All about Sea Urchins

Sea urchins are small, spiny sea creatures of the class Echinoidea found in oceans all over the world. Their shell, which is also called the "test", is globular in shape and covered with spines. The size of an adult test is typically from 3 to 10 cm. Typical sea urchins have spines that are 1 to 3 cm in length, 1 to 2 mm thick, and not awfully sharp. Diadema antillarum, familiar in the Caribbean, has thin, potentially dangerous spines that can be 10 to 20 cm long. Common colors include black and dull shades of green, olive, brown, purple, and red. Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry and move by means of hundreds of tiny, transparent, adhesive "tube feet". The pentamerous regularity is not obvious at a casual glance but is easily seen in the dried shell or test of the urchin.

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is defined by primarily having a globoid shape without arms or projecting rays, even if the sea cucumbers and the irregular echinoids have secondarily-evolved dissimilar shapes. Although many sea cucumbers have branched tentacles surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, starfish and brittle stars. Within the echinoderms, sea urchins are classified as echinoids. Particularly, the term "sea urchin" refers to the "regular echinoids," which are symmetrical and globular. The ordinary phrase "sea urchin" actually includes several different taxonomic groups: the Echinoida and the Cidaroida or "slate-pencil urchins", which have very thick, blunt spines, and others. Besides sea urchins, the Echinoidea also includes three groups of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins.