Clam Biology

Illustration: Clams

Taxonomy

  • Phylum: Mollusca
  • Class: Bivalvia
  • Order: Veneroida
  • Family: Veneridae

Life cycle

Butter clams

Generalized life cycle of the butter clam: Male and female clams spawn in May in the Strait of Georgia, later in north coast. Mass fertilization occurs in water column. Fertilized eggs develop into ciliated, motile larvae within 12 h of fertilization. Larval phase includes several stages (i.e. trochophore, veliger and umbone) and lasts for 3-4 weeks, during which time the larvae drift in the plankton and are dispersed by water currents. The larval phase ends when larvae settle from the plankton and attach themselves to gravel or broken shell by byssal threads, referred to as spatting. At approximately 5 mm the spat or juvenile clam creates a permanent burrow where it remains for life. Although butter clams may spawn every year, poor juvenile recruitment due to adverse environmental conditions, predation or competition can affect adult abundance. Adult clams may live to over 20 years and reach a maximum shell length of 130 mm.

Littleneck Clams

Generalized life cycle of the littleneck clam: Male and female clams spawn from May to September in the Strait of Georgia, and begin later in north coast. Mass fertilization occurs in water column. Fertilized eggs develop into ciliated, motile larvae within 12 h of fertilization. Larval phase includes several stages (i.e. trochophore, veliger and umbone) and lasts for 3-4 weeks, during which time the larvae drift in the plankton and are dispersed by water currents. The larval phase ends when larvae settle from the plankton and attach themselves to gravel or broken shell by byssal threads, referred to as spatting. At approximately 5 mm the spat or juvenile clam creates a permanent burrow where it remains for life. Although littleneck clams may spawn every year, poor juvenile recruitment due to adverse environmental conditions, predation or competition can affect adult abundance. Adult clams may live to 10 years and reach a maximum shell length of 75 mm.

Manila Clams

Generalized life cycle of the Manila clam: Male and female clams spawn between mid-June and September in the Strait of Georgia, during August in central coast. Mass fertilization occurs in water column. Fertilized eggs develop into ciliated, motile larvae within 12 h of fertilization. Larval phase includes several stages (i.e. trochophore, veliger and umbone) and lasts for 3-4 weeks, during which time the larvae drift in the plankton and are dispersed by water currents. The larval phase ends when larvae settle from the plankton and attach themselves to gravel or broken shell by byssal threads, referred to as spatting. At approximately 5 mm the spat or juvenile clam creates a permanent burrow where it remains for life. Manila clams require surface water temperatures in excess of 14 degrees C to permit gametogenesis, spawning and larval development, so establishment is largely along south coast. Adult clams may live to 10 years and reach a maximum shell length of 75 mm.

Razor Clams

Generalized life cycle of the razor clam: Male and female clams spawn in late May and June at Long Beach and in July and early August in Haida Gwaii. Mass fertilization occurs in water column. Fertilized eggs develop into ciliated, motile larvae within 12 h of fertilization. Larval phase includes several stages (i.e. trochophore, veliger and umbone) and lasts for about 3-4 weeks, during which time the larvae drift in the plankton and are dispersed by water currents. The larval phase ends when larvae settle from the plankton and attach themselves to sand or broken shell by byssal threads, referred to as spatting. At about 5 mm the spat or juvenile clam burrows into sand, but no permanent burrow can be constructed due to instability of sand. Recruitment of juvenile clams has occurred at low levels in Queen Charlotte beaches since 1971. Adult clams may live to over 10 years and reach a maximum shell length of 18 cm.

Distribution

  • Butter Clams: common in protected beaches in bays and estuaries along B.C. coast.
  • Littleneck Clams: common in protected beaches in bays and estuaries along B.C. coast, and near rocky outcrops on outer coast.
  • Manila Clams: exotic species; common in protected beaches in bays and estuaries in Georgia Strait and west coast of Vancouver Island; small isolated populations in Queen Charlotte Strait and around Bella Bella.
  • Razor Clams: occur only on surf-swept beaches in Long Beach region on the west coast of Vancouver Island and on beaches east of Masset on the northeast coast of Graham Island in Haida Gwaii.

Habitat

  • Butter Clams: variety of substrates but typically occur in beaches of porous sand, broken shell, gravel and mud; may occur in association with littleneck clam; burrow to a maximum depth of 25 cm; planktonic larvae are dispersed by currents; adults remain in same burrow for life.
  • Littleneck Clams: firm, gravel beaches; often in association with butter clam; burrow to a maximum depth of 15 cm, but usually 3-8 cm below surface; planktonic larvae dispersed by currents; adults remain in same burrow for life.
  • Manila Clams: sand-gravel beaches; burrow just below surface, to maximum depth of approximately 10 cm; planktonic larvae dispersed by currents; adults remain in same location for life.
  • Razor Clams: sandy, exposed beaches; usually burrows from just below surface to a depth of 25 cm; can burrow to depths greater than 60 cm in less than a minute; planktonic larvae dispersed by currents; do not form permanent burrows.

Tidal elevation

  • Butter Clams: lower third of intertidal zone; may occur to 15 m subtidal depth.
  • Littleneck Clams: slightly above mid-intertidal to subtidal; may occur to 12 m subtidal depth.
  • Manila Clams: from 1 m intertidal zone to well above mid-intertidal level; does not inhabit subtidal; limited spatial competition with native clams.
  • Razor Clams: mid-tide level to 20 m subtidal depth.

Food

  • Butter Clams: suspension feeder; mainly phytoplankton, but also zooplankton and detritus.
  • Littleneck Clams: suspension feeder; mainly phytoplankton, but also zooplankton and detritus.
  • Manila Clams: suspension feeder; mainly phytoplankton, but also zooplankton and detritus.
  • Razor Clams: filter feeder; mainly planktonic diatoms, but also zooplankton and detritus.

Predators

  • Butter Clams: crabs and fishes prey on juveniles; moon snail, birds and sea stars prey on adults.
  • Littleneck Clams: crabs and fishes prey on juveniles; moon snail, birds, and sea stars prey on adults.
  • Manila Clams: mainly water birds due to higher intertidal colonization.
  • Razor Clams: gulls, ducks, crabs and fishes.

Growth rate

  • Butter Clams: Slow; sexually mature at 38 mm (about 3 yr.); legal size of 63 mm is reached in 5-6 yr. in Strait of Georgia, 7-8 yr. in Alert Bay and after 9 yr. in north coast.
  • Littleneck Clams: Slow; sexually mature at 25 mm (about 2 yr.); legal size of 38 mm is reached in 3.5 yr. in Strait of Georgia and 5-6 yr. in north coast.
  • Manila Clams: Slow; sexually mature at 20 mm (about 2 yr.); legal size of 38 mm is reached in 3.5 yr. in Strait of Georgia and 5-6 yr. in north coast.
  • Razor Clams: Variable, fastest at lower beach levels; sexually mature after 1 yr. in south and 3 yr. in north; minimum legal size of 90 mm is reached in 1-2 yr. at Long Beach and 3-4 yr. on Haida Gwaii; largest and oldest clams found in B.C. are from Haida Gwaii.

Fishery

  • Butter Clams: Commercial, Native and sport intertidal fisheries; formerly an important commercial species but at present accounts for less than 10% (159 t in 1986) of commercial intertidal clam harvest; sewage pollution and paralytic shellfish poisoning (PSP) have closed large areas of coast to harvesting.
  • Littleneck Clams: Commercial, Native and sport intertidal fisheries; accounts for about 10% (285 t in 1986) of commercial intertidal clam harvest; sewage and paralytic shellfish poisoning (PSP) have closed large areas of coast to harvesting.
  • Manila Clams: Commercial, Native and sport fisheries; accounts for more than 65% (1894 t in 1986) of commercial intertidal clam harvest; sewage and paralytic shellfish poisoning (PSP) have closed large areas of coast to harvesting
  • Razor Clams: Commercial, native and sport fisheries; accounts for less than 5% (142 t in 1986) of commercial intertidal clam harvest.

References

Anon. (no date). Clams of British Columbia. Fish. Oceans Can., leaflet: 4 p.

Bernard, F.R. 1983. Catalogue of the living bivalvia of the eastern Pacific Ocean: Bering Strait to Cape Horn. Can. Spec. Publ. Fish. Aquat. Sci. 61: 55.

Bourne, N. 1986. Intertidal clams, p. 22-31. In G.S. Jamieson and K. Francis [ed.] Invertebrate and marine plant resources of British Columbia. Can. Spec. Publ. Fish. Aquat. Sci. 91.

Fitch, J.E. 1953. Common marine bivalves of California. Calif. Dep. Fish Game Fish Bull. 90: 102 p.

Jamieson, G.S. 1986. Paralytic shellfish poisoning, p. 44-46. In G.S. Jamieson and K. Francis [ed.] Invertebrate and marine plant resources of British Columbia. Can. Spec. Publ. Fish. Aquat. Sci. 91.

Kozloff, E.N. 1983. Seashore life of the northern Pacific coast. Douglas & McIntyre, Vancouver: 292-294.

Quayle, D.B. 1978. The intertidal bivalves of British Columbia. B.C. Prov. Mus. Handb. 17: 63-64.

Quayle, D.B., and N. Bourne. 1972. The clam fisheries of British Columbia. Fish. Res. Board Can. Bull. 179: 70 p.