Green Island Hammerhead Trip (Feb11) C 582.jpg
Shi Lang, Green Island -- Close-up of a ea urchin (Echinothrix calamaris) during a night dive.
Clearly visible is the urchin's spherical, semi-transparent cloaca that protrudes from the anal opening. It serves to expel waste matter and can be withdrawn into the shell.
Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. "Echinodermate" means "spiny skin" in Greek.
Like other echinoderms they have a fivefold symmetry and move by means of hundreds of tiny, transparent, adhesive "tube feet".
Sea urchins are nocturnal animals.
The spines, long and sharp in some species, protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are not dangerous. It is not clear if the spines are venomous.
Sea urchins feed mainly on algae, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaetes, sponges, brittle stars and crinoids.