This is a long spine sea urchin. It is also called long-spined sea urchin, lime urchin, or black sea urchin. Sea urchins are in a group of animals called echinoderms, which also include sea stars. Long spine sea urchins live in the Caribbean Sea and eat algae.
In 1983, throughout the Caribbean most of the long spine sea urchins died. No one knows why for sure, but it was probably some kind of disease. When that happened, the algae started to grow, grow, grow. When that happened, there was less coral on the coral reefs. When that happened, the fish and other animals that live at the coral reefs left. When that happened, not as many people came to the Caribbean to snorkel and dive.
Recently, scientists have discovered that long spine sea urchins are making a comeback.
I think it is interesting that when something affects one animal, it can affect other animals, too.