EBchecked/topic/177910/echinoderm

 plb36. (2013, February 12). Echinoderms Fun Facts And Trivia. Received from http://plb36.hubpages.com/hub/10-Things-You-Didnt-Know-About-Echinoderms

 Scripps Institution of Oceanography. (2011). Fossil Record of Echinoderms. Retrieved from http://echinotol.org/fossil-record-echinoderms

 Starfish (Sea Stars), Asteroidea. National Geographic. (n.d.). Retrieved from http://animals.nationalgeographic.com/animals/invertebrates/starfish/

Telford et. al. 2014. Phylogenomic analysis of echinoderm class relationships supports Asterozoa. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/24850925

Waggoner, B. (2001, January 19). Introduction to the Blastoidea. Retrieved from http://www.ucmp.berkeley.edu/echinodermata/blastoidea.html

 Wray, Gregory A. (1999, December 14). Spiny-skinned animals: sea urchins, starfish, and their allies. Retrieved from http://tolweb.org/Echinodermata/2497/1999.12.14

 Zubi, T. (2013, February 27). Multi-celled animals (Metazoa). Retrieved from http://www.starfish.ch/reef/echinoderms.html