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The Amazing Echinoderms

Tourists will find them on the beach and in shallow waters and many of their dried skeletons will go home as souvenirs. They are the echinoderms. Next to shells and corals, these are probably the most collected and displayed of sea animals. Many homes, restaurants and stores display starfish, sand dollars or sea biscuits.

This is the Brown Sea Biscuit (Clypeaster rosaceus). The living animal is covered with a dense 'fur' of short spines. 

Below: the spiny urchin (Diadema antillarum) Photos by Ronda Cox of Tropical Kayak Tours.

 

Anyone who has trodden upon a spiny sea urchin (known locally as a sea egg) will fully appreciate the name echinoderm, which means literally ‘spiny-skinned’. All of the animals in this large and diverse group have spiny skins, but few of them are as well developed as in the spiny urchin (Diadema antillarum), common in shallow Bahamian waters especially around rocky coasts.

The spines of this animal are made of crystalline calcium carbonate, the same substance from which the skeletons of corals and the rock of the Bahama Islands is made.

Diadema sea urchin - Photo by Ronda Cox of Tropical Kayak Tours.

It is a fragile substance, and while having strength along its length - as when forced into someone's foot when they tread upon it, it is very brittle when subjected to sideways forces. In short, this means that the spines will penetrate into your foot and then break off - much to your later discomfort. As if this were not bad enough, the spines are coated with a mucus irritating to human flesh!

Fewer and much thicker are the spines of the comparatively harmless pencil urchin (Eucidaris tribuloides). This shy urchin is usually found wedged into a hole in the rock, under a boulder or deep under the reef. In the Pacific, the spines of a closely related species are commonly used to make necklaces and wind chimes.

Probably the commonest of Bahamian urchins is the round 
whitish short-spined Green sea urchin (Lytechinus variegatus). This is a dead urchin washed up on the beach with just a few spines remaining.
Photo by Ronda Cox of Tropical Kayak Tours.

This is a close relative of the common Mediterranean urchin whose sexual organs are a gastronomic speciality. The animal is simply cut in half and the organs removed with a spoon and eaten raw. I must confess I have never been tempted to sample the organs of Bahamian Sea eggs!

 Because of the potential discomfort of handling sea urchins, most people leave them well alone. They are however, worth a closer look. We are accustomed to thinking of animals as having two sides; the echinoderms have five! Two claws are common in marine animals; the echinoderms have hundreds. As if this were not enough, the echinoderms move by means of hundreds or even thousands of tiny hydraulically operated suckers called tube feet.

The Rock boring urchin can excavate holes in the rock. These are just below the low water mark. Any potential predator would find removing these urchins an almost impossible task.

In the sea urchins, the tube feet run in five rows from the bottom of the globe-like spherical skeleton to the top. In the starfish (sea stars) and Brittle stars, the rows run from the centre underside out along each arm. These amazingly mobile tube feet can be best observed when a starfish or urchin climbs the sides of a glass aquarium. Not only are they used for locomotion, but also for manipulating tiny particles of food. Such mobility is especially surprising when one considers that the fact that echinoderms have no brain as such and only the simplest of sense organs.

Sea urchins are grazers, chewing off small seaweeds that grow upon the surface of the rock with five teeth that come together in the centre of the mouth on the underside. Star fish feed mainly on small clam shells that they prise apart with their arms, although the largest Bahamian species, the Cushion Star feeds on Sea grass.

The Basket Star entangles its arms around soft coral during the day. At night  it stretches them out to make a 'net' in which it can catch plankton floating by.

Rather less of a gourmet is the humble Sea Cucumber. He makes do with what biologists call detritus, particles of dead and decaying organic matter scattered on the sandy sea bed. Sea Cucumbers, which do somewhat resemble brown slimy cucumbers have a variety of impolite names in the islands. They are host to one of the strangest of Bahamian fish. 

The Pearlfish lives in the back passage of the Sea Cucumber. The late Jim Bohlke examined more than 100 Sea Cucumbers in the harbour at Bimini and found that more than one in three had Pearlfish inside them!

It is not entirely clear whether the Pearl fish actually feeds on the internal organs of the Cucumber, or whether it only uses its host for shelter-venturing out at night to feed. The former could well be the case, for the Sea Cucumber can regrow its organs. In fact, if harassed by a predator, one of its tricks is to regurgitate its internal organs as a sort of diversionary snack for the predator. Meanwhile, the Sea Cucumber crawls away and grows another set!

Starfish and Brittle Stars can also grow new legs of one of them is bitten off. They will sometimes also grow a new leg in response to damage and will end up with more than the five legs they started life with!

The Sand Dollar (Mellita quinquiesperforata) and the several species of 'Sea Biscuit' common in Bahamian waters are all adapted for burrowing under the sand. Their flattened shapes - particularly pronounced in the case of the Sand Dollar, allow them to slide slowly beneath the surface of the sand. 

They all have spines, but these become progressively smaller and more numerous as the flattening increases until in the Sand Dollar they have the appearance of a fine brown fur.

Reproduction is a communal affair in echinoderms and occurs when a female releases her several million eggs into the water. It is believed that a hormone - a chemical substance released at the same time, stimulates other females to release eggs and males to produce sperms. Although fertilisation is a haphazard business in the open sea,  it is nevertheless effective because of the enormous numbers of eggs and sperms involved. Such huge numbers are necessary because the vast majority of them will be eaten by other organisms. This method however, ensures a wide dispersal of the species and has several other biological advantages.

Scientists have theorised that the lowly echinoderm may be ancestral to all of the vertebrates or animals with backbones - including Mankind. Not only does the larva of some echinoderms bear an uncanny similarity to the larva of primitive vertebrates, but biochemical similarities have also been discovered.

Echinoderms may not be the most exciting animals, but they are some of the strangest, their odd irregularity giving them certain decorative values. Their true value though, lies in lies in the role they play in the rich diversity of marine life. Like little vacuum cleaners, the urchins and Sea Cucumbers clean up the sea bed, while their juvenile stages make up a large and important part of the plankton - providing food for small fish and other animals in the upper layers of the oceans.

Next time you are on the beach, take a second look, see the tube feet and the tiny 'biters' or pedicellariae between the spines which serve to keep the urchin clean and sometimes to grip on to pieces of seaweed for camouflage. Observe, think and enjoy, but whatever you do, don't tread on a spiny urchin!

 

 

© R. Attrill 2000