One Shell of a Phylum

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!

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.

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.

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.

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.

 

 

2.    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.

3.      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.

 

4.    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/