Excerpts from Jim Conrad's
from the May 29, 2011 Newsletter issued from Mayan Beach Garden Inn 20 kms north of Mahahual, Quintana Roo, México
At the rocky point below us a ringed or segmented object about two inches long (5cm) was either stuck to or else protruded from an algae-covered rock regularly swept by waves. It looked so rocklike that I thought it was a fossil. Scratching it with my fingernail, it felt so dense and flinty I grew convinced it was. Then I noticed a kind of hairy-leathery fringe around the protrusion. Between waves I got the picture shown above.
Before long I'd found a dozen or so of these creatures, all on rocks or sand in very shallow water or submerged only when waves swept in. They were like slugs with stony, straight, segmented shells.
On the bike ride back home, the name "chiton" suddenly dawned on me. It was a word from Biology 100 class many years ago. Chitons are primitive "living fossils" appearing at least 400 million years ago, during the Devonion Period. That was very long before dinosaurs, in fact the first fish with hinged jaws were just arising in the oceans, and on land the first gymnosperms were just getting around to forming the Earth's first forests.
Soon the Internet confirmed that they were chitons. In fact, my picture matches the appearance of what often is called the West Indian Fuzzy Chiton, ACANTHOPLEURA GRANULATA, commonly distributed throughout the tropical Western Atlantic. In English speaking Caribbean countries sometimes chitons are referred to as "curb," and their fleshy parts are eaten. The "hairy-leathery" ring surrounding the stony segments is known as the girdle. It's not clear what girdles do for the chiton, since some chitons get along well without them.
About a thousand chiton species are recognized. Wikipedia's page all about them is at http://en.wikipedia.org/wiki/Chiton.