Dive Dry with Dr. Bill

#316: Chitons... The Undersea Armadillos

During part of my youth I lived in northern Florida, where I undoubtedly first developed my interest in marine life. I have memories of seeing armadillos there, often as road kill (don't blame me, I wasn't driving). Occasionally when diving in our waters, I am reminded of these strange mammals when I see an equally strange form of snail, the chiton. Their appearance is superficially similar, although their ecological roles are, of course, quite different.

Chitons are a unique group of snails found from the Arctic to the tropics. They are most common in temperate waters like our own where they usually find rocky substrate to live on and plenty of algae to feed on. They represent a very old group, originating about half a billion years ago in the upper Cambrian. For some reason they have begun radiating, creating new species, during the recent geologic era. There are now some 600 species world-wide with about 120 known from Alaska to California.

Unlike most snails, which either have a single shell or no shell at all in the case of the nudibranchs, chitons have jointed shells composed of eight plates or valves. These plates are embedded in the tough, fleshy mantle that makes up the bulk of the animal's body. This armadillo-like shell does not completely cover the mantle and it extends out on either side of the plates. The exposed mantle on the edge of the snail is referred to as the girdle. Hmmm, do women still wear such things? I hope not. The girdle may have a mossy or woolly appearance (on the chiton, not on women). On the undersurface is a broad muscular foot like that of other snails. It is used to cling tightly to the substrate, and to move about in search of food. Gills are located in the groove on either side of the foot.

Chitons are primarily herbivores or scavengers. The diet of individuals in the same species may vary based on what kind of habitat they live in. Like many snails, they have a scraping structure known as a radula. With it they scrape "munchies" off rocks and other substrates, and bits of tissue from seaweeds and kelp, leaving obvious scars. Now scraping food off of rocks can be pretty hard on your teeth. Chitons make their radula teeth much harder by incorporating the mineral magnetite in them. They are able to create this substance themselves through a process known as biomineralization. I guess I could try collecting chitons in the field with a strong magnet!

Some chitons feed only at night, roaming about the substrate before returning to a permanent "home spot" where they rest during the day. Some seek home spots in shallow depressions among the rocks, giving them protection against being dislodged by waves or swell. These depressions may be used by generation after generation of chitons. Here, firmly secured to the rock by their muscular foot, they are protected by the armadillo-like plates of the shell. Should they get dislodged by the surf, or a potential predator, they rely on a secondary line of defense. They roll up like an armadillo to protect their tasty foot and soft mantle. If not eaten, some species may live as long as 20 years.

Plates from chitons have appeared in the kitchen middens of early Native Americans in southern California. The larger species were eaten just like other snails including abalone and wavy tops. Back in my Euell Gibbons era, I led my Toyon students on a survival hike to Little Harbor where we were to eat off the land for 24 hours. I made a stew from kelp, cactus, limpets and chitons. I survived, and it really wasn't much worse than the rest of my cooking. Of course my students snuck in candy bars or stopped at the Airport-in-the-Sky for a buffalo burger... and thereby failed the test!

A individual chiton is usually either male or female, although some may be hermaphrodites. Now I'm not sure how much sex chitons get. After all, both the males and the females wear girdles. Perhaps because of this, the eggs and sperm are released into the water where they fertilize, although some species may brood the developing eggs. The eggs of those that don't brood drift in the currents with the larvae hatching to become part of the plankton before settling to the bottom. Based on this, brooding species may not have an easy means of dispersal unless they catch a ride on a drifting kelp raft. Maybe that's how some of them got to Catalina.

© 2008 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

Three species of chitons, and one beginning to roll up like an armadillo so I don't munch on it!

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia