The Gumboot Chiton (Cryptochiton stelleri)

The Gumboot Chiton

The Pacific Coast of North America is home to the world's largest chiton, a class of animals in the Phylum Mollusca. The top picture shows a typical individual of this species next to the size 12 gumboot of the author. The genus name for this animal, Cryptochiton, means 'cryptic' or 'hidden' referring both to ithe fact that it looks rather like a clump of rock in its natural environment, and the eight bony plates normally seen on the back surface of chitons are hidden from view by the thick red mantle. The species hame, stelleri, is in honor of the great German explorer scientist, Georg Steller, one of the first great naturalists to study life on the North American West Coast around 1750.

Chitons are often found washed up on sandy beaches in various stages of decay. The middle photo shows one such rotting chiton revealing 5 of the 8 bony plates exposed by the removal of the foot and internal organs. Many members of phylum Mollusca have protective shells, but only members of this Class of chitons, (Class Polyplacophora - meaning 'many shelled') have the 8 bony plates.

Most chitons shed their eggs and sperm directly into the sea water. The larva that develops is called a 'trochophore' larva: it swims around propelled by circular bands of cilia. Types of trochophore larvae are found in the life history of several phyla of animals including annelids, molluscs, and sipunculids, sugggesting these groups may share common ancestors. Eventually, the chiton trochophore develops a kind of segmentation revealing the 8 portions of the body that will each secrete one of the bony plates. Like many other marine animals, the chiton spends several days to weeks of its life developing in the plankon before it is ready to settle and metamorphose into the adult form.