Chitons

Phylum Mollusca Class Polyplacophora Number of families 10

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Mollusks with a flattened, ovoid shape, broad ventral foot, and eight (sometimes seven) dorsal shell plates that overlap one another and allow the animal to bend and mold itself onto a rock to avoid wave dislodgement

Photo: The lined chiton (Tonicella lineata) is a slow moving mollusk with a shell composed of multiple plates. (Photo by David Hall/Photo Researchers, Inc. Reproduced by permission.)

Evolution and systematics

The earliest fossil chitons occur in the Upper Cambrian, dating the group back nearly half a billion years. The fossil record of this group of mollusks is relatively sparse, with approximately 350 described fossil species. Chitons diversified more rapidly in recent (Cenozoic) times, and today there are approximately 1,000 living species worldwide. One-fifth of the species are found on the Pacific coast of North America, distributed from Alaska to Southern California, more than on any coast of comparable length in the world. Roughly half of all living species live in the intertidal or shallow subtidal zones.

Within the phylum Mollusca, the chitons are unique in possessing eight shell valves. However, the class Monopla-cophora, with a single shell, shares several characteristics with the chitons, including eight pairs of dorsoventral pedal retractor muscles. Repeated pairs of many organs, including one to three pairs of gonoducts, three to seven pairs of excretory nephridiopores, three to six pairs of gills, and two paired atria in monoplacophorans may suggest that mollusks as a whole evolved from a segmented ancestor not unlike the chitons. Larval aplacophorans, larval polyplacophorans, and adult polyplacophorans possess seven or eight transverse dorsal rows of spicules, further strengthening the link between these two classes of mollusks. Finally, recent analyses of 18S rDNA gene sequences suggest that the mollusks are united with other eu-trochozoans that possess a trochophore larva, including the annelids. The analyses also support the theory that mollusks arose from a segmented ancestor.

Nevertheless, other theories currently lean toward a non-segmented ancestor for the phylum Mollusca, based on the unsegmented coelom (unlike that of annelids), the lack of agreement in the number of paired organs in basal mollusks (e.g., aplacophorans versus polyplacophorans), and the lack of evidence of segmentation in many other classes within the phylum. These theories support the placement of the apla-cophorans, with a vermiform body that lacks a shell (and often lacks a foot, radula, and most gills), as near the base of the phylogenetic tree of mollusks.

In addition, apparent differences between the shell valves of polyplacophorans and the so-called dorsal plates found in aplacophorans suggest that chitons stand alone as a uniquely derived group that arose early in the evolution of mollusks.

In conclusion, it is unclear where, within the spiralian protostomes (including numerous phyla such as the Platy-helminthes, Nemertea, Sipuncula, Echiura, Annelida, Ony-cophora, and Arthropoda), the mollusks arose, although the presence of a reduced coelomic area surrounding the heart (the pericardial space) suggests a coelomate (rather than acoelomate) ancestor.

Physical characteristics

Chitons are distinct in possessing eight (sometimes seven) overlapping transverse shell plates (hence, the name Polyplacophora, which means "bearer of many plates") that permit

A juvenile chiton on a cone shell. (Photo by Bill Wood. Bruce Coleman, Inc. Reproduced by permission.)

the ovoid, dorsoventrally reduced body to conform to the irregular, rocky shores on which they are most often found. Strong, paired pedal retractor muscles extend from the foot to each shell valve, which is often wing-like or butterfly-like in shape, with two lateral, anterior extensions where these muscles attach. The shell plates are composed of four layers: an outer, organic periostracum; an inner tegmentum composed of calcium carbonate and proteinaceous material (con-chiolin); an inner articulamentum below the tegmentum, comprised of pure calcium carbonate (aragonite) that extends laterally, free of the tegmentum layer to form the insertion plates of each valve; and the innermost hypostracum, lying against the mantle.

The mantle, a thick, stiff tissue layer that secretes the shell, extends beyond the shell plates (and sometimes over them, such as in Cryptochiton stelleri). This tissue layer secretes a thin glycoprotein cuticle on the dorsal surface of the body. This cuticle may bear scales, bristles, or calcareous spicules similar to those in the class Aplacophora.

Ventrally, a broad muscular foot is bordered laterally on each side of the body by a pallial groove between the edge of the foot (medially) and the edge of the mantle (laterally), forming a chamber in which the gills (or ctenidia) are located. Within these mantle cavities, anywhere from six to 88 pairs of ciliated, bipectinate gills are located. A current of water en ters alongside the anterior end of the body, ventrally, on both sides of the body near the head, and travels through each of these grooves posteriorly, exiting out of the body beyond the tail end of the digestive track (the anus). This water current carries oxygenated seawater, and rids the animal of egested feces (released from the anus) as well as urine released out of a pair of nephridiopores that open laterally in the posterior mantle cavity. The nephridiopores represent the exit points of two large nephridia that filter out wastes within the coelomic cavity surrounding the heart, which is filled with blood (and often termed a heomocoel).

Most chitons feed on microalgae, scraping the surface of the rocks on which they sit with a long radular belt of 17 recurved teeth, arranged in transverse rows, that are capped with magnetite (an iron-containing hardening material) in some species. Some variation in feeding exists. Species such as Katharina tunicata feed on large macroalgae, including kelps (Hedophyllum). Other species possess spectacular modifications of the anterior portion of the girdle to trap small crustacean prey, allowing the evolution of carnivory in an otherwise completely herbivorous (or omnivorous) group.

The chiton nervous system consists of a circumenteric nerve ring around the gut, leading to ladder-like nerve cords that radiate posteriorly towards the end of the body along four lines: two paired pedal cords and two paired visceral cords. These four nerve cords are connected by a series of transverse rung-like commissures, yielding the ladder-like form of the overall system.

Reproduction in chitons involves a single gonad, formed in the dorsal hemocoel, which empties gametes (either eggs or sperm, depending on the sex of the individual) by way of two gonoducts into the mantle cavity just anterior to the openings of the nephridiopores on both sides of the foot. Eggs (which are coated with a spiny envelope) are released either singly or in strings, and are fertilized externally in the water column. Development usually leads to a lecithotrophic (yolk-filled) trochophore larva. There is no veliger stage. Some species have larger, direct-developing eggs that are brooded in the female's mantle cavity, from which a juvenile chiton is formed. The shell gland develops with seven regions on the dorsum of the juvenile; the seventh band divides later to form an eighth. Thus, eight shell plates are formed.

Distribution

Chitons are common rocky intertidal inhabitants, occurring particularly in the temperate zone.

Habitat

Chitons are found primarily on hard substrates, molding their body to the contours of the rock. They are found from the high intertidal zone to depths of more than 13,123 ft (4,000 m), and occur in tropical, temperate, and cold polar seas. Most abundant on hard substrates, especially rocks, chitons graze on surface microalgae and encrusting organisms.

Behavior

Cryptochitons and other chitons roll up when dislodged from a rock, about the only defensive trick these animals have. Several species, such as the mossy chiton (Mopalia muscosa), have "home scars", or areas on a rock that they return to following excursions for feeding; these place are often particularly well situated for the chiton to grasp onto to avoid dislodgement by waves as the tide comes in. Except for the predatory chiton, Placiphorella velata, which can quickly trap prey with its "head-flap", most chitons are highly sedentary animals that remain stationary when the tide is out, mostly feeding at night when low tides or full water submergence keeps them moist.

Feeding ecology and diet

The radula, a chitonous ribbon covered with many rows of hard, recurved teeth, is used for feeding, most frequently on the microalgae that coat the rocks on which chitons are found. In some species, this radula contains iron, part of it as magnetite, the only known example of biological production of this common mineral. Magnetite greatly strengthens the radula, allowing many chitons to feed on hard, encrusted coralline algae.

Besides the microscopic and macroscopic algae, chitons are also known to feed on other encrusting organisms (such as bryozoans). One genus (Placiphorella) has evolved the remarkable ability to capture live prey such as worms and small crustaceans by using an expanded "head-flap" created by an anterior extension of the girdle, which is held above the substrate until unwary victims wander in, at which time the head-flap is rapidly closed down over the prey.

Reproductive biology

The reproductive system of chitons consists of a gonad located in front of the heart dorsally, with a pair of ducts that open into the mantle cavity at the posterior end of the body. The sexes are always separate, with sperm shed into the sea. Eggs are either shed into the sea or retained in the female's mantle cavity, where sperm that enter with the respiratory currents fertilize them. The eggs are then brooded until embryos become well-developed young chitons.

Conservation status

No chitons are listed by the IUCN.

Significance to humans

Native Americans of the Pacific Coast of North America used to eat the giant chiton, Cryptochiton stelleri; shell valves of this species can be found in prehistoric kitchen middens.

Dermataolgy The Sea
1. Tonicella lineata; 2. Placiphorella velata; 3. Mopalia muscosa; 4. Katharina tunicata; 5. Cryptochiton stelleri. (Illustration by Joseph E. Trumpey)

Gumboot chiton

Cryptochiton stelleri

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