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Sea Stars or Starfish?

The fascinating world of the echinoderm.

By Melissa Heres, Waterfront Assistant, The School for Field Studies, Center for Marine Resource Studies ~ Photos By Anna Handte-Reinecker

The name “echinoderm” might not bring much to your mind—perhaps unwelcomed trips to the dermatologist or a whiff of echinacea. But by taking the word apart we learn that echino- translates to “something prickly,” while -derm is a Greek root that means “skin.” So, what exactly are these prickly-skinned creatures? 

This brittle star, with its thin arms and distinct central disk, is seen moving across a sandy floor.

Echinodermata is a phylum or grouping of organisms with similar traits and genes. The traits that an organism must possess to fall into the illustrious category of Echinodermata are as follows: they usually have five point radial symmetry, tube feet, a calcified skeleton and a water vascular system, which acts almost as a hydraulic system to move their tube feet. Having prickly skin isn’t a necessity to be considered an echinoderm. Interestingly, these creatures don’t have eyes or a brain, but they do have an incredible ability to regenerate—sea stars in particular are well known for losing an arm and re-growing it within a year or so. 

Echinoderms are divided into five smaller groupings, called classes. These include Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (feather stars) and Holothuroidea (sea cucumbers). Class by class, we can learn about the lives of these incredible and often underappreciated animals that are found right here in the Turks & Caicos Islands.

Echinoderm biology is truly, spectacularly weird. Since echinoderms lack a head, they do not necessarily have a front or back portion of their body. Echinoderms do, however, have a bottom and top portion, called “oral” (or mouth bearing) and “aboral” (non-mouth bearing), respectively. The water vascular system mentioned earlier consists of fluid filled canals that lead to the echinoderm’s tube feet. These work in a sort of hydraulic system, where fluid is pumped into the tube foot via a one-way valve. Echinoderms can possess as many as 2,000 of these tube feet, and we know little about how the coordination of all of these 2,000 contractions and retractions work in their entirety. We do know that echinoderms use a duo-gland adhesive system which allows their tube feet to stick to the surface that they are walking on. The echinoderm’s tube feet secrete an adhesive to attach themselves to whatever they’re crawling on, and then secrete another chemical that breaks this adhesion to detach their tube feet from the substrate. Another unique characteristic that echinoderms possess is a mutable connective tissue, which allows them to alter the degree of stiffness of their tissue, reverting from stiff to nearly liquid in a matter of seconds.

Perhaps the most well known and loved of all the echinoderms is the sea star, in the class Stelleroidea and subclass Asteroidea. Sea stars—often misleadingly termed starfish (even though they are invertebrates and not fish) have a star-shaped body. Sea stars use their tube feet, not their arms, to move around. Their main prey includes small invertebrates such as sponges, worms, bivalves or coral polyps, but sometimes these sea stars prey upon small fish, and even other echinoderms! When presented with a rather large meal, sea stars have an interesting way of feeding: they can invert their stomach out through their mouth and digest their prey externally!

Closely related to the sea stars are the brittle stars, also in the class Stelleroidea but in the subclass Ophiuroidea. These creatures are characterized by a central disk with five slender arms extending outwards. Some brittle stars are deposit feeders, meaning that they ingest sand and filter out the organic bits to eat. Other brittle stars are suspension feeders, meaning that they filter food particles from the water. Yet other brittle stars are carnivores or scavengers. Brittle stars are usually found under rocks and crevices during the day, coming out at night to feed.

This West Indian sea egg hides from predators in a patch of turtle grass.

The oldest known echinoderms are in the class Crinoidea, also called crinoids. This class contains sea lilies and feather stars, wherein sea lilies are stationary and feather stars can move. Feather stars are able to swim short distances by moving their arms, or “feathers,” down forcefully in a beautiful display. All crinoids are suspension feeders and use their arms and a series of mucus-covered, tubular pinnules to catch their food.

Sea biscuits, sea urchins, and sand dollars all fall within the class Echinoidea. This group of echinoderms have spines that attach to their tests (their version of skeletons) via ball and socket joints. Spines of echinoids serve several purposes: usually for defense, gathering food or bracing themselves when they get stuck in tight crevices. Echinoids can be either regular, meaning that they have a perfect, spherical shape, or irregular, meaning that they have some degree of bilateral symmetry. All sea urchins are regular, while heart urchins or sea biscuits are irregular, displaying a more elongated body shape and a distinct front and back part of their body.        

Last, but certainly not least, are the sea cucumbers, which are in the class Holothuroidea. Sea cucumbers can be thought of as elongated sea urchins, minus the spines. Their tentacles, however, are modified tube feet that can be outstretched from their mouth to capture food. Most are suspension feeders that dig through the sand to find bits of organic material.

Echinoderms spanning all of the classes have an incredible ability to regenerate. Sea cucumbers, when threatened, have the ability to expel their entire digestive system! In order to do this, they liquify and rupture the connective tissue holding their digestive system in place. All of these body parts are eventually reformed.  Similarly, sea stars can also regenerate body parts. When disturbed, these creatures can sever their limbs and eventually regenerate them over time.

A sea urchin test, or skeleton, is washed ashore.

These amazing creatures can be seen all around the waters of the TCI. Golden crinoids can often be seen hiding in crevices of reefs. The cushion sea star is commonly found in the flats, in shallow, predominantly sandy areas. Brittle stars of all kinds can also be found, usually in the crevices of corals or sponges. The giant basket star, a kind of brittle star, is inconspicuous during the day, but extends its long, thinly branched arms out at night to feed. Sea urchins can be found among the reef, like the long-spined urchin, or in predominantly sandy or mangrove areas, like the West Indian sea egg. Heart urchins, sand dollars and sea biscuits can all be found in shallow, sandy areas. Sea cucumbers can be found in shallow sandy patches or deeper reefs.

Students at the SFS Center for Marine Resource Studies on South Caicos have the wonderful opportunity to witness these amazing creatures in action on a weekly basis. Sightings of such varied echinoderms as tessellated cushion stars, ruby brittle stars, reef urchins and tiger tail sea cucumbers are never in short supply. These creatures have been spotted during recreational snorkels and dives, field research exercises and extraordinary night dives near the wall that leads down to the Columbus Passage (Turks Head Passage). Taking time to snorkel or dive on the beautiful reefs of the Turks & Caicos Islands is truly a magical experience; take some time and explore them for yourself and see how many echinoderms you can spot!

For more information, contact SFS Center Director Heidi Hertler, PhD at hhertler@fieldstudies.org.



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