echinoderms

echinoderms, a group of invertebrate animals including starfish (asteroids), sea urchins (echinoids), sea cucumbers (holothurians), brittle stars (ophuiroids), and sea lilies (crinoids). The basic design of the bodies is a radial symmetry with five arms as seen in starfish. They have internal skeletons constructed of plates of calcium carbonate elaborately slotted together. This skeleton provides support for their unique mode of movement based on tube feet. Each tube foot is a tiny, extensible, tubular structure that ends in a sucker and is powered by a hydraulic system. Individually, these are weak, but when acting in concert they are remarkably powerful.

Starfish are mostly predatory and can pull apart the shells of a clam far enough to evert their stomach into the victim and start to digest its flesh in situ. The most notorious starfish is the crown of thorns (Acanthaster planci) which eats the soft tissue of corals and causes considerable damage to coral reefs. They are nocturnal, but if found should be handled with great care because their spines are poisonous.

Most other types of echinoderm either eat seaweeds or feed on detritus by filtering water, or by swallowing mud. Many sea urchins graze on seaweeds using the teeth of the Aristotle's Lantern, the elaborate mouthparts on the underside of the body. Others burrow in sand and mud and have become asymmetric; the sand dollars (e.g. Mellita spp.) for example are very flattened and bury themselves just beneath the surface of clean sandy beaches. In the kelp forests fringing the coasts of California, sea urchins feed on the kelp and in turn are eaten by a marine mammal, the sea otter. In some species of urchins the gonads, which hang around the insides of the shells like washing, are considered to be delicacies by gourmets. Urchins are also exploited for their shells, which are cleaned and sold to tourists as souvenirs. Many sea urchins need to be handled carefully, as the long spines of some readily break off and cause festering wounds, and others can inflict painful stings using special clawlike structures.

Sea cucumbers, which are almost devoid of any traces of radial symmetry, are more wormlike in their feeding behaviour. They either burrow in the sediment or crawl over the seabed, feeding by either swallowing mud or filtering particles from the water. A few deep-water species can swim. They are often abundant in the lagoons on the landward side of coral reefs. In Asia and Mediterranean countries they are collected, dried, and eaten as trepang or bêche-de-mer.

Brittle stars move by rowing themselves along using their arms, and feed on small food items. In areas where there is a rich supply of food they can form thick layers of animals several centimetres thick. Basket stars (Astroboa spp.) can be nearly a metre across and their writhing arms are repeatedly divided and used to trap particles suspended in the water.

Sea lilies either have their bodies on the end of long stalks or have clawlike appendages so that they sit with their mouths uppermost and use their long arms to entrap fine food particles from the water. Their limy skeletons preserve well and so there is a rich fossil record of echinoderms that stretches back 600 million years to the Cambrian era.

M. V. Angel