Phylum Mollusca: Subtidal Bivalves, Cephalopods, & Chitons

Snails, sea slugs, bivalves, octopus, chitons. Second only to arthropods in numbers of species, molluscs have shown enormous evolutionary flexibility in their basic body plan of a head, a broad creeping foot, and a visceral mass containing their gut & internal organs.  ~100,000 living species.
| Kelp Forest Index | Rocky Shore Molluscs |               | Rocky Shore Index | SeaNet Home |

Class Bivalvia (Pelecypoda)
Bivalves comprise ~20,000 living species. It is the second largest class of molluscs after the gastropods (snails & slugs).  Unfortunately, most are cryptic, buried in sand or nestled in rocky crevices.  The few listed here are ones most often encountered in the field by divers, but are only a tiny fraction of the bivalves that occur in Monterey.  

Subclass Pteriomorphia
  Order Ostreoida
Subclass Heterodonta
  Order Veneroida

  Order Myoida
  Order  
  Pholadomyoida
Chlamys hastata Crassadoma gigantea Pododesmus macrochisma Leukoma staminea Chlamydoconcha Chaceia ovoidea Parapholas californica Mytilimeria nuttallii
Chlamys
hastata
Crassadoma
gigantea
Pododesmus macrochisma Leukoma staminea Chlamydoconcha orcutti Chaceia
ovoidea
Parapholas californica Mytilimeria nuttallii

 
Class Cephalopoda
                                                                                | kelp forest index | Rocky Shore Molluscs | SeaNet Home |
Squid and octopus arose from gastropod ancestors to become some of the most highly evolved invertebrates. Their relative brain size rivals that of vertebrates and is far larger than any other invertebrate group. Squid rival fishes in their mobility and dominance as open-ocean predators. 1000 living species.
Order Teuthoidea Order Octopoda
Doryteuthis opalescens
Octopus rubescens
Doryteuthis opalescens
Octopus rubescens
 
Class Polyplacophora
                                                                             | kelp forest index | Rocky Shore Molluscs | SeaNet Home |
Ancient primitive molluscs with 8 separate dorsal shell plates. 1000 living species.
  Order Chitonida
Cryptochiton stelleri
Lepidozona mertensii Mopalia ciliata Placiphorella velata Stenoplax fallax Stenoplax heathiana Tonicella lineata Tonicella lokii
Cryptochiton stelleri Lepidozona mertensii Mopalia
ciliata
Placiphorella
velata
Stenoplax
fallax
Stenoplax heathiana Tonicella
lineata
Tonicella
lokii
Tonicella undocaerulea
Tonicella undocaerulea
















Crassadoma gigantea
Crassadoma gigantea  (Gray, 1825)
Phylum Mollusca, Class Bivalvia, Subclass Pteriomorpha, Order Ostreoida, Family Pectinidae

Rock scallop; shell 4-15 cm, becoming irregular with age; right-hand shell attached to rocks; radial ribs on upper (left) shell with scaly shingle-like spines; mantle margin orange with numerous blue eyes; hinge area purple on interior of shell. 

Common, but cryptic due to overgrowth & fouling of shell; in crevices and on wharf pilings  Juveniles free living & swim by clapping shells but become attached at ~25 mm diameter; can live >25 years.

Geogr. Range: British Columbia to Baja
Synonyms: Hinnites giganteus
Similar species:

Image: Serpula (below scallop), siphon of Pyura (far left)

back to spp. list | more info | refs. | kelp forest index |

Chlamys hastata Chlamys hastata (Sowerby, 1842)
Phylum Mollusca, Class Bivalvia, Subclass Pteriomorpha, Order Ostreoida, Family Pectinidae

Spiny scallop; to 6 cm across with spiny radiating ribs; auricles (flaps near hinge) unequal (anterior one larger). Mantle margin lined with short tentacles & blue eyes. Shell often covered with sponge Mycale adhaerens (pictured) or Myxilla incrustans

Moderately common, but sporadic on rock. Juveniles attach right-hand shell to substrate using byssal threads; adults unattached; can swim using vigorous clapping of shells to escape predators. Lifespan to 5 yr.

Geogr. Range: Alaska to San Diego
Synonyms: Pecten hericeus
Similar species: Chlamys rubida has less pronounced ribs and usually occurs in deeper water on soft substrate; rare south of Puget Sound.

back to spp. list | more info | refs. | kelp forest index |

Pododesmus macrochisma Pododesmus macrochisma (Deshayes, 1839)
Phylum Mollusca, Class Bivalvia, Subclass Pteriomorpha, Order Ostreoida, Family Anomiidae

Rock jingle; irregularly circular shell 8 cm diameter with irregular branching ribs; attached with right shell down by modified foot retractor muscle & calcified byssal threads projecting through distinctive hole in right shell; interior of upper (left) shell often dark pearly green.

On rocks; cryptic due to overgrowth; detectable from circular outline with fringe of short mantle tentacles; preyed on by Pisaster giganteus and other sea stars.

Geogr. Range: SE Alaska to Baja
Synonyms: Pododesmus cepio
Similar species:

Image: Salmacina (white tubes at rt.), Lagenicella (top cntr.), Crisia (upper left), many spirorbids on shell (tiny white coils with red tentacles), red is unidentified sponge.

back to spp. list | more info | refs. | kelp forest index |

Leukoma staminea Leukoma staminea  (Conrad, 1837)
Phylum Mollusca, Class Bivalvia, Subclass Heterodonta, Order Veneroida, Family Veneridae

Littleneck clam. Broadly oval, nearly circular 7 cm across; many fine radial ribs & usually with faint concentric ribs. 

Very common but encountered mostly as empty shells; in sheltered sandy sites 3-8 cm below surface and nestled in crevices among rocks on outer coast.

Geogr. Range: Aleutian Islands to Baja
Synonyms: Protothaca staminea
Similar species:

back to spp. list | more info | refs. | kelp forest index |
Chlamydoconcha orcutti Chlamydoconcha orcutti  Dall, 1884
Phylum Mollusca, Class Bivalvia, Subclass Heterodonta, Order Veneroida, Family Galeommatidae

Naked clam; 1-3 cm; at first glance resembles an unusual opisthobranch; shells reduced & overgrown by mantle; color dense opaque white; foot is elongate & motile, but usually attached by small byssus threads to undersides of rocks or crevices.

Uncommon & rarely encountered due to cryptic under-rock, nestling habit; filter feeder.  Hermaphroditic, maturing first as dwarf commensal male (~1 mm long) that lives embedded in mantle of adult (just posterior to dense white ring visible on specimens in photo). 

Geogr. Range: Sonoma County, CA to Baja
Synonyms:
Similar species:

Image: specimens found by F. Sommer

back to spp. list | more info | refs. | kelp forest index |
Parapholas californica Parapholas californica  (Conrad, 1837)
Phylum Mollusca, Class Bivalvia, Subclass Heterodonta, Order Myoida, Family Pholadidae

Shale-boring clam; siphon tips a circular disc 2-3 cm diameter usually flush with substrate, pale mottled tan; inhalant siphon larger with frill of tentacles around margin. Shell to 15 cm long, drills burrow to 30 cm deep.

Geogr. Range: Bodega Bay to Baja
Synonyms:
Similar species:

back to spp. list | more info | refs. | kelp forest index |
Chaceia ovoidea Chaceia ovoidea  (Gould, 1851)
Phylum Mollusca, Class Bivalvia, Subclass Heterodonta, Order Myoida, Family Pholadidae

Shale-boring clam; siphons deep purple-mahogany 5-8 cm tall; inhalant siphon flared at tip, exhalant siphon tubular. Shell 11-12 cm long; burrows >1 m deep, often directed horizontally into rock shelves.

Abundant where softer shale occurs (e.g. Shale Beds east of Monterey Wharf).

Geogr. Range: central California to Baja
Synonyms:
Similar species:

Image: Peltodoris nobilis (rt., with gills expanded), Salmacina (cntr. bottom), probably Cystodytes (left & cntr near siphons)

back to spp. list | more info | refs. | kelp forest index |
Mytilimeria nuttallii Mytilimeria nuttallii  Conrad, 1837
Phylum Mollusca, Class Bivalvia, Subclass Heterodonta, Order Pholadomyoida
(Anomalodesmata), Family Lyonsiidae

Commensal clam embedded in compound tunicates (e.g. Cystodytes, Aplidium, Eudistoma) with only shell gape and short siphons visible (arrow); shells bright white to 4 cm, thin & fragile, globose (inset); hermaphroditic broadcast spawner. 

Common but inconspicuous, low intertidal zone and subtidal.  Larvae settle on compound ascidians & attach with byssus threads, then are overgrown by the tunicate colony; tunicate usually outlives the clam. 

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

back to spp. list | more info | refs. | kelp forest index |
Doryteuthis opalescens Doryteuthis opalescens  (Berry, 1911)
Phylum Mollusca, Class Cephalopoda, Order Teuthoidea, Family Loliginidae

Market squid; 10 appendages (8 arms, 2 long tentacles) mantle length 17-19 cm.

Seasonally abundant near shore; spawning aggregations lay large communal egg masses on sand in deeper water.

Geogr. Range: British Columbia to Baja
Synonyms:  Loligo opalescens
Similar species:

back to spp. list | more info | refs. | kelp forest index |
Octopus rubescens Octopus rubescens  Berry 1953
Phylum Mollusca, Class Cephalopoda, Order Octopoda, Family Octopodidae

Common octopus; 8 arms, mantle length 10 cm; body red-brown but capable of altering color, color pattern & body texture very rapidly; lacks obvious paired eye spots below eyes near base of arms. 

Common but very cryptic & rarely seen. Nocturnal. Semi-permanent dens under rocks, in holdfasts, and other sheltered sites.

Geogr. Range: Alaska to Baja
Synonyms:
Similar species: species of octopus are difficult to distinguish, especially when small.

back to spp. list | more info | refs. | kelp forest index |
Cryptochiton stelleri Cryptochiton 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:

back to spp. list | more info | refs. | kelp forest index |
Mopalia ciliata Mopalia 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)


back to spp. list | more info | refs. | kelp forest index |
Tonnicella lokii Tonicella 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.

back to spp. list | more info | refs. | kelp forest index |
Tonicella undocaerulea Tonicella 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

back to spp. list | more info | refs. | kelp forest index |
Tonicella lineata Tonicella 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

back to spp. list | more info | refs. | kelp forest index |
Placiphorella velata Placiphorella 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:

back to spp. list | more info | refs. | kelp forest index |
Stenoplax fallax
Stenoplax 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.

back to spp. list | more info | refs. | kelp forest index |
Stenoplax heathiana Stenoplax 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.

back to spp. list | more info | refs. | kelp forest index |
Lepidozona mertensii
Lepidozona 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)

back to spp. list | more info | refs. | kelp forest index |


Page created by J.M. Watanabe
All images copyright James Watanabe unless otherwise indicated
Contact:  watanabe_at_stanford_dot_edu                                                                                                                                       Last  update: 16 June 2014