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Class Polyplacophora

Ancient primitive molluscs with 8 separate dorsal shell plates. 1000 living species.

Order Chitonida


Cryptochiton stelleriCryptochiton stelleri  (von Middendorff, 1847)

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Mopaliidae

Largest chiton in the world, to 30 cm. Girdle brick red, overgrows 8 shell plates completely.

Abundant, on rocks or sand; herbivorous on red algae with some brown algae. Many have a commensal scale worm Arctonoe vittata and/or a pea crab Opisthopus living in pallial groove between side of foot & inner edge of girdle; several species of filamentous red algae also grow on dorsal portion of girdle.

Geogr. Range: Aleutian Islands to Channel Islands (Santa Barbara)
Synonyms:
Similar species:

 

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Mopalia ciliataMopalia ciliata  (Sowerby, 1840)

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Mopaliidae

Broadly oval 6 cm long with wide spiny girdle; highly variable in color: green with red, orange, blue or white markings.

Common, on rocks in shallow subtidal. Herbivorous.

Geogr. Range: Aleutian Islands to Baja, rare north of Monterey
Synonyms:
Similar species: there are nearly 20 species of Mopalia in Monterey regions; microscopic examination of girdle bristles is required to distinguish most of them.

Image: Balanus crenatus (barnacles)

 

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Tonicella lokiiTonicella lokii  Clark, 1999

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Mopaliidae

Pink lined chiton, 5 cm; head valve (arrow) with continuous zig-zag concentric lines, pink, white, & dark maroon or black, sometimes brilliant blue.

Common but somewhat cryptic; on crustose coralline algae often covered by other algae. Grazes on crustose corallines similarly to the limpet Acmaea mitra.

Geogr. Range: northern and central California
Synonyms:
Similar species: Tonicella undocaerulea also has zig-zag concentric lines on head plate, but lacks continuous dark lines.  Tonicella lineata has smoothly arched concentric lines rather than zig-zags on head valve.  Tonicella venusta is small (1.5 cm), lines on head valve are hard to see and lines on middle valves often expand into flame-like markings.

 

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Tonicella undocaeruleaTonicella undocaerulea  Sirenko,1973

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Mopaliidae

Pink lined chiton, 2.5 cm; head valve with continuous zig-zag concentric lines of pink or orange & white; if dark maroon or black markings present, they do not form continuous lines (photo of preserved specimen shows color pattern but not true colors).

Moderately common; on crustose corallines.

Geogr. Range: Aleutian Islands to Channel Islands (Santa Barbara)
Synonyms:
Similar species: Tonicella lokii also has zig-zag concentric lines on head plate, but with continuous dark lines.  Tonicella lineata has smoothly arched concentric lines rather than zig-zags on head valve.  Tonicella venusta is small (1.5 cm), lines on head valve are hard to see and lines on middle valves often expand into flame-like markings

 

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Tonicella lineataTonicella lineata  (Wood, 1815)

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Mopaliidae

Pink lined chiton, 5 cm; head valve (arrow) with smoothly arched (not zig-zag) concentric lines of pink or orange, white, dark maroon or black, & sometimes brilliant blue.

Moderately common, low intertidal & shallow subtidal on rocks with crustose coralline algae; grazes on crustose corallines.

Geogr. Range: Aleutian Islands to central California
Synonyms:
Similar species: Tonicella undocaerulea  has zig-zag concentric lines on head plate, but lacks continuous dark lines.  Tonicella lokii  has zig-zag lines on head plate with continuous dark lines. Tonicella venusta is small (1.5 cm), lines on head valve are hard to see and lines on middle valves often expand into flame-like markings

 

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Placiphorella velataPlaciphorella velata  Dall, 1879

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Mopaliidae

Broadly oval 5 cm long, with anterior margin of girdle extended into a flap used to capture small motile prey; valves much shorter than width; color pattern a mixture of green, brown, pink, tan & blue.

Common, but very cryptic nestled among algae & sessile invertebrates with anterior margin of girdle raised.

Geogr. Range: Alaska to Baja
Synonyms:
Similar species:

 

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Stenoplax fallaxStenoplax fallax  (Carpenter in Pilsbry, 1892)

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Ishnochitonidae

Elongate oval 8 cm or more; bottom of foot orange; valves purple to red, speckled with brown or white.  Adults on sides of rocks buried in sand; a fast-moving, photonegative chiton.

Common but cryptic under rocks in coarse sand; mostly subtidal.

Geogr. Range: northern California to Baja
Synonyms: Ishnochiton fallax
Similar species:  Stenoplax heathiana is more white or tan with a cream-colored foot.

 

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Stenoplax heathianaStenoplax heathiana Berry, 1946

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family Ishnochitonidae

Elongate oval 8 cm or more; bottom of foot cream; valves white or tan with brown or green speckles or streaks; photonegative; moves quickly back toward underside of rock when disturbed.

Common, but cryptic under rocks in sand; more abundant in mid- to low intertidal zone than in subtidal.

Geogr. Range: northern California to Baja
Synonyms: Ishnochiton heathiana
Similar species:  Stenoplax fallax has an orange foot and its valves are more purple or reddish.

 

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Lepidozona mertensiiLepidozona mertensii

Phylum Mollusca, Class Polyplacophora, Order Chitonida, Family

Broadly oval to 5 cm; central areas of intermediate valves a lattice work of fine ribs; lateral areas with rows of prominent pustules; color variable but usually reddish or orange, often with white blotches; girdle with minute scales & usually reddish with narrow light bands.

Moderately common, but cryptic; usually on undersides of cobbles in kelp forest depths.

Geogr. Range: Alaska to Isla Guadalupe, Mexico
Synonyms:
Similar species: Lepidozona cooperi is similar in size and shell sculpture but is usually uniform dull olive green or blackish-brown (never red).

Image: Membranipora fusca (rt. & top sides)

 

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