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Class Bivalvia (Pelecypoda)

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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

 

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Subclass Pteriomorphia
  Order Ostreoida

 

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)

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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.

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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.

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Subclass Heterodonta
  Order Veneroida

 

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:

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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

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  Order  Myoida

 

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:

 

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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)

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  Order Pholadomyoida

 

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:

Image:

 

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