This sea urchin is plastic. No, really!
Plasticity describes the ability of a species to grow into different shape, size or other attributes depending on the conditions they live in.
Echinometra lucunter was studied in different sites in Barbados, and individuals in Little Bay were quite different from those at Graves End: their bodies were flatter to the ground and more oval (possibly because higher wave action in Little Bay made it important to by hydrodynamically shaped), and their coloring was different (perhaps because they had a different array of algae to feed on).
This species is also pretty widely distributed, mostly in the Caribbean and Atlantic. The plasticity and the wide distribution may be why these critters have cost taxonomists so much work. At least fifteen different species have been discovered and named, only to turn out to be E. lucunter.
Check out the list: http://eol.org/data_objects/11594077
and the Barbados study: http://www.int-res.com/articles/meps/15/m015p207.pdf
(photo: Simon Coppard via The Echinoderms of Panama)