Polyplacophora

(Chitons)

Phylum Mollusca

Class Polyplacophora

Number of families 10

Thumbnail description
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

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 Monoplacophora, 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 eutrochozoans 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 nonsegmented 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 aplacophorans, 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 Platyhelminthes, Nemertea, Sipuncula, Echiura, Annelida, Onycophora, 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

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 (conchiolin); 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 enters 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.

Species accounts

List of Species

Gumboot chiton

Cryptochiton stelleri

order

Neoloricata

family

Acanthochitonidae

taxonomy

Cryptochiton stelleri Middendorff, 1846, Kamtschatka, Russia.

other common names

English: Gumshoe chiton.

physical characteristics

Largest chiton in the world, reaching 13 in (33 cm) in length and 5 in (13 cm) in width. Has the general appearance of a wandering "meatloaf." It is distinguished both by its size and its brick-red colored, leathery mantle, which extends up and over the shell valves, obscuring them from sight. The mantle is covered with closely spaced fascicles of very short, spreading spines, or spicules. The white, butterfly-shaped shell valves are hard and frequently wash up on beaches intact.

distribution

From the Aleutian Islands (Alaska) south to San Miguel Island and San Nicolas Island of the Channel Islands National Park in California; northern Hokkaido Island, Japan; and Kurile Islands, Kamchatka.

habitat

Found on rocky shores as well as soft bottoms, in relatively protected sites near deep channels, from the low intertidal zone down to a depth of roughly 70 ft (21.3 m) in kelp beds.

behavior

It has a relatively weak foothold on the rocks at low tide, and individuals can frequently be found lying near the base of a rock from which they have fallen at low tide. Individuals are not gregarious, and a study on the Oregon coast found that marked individuals remain within 65.6 ft (20 m) of the point of release even after two years of time. They often harbor a commensal polychaete worm, the scaleworm (Arctonoe vittata), ventrally in the pallial groove (mantle cavity) on one side of the foot. The commensal feeds on plankton and detritus brought in by the respiratory currents of the host.

feeding ecology and diet

Uses its many transverse rows of 17 teeth capped with magnetite, with a central tooth flanked by eight marginal teeth on either side, to feed on red algae, including Gigartina, Iridaea, Plocamium, and various corallines. Individuals will also eat sea lettuce (Ulva), kelp (Macrocystis), and small Laminaria. They grow slowly and may live 20 years or more. They have few enemies: the predaceous snail, Ocenebra lurida, is the sole exception; it rasps pits 0.4 in (1 cm) in diameter and 0.11–0.15 in (3–4 mm) deep into the dorsal surface of a chiton's body, exposing the yellow flesh over the valves.

reproductive biology

Spawning in California occurs between March and May, may last over a week, and results in individuals losing up to 5% of their body mass. The cinnamon-red eggs are laid in gelatinous spiral strings up to 3.3 ft (1 m) long, which do not stick to the substratum and are broken up by waves. Release of eggs by females triggers the release of sperm by males. Trochophore larvae are liberated from the egg roughly five days post-fertilization, following a free-swimming period of up to 20 hours, then settle and begin metamorphosis.

conservation status

Not listed by the IUCN.

significance to humans

Amerindians of the Pacific Coast of North America used to eat this species; shell valves are frequently found in prehistoric kitchen middens.

Lined chiton

Tonicella lineata

order

Neoloricata

family

Ischnochitonidae

taxonomy

Tonicella lineata Wood, 1815, Sitka, Alaska, United States.

other common names

None known.

physical characteristics

Body low, elongate-oval, usually less than 1.2 in (3 cm) long. Possesses shiny shell valves distinctly demarcated with zigzag lines of alternating dark and light red, light (or dark) blue and red, or whitish and red colors. Mantle girdle naked and leathery, usually yellow to green in color, sometimes banded.

distribution

From Alaska (in the Aleutian Islands) south to San Miguel Island in the Channel Islands National Park in California, in the Sea of Okhotsk (Russia) to northern Japan.

habitat

Found on temperate rocky shores, on rocks covered with erect or crustose coralline algae, in the mid to low intertidal zone down into the subtidal to depths of 180 ft (54.8 m). On the Monterey Peninsula in California, subtidal individuals show consistent color and size differences, being smaller (0.39–0.78 in [1–2 cm] long) with purple lines on the girdle. On the Oregon coast, it is frequently found living under purple sea urchins (Strongylocentrotus purpuratus) in the burrows that the urchins dig out of rock. The color pattern frequently matches, to some degree, the coralline algal substrate, lending chiton some degree of protective camouflage.

behavior

Activity patterns vary with habitat: individuals near Monterey, California, remain stationary in the intertidal zone when exposed at low tide, while subtidal individuals follow a diurnal

rhythm characterized by more twice as much movement at night than during the day.

feeding ecology and diet

Feeds on crustose coralline algae (Lithothamnium), which it scrapes with its strong radula, consuming the more superficial layers and removing the film of diatoms and other small organisms coating the surface of this alga. Species that feed on the lined chiton include the sea stars (Pisaster ochraceus and Leptasterias hexactis). However, in Monterey Bay, California, sea stars have rarely been observed to feed on chitons except for those that are removed from the rocks.

reproductive biology

Females release eggs into the water column in April along the coasts of California and central Oregon, whereas populations on San Juan Island (off the Washington coast) release eggs in May and June. Cleavage divisions and gastrulation lead to the formation of a trochophore larvae 16–24 hours post-fertilization (depending on temperature), and they hatch roughly 43–44 hours after fertilization. Development of the trochophore larva stops sometime within 150–160 hours post-fertilization, and further development depends on contact with crustose coralline algae (or an extract thereof). Larvae undergo metamorphosis to the adult within 12 hours of settlement, becoming juvenile chitons that begin to feed around 30 days after settlement.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Black katy chiton

Katharina tunicata

order

Neoloricata

family

Mopaliidae

taxonomy

Katharina tunicata Wood, 1815, west coast of North America.

other common names

None known.

physical characteristics

Up to 4.7 in (12 cm) long, with a highly elongate-oval shape. Girdle thick, shiny, black, and leathery; white shell valves are deeply embedded in the girdle, exposed only in the mid-dorsal area.

distribution

Found in the Aleutian Islands (Alaska) to the faunal break at Point Conception (Santa Barbara County in California), and in Kamchatka, Russia.

habitat

Common on mid to low intertidal rocky shores exposed to strong wave action, often in direct sunlight, as well as inland waters associated with swift currents in the Pacific Northwest.

behavior

Remains at a particular level in the tide zone, and is one of the tougher chitons, being highly resistant to wave splash and exposure to sun.

feeding ecology and diet

Uses hard, magnetite-capped radular teeth to consume diatoms and red and brown algae, including kelp (Hedophyllum).

reproductive biology

Reaches sexual maturity at a mass of 0.14 oz (4 g) and length of 1.3–1.4 in (33–36 mm). Depending on latitude, individuals spawn between March and July; later spawning occurs in colder regions. The eggs are green. Lifespan is roughly three years.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Mossy chiton

Mopalia muscosa

order

Neoloricata

family

Mopaliidae

taxonomy

Mopalia muscosa Gould, 1846, Puget Sound, United States.

other common names

None known.

physical characteristics

Oval to oblong body up to 3.5 in (9 cm) long with a moderately developed dorsal ridge down the midline; brown to olive or gray valve surface is lusterless, sometimes sculptured with wavy, crenulated riblets, but often eroded or overgrown by algae or worm tubes. Tan-colored girdle covered by dense assemblage of stiff, mossy, brownish red hairs or bristles with a slight notch carved at the posterior end of the girdle.

distribution

Found in the Queen Charlotte Islands (British Columbia, Canada) to Isla Cedros (Baja California).

habitat

Does well in estuaries because it tolerates a wide range of environmental conditions. Found on rocks protected from heavy surf and in tidepools in the mid to low intertidal zone.

behavior

More tolerant of light than many other chitons, and can be seen out in the open on foggy days on the Pacific Coast of North America. It spends most of the daylight hours under rocks, however, and moves primarily at night when the animal is wet or submerged. Individuals that are exposed at low tide show homing behavior, moving within a radius of roughly 19.6 in (50 cm) from home base, frequently returning by a set pathway to original resting place when immersed by tide. Individual home ranges do not overlap, but chitons living in tidepools (which are never exposed) do not appear to maintain home ranges.

feeding ecology and diet

Feeds on algae, primarily the reds (Gigartina papillata and Endocladia muricata) as well as green alga (Cladophora) when available, but the gut may contain up to 15% animal matter.

reproductive biology

In Central and Northern California, spawning has been noted from July to September, whereas a winter spawn has been recorded in Santa Monica Bay (Los Angeles County, California) and a spring spawn has been observed in Monterey Bay, with eggs and sperm shed into large tidepools. The green- or golden brown-colored eggs are 0.011 in (0.29 mm) in diameter and, following fertilization, develop through three cleavage divisions in 2.5 hours. Hatching occurs in 20 hours, and the larvae swim freely for several days, with settlement occurring about 11.5 days post-fertilization, provided an appropriate substratum is present. The first seven shell plates are visible within 13.5 days, with the last (eighth) shell plate appearing only about six weeks after fertilization. Sexual maturity is reached in approximately two years.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Veiled chiton

Placiphorella velata

order

Neoloricata

family

Mopaliidae

taxonomy

Placiphorella velata Dall, 1879, Humboldt Bay, California; Monterey Bay, California; Todos Santos Bay, California, United States.

other common names

None known.

physical characteristics

Distinguished by the girdle around the margins of shell valves, which is greatly expanded anteriorly to form a "head flap," ventrally pigmented with red and green. The body is up to 1.9 in (5 cm) long, with brown to red shell valves that are short and wide and variously mottled and streaked with green, beige, white, and brown.

distribution

Although uncommon, found from Forrester Island (Alaska) to Isla Cedros (Baja California) and the upper Gulf of California.

habitat

Found associated with coralline algae very low in the intertidal zone, in shaded depressions, or in crevices on or under rocks from the low tide line down to a depth of 50 ft (15.2 m).

behavior

Only chiton genus to have adapted a means for capturing live prey. Other than the fast movement of the girdle clamping down on unsuspecting victims that wander into the cavity below "head flap," adults are highly sedentary.

feeding ecology and diet

May browse on encrusting sponges and algae, scraping the rocks with radula like other chitons, or acts as a predator, an unusual feeding mode among chitons. Captures worms and small crustaceans by clamping down on them quickly (within one second) with the extended "head flap," which is a very large anterior expansion of the mantle that is lifted to form a trap for unsuspecting prey. Flap is pigmented on both sides and fringed with bristles, and thus resembles a blade of red algae. The rest of the body is often overgrown with bryozoans and algae, making it highly camouflaged as well.

reproductive biology

In California, it spawns in September.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Brusca, G. J., and Brusca, R. C. A Naturalist's Seashore Guide: Common Marine Life of the Northern California Coast and Adjacent Shores. Eureka, CA: Mad River Press, 1978.

Gotshall, D. W. Guide to Marine Invertebrates: Alaska to Baja California. Monterey, CA: Sea Challengers, 1994.

Kozloff, E. N. Seashore Life of the Northern Pacific Coast: An Illustrated Guide to Northern California, Oregon, Washington, and British Columbia. Seattle: University of Washington Press, 1983.

Meinkoth, N. A. The Audubon Society Field Guide to North American Seashore Creatures. New York: Alfred A. Knopf, 1992.

Morris, R. H., Abbott, D. P., and Haderlie, E. C. Intertidal Invertebrates of California. Stanford, CA: Stanford University Press, 1980.

Pearse, V., Pearse, J., Buchsbaum, M., and Buchsbaum, R. Living Invertebrates. Boston, MA: Blackwell Scientific Publications, 1997.

Sean F. Craig, PhD