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

 

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

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).

 

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).

 

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).

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