File:Diadema antillarum (hatpin urchins) (Bimini Island, Bahamas) (16144456455).jpg

Diadema antillarum (Philippi, 1845) - hatpin urchins. (photo taken in 1964 by Lee & Mary Ellen St. John)

Sea urchins (regular echinoids) are starfish relatives with a pentaradially-symmetrical, subglobose, calcareous skeleton (test) covered in spines. The mouth is underneath the urchin, close to & at the water-substrate interface. Sea urchins are algae grazers - they use a pentaradially-structured jaw, called an Aristotle's lantern, to scrape algae and biofilms from hard substrates.

Hatpin urchins used to be abundant in the Caribbean (see 1964 photo above). Their long, sharp spines were a common hazard to swimmers and snorkelers (I’ve seen spine tips break off in someone’s flesh). In 1983 and 1984, a disease-induced near-extinction of the species occurred. At any one locality, over 1 or 2 days, numerous dead hatpin urchin skeletons (tests) washed ashore onto beaches. The sequence of deaths followed a current in the Caribbean. The near-extinction was caused by a still-unidentified pathogen. Around San Salvador Island in the eastern Bahamas, Diadema is making a comeback - I saw it for the first time in March 2009. I saw live specimens on carbonate hardgrounds in very shallow water landward of Telephone Pole Reef and dead specimens on the adjacent beach.

The decline in the health of reefs throughout the Caribbean since the 1980s has been attributed to, in part, the absence of Diadema urchins. Diadema is an algal grazer. After its near-extinction, the lack of algal grazing resulted in reefs being overwhelmed by benthic algae.

The large fish at left in the above photo is a stoplight parrotfish (Sparisoma viride).

Classification: Animalia, Echinodermata, Echinoidea, Diadematidae