Chitons are small mollusks, oval in outline, with a broad foot, and a mantle that secretes, and sometimes extends over, the shell. They live on rocky seashores in much the same life-style as limpets. They are easily distinguishable from limpets, however, by their shell made of eight plates (or valves) with transverse sutures. Also, unlike limpets and other snails, the chitons have no tentacles or eyes in the head region, just a mouth and a radula. The shell is so different from those of other mollusks that one might think chitons are segmented (or metameric), but contrary to the general rule, this is incorrect. Internally, there is no evidence of segmentation, and the eight valves are actually derived from a single embryonic shell.
Except for the color, the uniformity of external appearance tempts one to regard chitons as races of a single species, but the small variations are very important to other chitons and to chiton specialists, who like to count the notches and slits along the valve edges. The word chiton is a Greek word meaning a gown or tunic, usually worn next to the skin, and covered with a cloak on going outdoors. The chiton was worn by both men and women, just as the eight plates are worn by both male and female chitons. There are about 600 species of chitons in all, about 75 of them are on the U.S. Pacific Coast. Among the most common species are Chaetopleura apiculata of New England and Mopalia muscosa of California.
Chitons are classified as subclass Polyplacophora in the class Amphineura, one of the six classes of mollusks. The other subclass contains the Aplacophora, a group of wormlike mollusks lacking a shell, but possessing in some genera calcareous spicules embedded in the mantle. Amphineura means nerves on both sides, and Polyplacophora means bearing many plates; chitons have two pairs of parallel nerve cords running the length of the body. The nervous system is simple and straight, not twisted as in prosobranch snails. In spite of their anatomical simplicity, there is no reason to suppose that chitons represent a form ancestral to all the mollusks. Rather the opposite, the fossil record suggests that they followed the gastropods and bivalves in evolution, and lost some structures or traits as they became adapted to a restricted niche. The lack of tentacles and eyes, for example, means that chitons cannot function as predators. The shell is obviously defensive. When pried loose from their preferred spot, chitons roll up in a ball, much like certain isopod crustaceans, called pill bugs, and like the armadillo, an armored mammal.
Most chitons are 0.8-1.2 in (2-4 cm) long, but there is a giant Pacific coast species, Cryptochiton stelleri, up to 11.8 in (30 cm) long. This species is unusual also for the mantle or girdle that completely covers the shell (crypto = hidden). Other surprises include a species of Callochiton septemvalvis (seven valves). Eggs are laid singly or in a jelly string, and are fertilized by sperm released into the sea water. The larvae of a few species develop within the female, but most larvae are planktonic.
The giant chiton Cryptochiton stelleri was included in a classic study of nucleotide sequences in RNA of a great variety of animals, in which the goal was to establish relations of the phyla. On the resultant phylogenetic tree, the chiton appeared at the end of a branch close to a polychaete worm and a brachiopod, and not far from two clams. Another analysis of the same data put Cryptochiton on a branch next to a nudibranch Anisodoris nobilis and ancestral to the two clams. There is reason to suspect that living chitons are highly evolved creatures, and not good subjects for deductions about the initial metazoan radiation in the pre-Cambrian. The reasoning is as follows: Many marine mollusks have oxidative enzymes that use an amino acid to produce products such as octopine, alanopine, etc. while serving to reoxidize coenzyme, and keep anaerobic metabolism going. These opine enzymes are most varied in archaeogastropods, which are regarded as primitive on numerous grounds. The trend in evolution has been to lose some or all of the opine enzymes, and come to depend entirely on lactic acid production for their function. This is what has happened in a few bivalves and polychaete worms, and in fishes and other vertebrates. It is also the case with the chitons Chaetopleura apiculata and Mopalia muscosa, which have only a lactate oxidase and no opine enzymes. The earliest chitons may have had a great variety of genes that modern species no longer possess, but this is something that would be difficult to investigate.
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