There are not many echinoderm species in Icelandic waters compared to the other invertebrate groups. But they are large and conspicuous. Echinoderms have a skeleton composed of calceous plates to protect themselves. Sometimes this forms a rigid skeleton, like most sea urchins, while for others the plates are small and loosely or not attached to each other, for example the soft sea cucumbers. The five groups of echinoderms have quite different habits.
Feather stars (crinoidea) are the most primitive of the echinoderms, they are almost sessile on the bottom and filter microscopic food from the ocean. They are mostly found in the deep sea and therefore rarely seen while diving.
Sea stars (asteroidea) are probably the best known group. Although innocent looking, they are in fact voracious predators on other slow moving or sessile animals, especially bivalves. Fishermen consider them a pest when they invade scallop beds and mussel growing lines. Several species are found in North Icelandic waters but by far the most common species is the aptly named common starfish (Asteria rubens).
Brittle stars (ophiuroidea) are closely related to sea stars. They are much smaller and have a circular central plate with 5 narrow arms extending from the plate. They can walk on these arms and are quite mobile compared to other echinoderms. Brittle stars are harmless detritus feeders.
Sea cucumbers (holothuroidea) are leathery and soft unlike other echinoderms. They vary in shape and size but the largest and most common is the orange-footed sea cucumber (Cucumaria frondosa). It is the size and shape of a large eggplant. Sea cucumbers are detritus or filter feeders.
Sea urchins (echinoidea) are the best studied echinoderms in Icelandic waters. The most common species in shallow waters are the red sea urchin (Echinus esculentus) and the even more common but smaller green sea urchin (Strongylocentrotus droebachiensis).