SeaShell News, 3-17-15, Sea Urchin, Shelling, Sanibel.
“Sea urchins or urchins (/ˈərtʃɪnz/), archaically called sea hedgehogs, are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. About 950 species of echinoids inhabit all oceans from the intertidal to 5000 m deep. The shell, or “test”, of sea urchins is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 in) across. Common colors include black and dull shades of green, olive, brown, purple, blue, and red. Sea urchins move slowly, and feed on mostly algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins. Their roe is a delicacy in many cuisines. The name “urchin” is an old word for hedgehog, which sea urchins resemble.
Sea urchins have conquered most sea habitats, on an extremely wide range of depths. Some species, such as Cidaris abyssicola, can live down to several thousands of meters deep. Many genera are totally indentured to the abyssal zone, such as many cidaroids, most of the genera in the Echinothuriidae family, or the strange genus Dermechinus. One of the deepest-living family is the Pourtalesiidae, strange bottle-shaped irregular sea urchins that live only in the hadal zone, and have been collected as deep as 6850 meters deep in the Java trench. Nevertheless, this makes sea urchin the class of echinoderms living the less deep, compared to sea cucumbers and crinoids that remain abundant below 8000m deep
Sea urchins can be found in all climates, from the warmest seas to the freezing polar seas (like the polar sea urchin Sterechinus neumayeri). They adapt their diet to their environment : in rich ecosystems they feed mainly on algae, that allow a quick growth ; at the contrary in less rich bottoms they adopt a slower metabolism, adapted to a less calorific diet.
The shingle urchin (Colobocentrotus atratus), which lives on exposed shorelines, is particularly resistant to wave action. It is one of the few sea urchin that can survive many hours outside from the water.
Despite their presence in nearly all the marine ecosystems, most species are encountered on temperate and tropical coasts, between the surface and some tens of meters deep, close to photosynthetic food sources.“
Source: Sea Urchins.