In anticipation of Linda's and my vacation to the Dominican Republic in a few days, I give you Diadema antillarum, the long-spined sea urchin. This urchin is a keystone herbivore on coral reefs throughout the Caribbean and one I hope to see during our vacation.
This particular species was once a dominant part of coral reef communities. However, a catastrophic die-off in 1983, the result of an unknown pathogen, resulted in an over 97% mortality rate Caribbean-wide - the most severe mass mortality ever recorded for a marine organism. The disappearance of D. antillarum from the reefs lead to the proliferation of macro-algae which quickly overtook the coral reefs. This unchecked algal cover resulted in coral die-off and prevented new coral recruitment. The result was an overall deterioration of reef communities. Although there have been reports of localized D. antillarum and subsequent coral reef recovery (Edmunds and Carpenter 2001), populations remain low even decades later (Lessios 2005 - pdf). Continued research and coral reef restoration using reared and/or transplanted D. antillarum are currently underway.
Changing tacks a bit, below is a stunning video of D. antillarum development from Diadema.org. The video is a bit long, but it comes complete with a soundtrack by Enya (I believe) and contains amazing footage of a metamorphosing larva - if nothing else, you need to watch for this (about a third to half-way in).