Echinoderms

Echinoderms – Starfish, Sea Urchins and Sea Cucumbers

Starfish in a rock pool

Photograph of a starfish in a rock pool on Clogherhead Beach

Starfish in a rock pool

Starfish in a rock pool

Photograph of a starfish in a rock pool on Clogherhead Beach

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Starfish, Sea Urchins and Sea Cucumbers have no bony skeleton and instead give their bodies rigidity by pumping water at high pressure down a network of tubes (in much the same way that a hydraulic digger uses pressurised fluids to power its metal arms). All echinoderms are based around the pattern of a five-pointed star, either laid out flat as with starfish, or folded over like the segments of an orange skin to form a sphere, as with sea urchins. They move by means of flexible sucker feet, powered by the ‘hydraulic fluid’ in the tubes inside their bodies.

Common starfish
Taken by Kieran Boyce, Dublin University Sub-Aqua Club

• The Common Starfish (Asterias rubens) has become a pest on many oyster and mussel beds around the Irish coast. It feeds on these shellfish by crawling over them, firmly attaching its arms to each of the two segments of their shell and pulling relentlessly until the unfortunate mollusc weakens. Then, as soon as a gap appears in the shell, the starfish extrudes part of its stomach inside and digests the shellfish inside its shell. The ability of starfish to regrow a severed arm, or even to create two new starfish from one that has been torn apart, has foiled attempts by fishermen to eradicate them from valuable shellfish beds.

•  The Cushion Star (Asterina gibbosa) is one of the commonest echinoderms found on Irish shores. Its small size (rarely more than 3 cm across), and its habit of living under rocks makes it difficult to find. It feeds on barnacles, sedentary worms and other encrusting animals.
   

• The Black Sea Urchin (Paracentrotus lividus) lives in shallow pits that it carves out of rocky surfaces on the lower shore. Its colour ranges from black, through purple and dark green or brown and it feeds mainly on seaweed. In Ireland , it is now farmed and sold abroad as luxury seafood since the gonads inside are considered a delicacy by gourmets on the Continent.
   

• The Edible Sea Urchin (Echinus esculentus) has a large white, pink or purple shell, up to 18 cm across the spines and equipped with rows of tube feet which can extend several centimetres out from the body. It is normally found at extreme low water mark and its empty shells are often washed up on the shore.