What are Chitons

Ah the Amphineura. What are they and does anybody care? I do, because I like primitive forms and the 8 plated chitons are the second most primitive members of a pretty important invertebrate phylum, the Mollusca. Only the mysterious solenogasters, the aplacophora, may be more primitive but may only more aberrant. Of the two, chitons are much more accessible. I have found them living on most of the rocky intertidal zones I have walked on in the past fifty years. They are usually nestled in a depression in the rocks that they made to suit their purpose: a place where they can hold on so tight that predators cannot get under the shell to the sweet meat inside. They show their identity and primitive nature in the 8 overlapping plates on their backs. All other shelled molluscs have the plates fused into single shells like the gastropods or two hinged shells like the bivalves.

On Heron Island, part of the Great Barrier Reef, the chitons are large, as are their chemically dissolved depressions. During the day they are firmly lodged in their homes. If you grab one and try to pry it off, it grips with that strong molluscan foot and won’t budge. But if you come back at night with a torch, you can catch them crawling about in the shallows, grazing on the algaes that grow there. By morning they are back in their depressions. The same one every night? Perhaps not, and it sounds like a good topic for a little study some time.

From a distance chitons appear drab and are usually only a few centimeters long, but under a microscope they often have beautiful patterns that are valuable camouflage in the wild. About a thousand species have been identified and they are found in the seas from polar to tropical regions and down to 4000 meters deep. They are most abundant in shallow waters with rocky bottoms but they have also conquered the rocky intertidal zone, which is a tough environment at the best of times.

Meglitsch (p 291) states that “Chitons are homebodies, and move about very little unless disturbed. They usually stray away from a definite home site only on short foraging expeditions, coming unerringly to the exact spot they call home, and to which they may have adapted by slight shell modifications.” So that question is answered: My Heron Island chitons are going back to the same depression night after night. How depressing… or not, for a chiton. He also mentions that they are photonegative and I have noticed that too. If you turn their rock over and expose them to the sun, they will laboriously crawl back around to the other side. Meglitsch goes on to say: “They adhere to the rocks with the broad flat foot unless one tries to detach them, when the tough girdle is thrust against the rock and the foot is retracted. This forms a suction cup and enables them to hold tenaciously. If they are detached from their rock, they roll up like an armidillo, with their dorsal plates forming a continuous protective covering.”

I think there are a lot of children and former children out there who have had that experience of finding a little chiton on a rock on a trip to the beach, of prying it off and watching it roll up like a little armidillo and then eventually putting it down to watch it unroll and crawl off, looking for the dark. So perhaps there are other people out there who care about these funny little animals.

Reference: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.