Clams & Mussels

Bivalves are a large group of animals each of which is covered by a pair of shells (valves).  There are 10,000 species of bivalves worldwide, including mussels, oysters, clams, scallops, and shipworms.  Some live in fresh water, but the majority are found in salt water environments, buried in sand and mud (clams), rock (piddocks) and wood (shipworms).  Others have evolved to become mobile and capable of swimming (scallops).  Bivalves have come to occupy very diverse environments, but remain relatively unchanged through time. 

 

     
 Pacific Blue Mussel (Mytilus trossulus)  
 Blue Mussel Description: Found in dense colonies. Blue, black, brown in color. Interior pearly. Soft body protected by two hinged, calcified shells (valves). Siphons draw in water that is filtered for food and oxygen. Attach to rocks and pilings by strong "byssal threads." Grows to 2.5 in (6 cm) locally. This species of mussel can live 1 - 2 years.  
 Food: Water drawn into the siphons is filtered for plankton and detritus.  
 Reproduction: Large numbers of eggs produced by female.  
 Fun Facts: This species can pump up to 3 quarts of water an hour through its gills in order to filter out plankton.  
   

 

California Mussel

 

(Mytilus californianus)

 
 California Mussel Description: Found in dense colonies. Prefers very strong wave/tidal action. Shell blue, black or brown. Interior grayish-blue. Soft body protected by two hinged calcified shells (valves). Siphons draw in water that is filtered for food and oxygen. Attached to rocks and pilings by strong "byssal threads." Shells thicker and rougher than blue mussels. Grows to 8 in (20 cm) locally, growing 3 in (8 cm) a year.  
 Food: Water drawn into the siphons is filtered for plankton and detritus.  
 Reproduction: Up to 100,000 eggs produced annually by female.  
 Fun Facts: Often host to immature pea crabs. This mussel can contain tiny pearls of varying shapes.  
   

 

Rock Oyster

 

(Pododesmus cepio)

 
 Rock Oyster Description: Also known as a "jingle shell." Nearly round when viewed from above, this oyster has a thin lower shell that conforms to the shape of the rock to which it is anchored. The upper shell is curved and thicker. Anchored through hole in lower shell. To 5 in (13 cm) or more.  
 Food: Filter feeder. When shells open and actively pumping water to filter for plankton, the bright reddish-orange flesh of the animal is visible.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: The name "jingle shell" comes from the sound a handful of these shells make in your hand.  
   

 

Olympia Oyster

 

(Ostrea lurida)

 
 Olympia Oyster Description: Also known as the "native Pacific oyster." Found in sheltered Puget Sound bays. Attaches to rocks. Gray to white, gnarled shells. Pollution, over-harvesting and slow maturation have resulted in sharply reduced numbers of this species. Grows up to 3.5 in (9 cm).  
 Food: Filter feeder.  
 Reproduction: Alternates between male and female each breeding season. Maturation in 4 to 6 years.  
 Fun Facts: The female holds young inside her mantle until their shells develop.  
   

 

Pacific Oyster

 

(Crassostrea gigas)

 
 Pacific Oyster Description: Also known as the "Japanese oyster." Introduced from Japan in the early 1900's, this exotic species became naturalized and is an important commercial crop. Thick, fluted white or gray shells. Grows up to 12 in (30 cm).  
 Food: Filter feeder.  
 Reproduction: The natural reproduction of this species is poor in our area, so commercial growers import tiny oysters each year.  
 Fun Facts: This oyster may live up to 20 years, and sometimes contain irregular pearls.  
   

 

Heart Cockle

 

(Clinocardium nuttallii)

 
 Heart Cockle Description: Oval shells with heart-shaped cross section. Prominent ribs radiating from the hinge. On older individuals the ribs can be worn smooth. Color varies from tan to dark browns. Mottling common. Found in tidal flats near surface. Paired siphons. Can live up to 16 years and reach 5.5 in (14 cm) in size.  
 Food: Filter feeder.  
 Reproduction: Maturation in 2 years. Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: Thick shell is brittle and cracks open easily when dropped onto rocks by gulls.  
   

 

Native Littleneck

 

(Protothaca staminea)

 
 Native Littleneck Description: Also known as the "steamer clam." Found just below the surface of tidal flats. Fused siphons are short. Similar in shape to heart cockle, but shells thinner and more flattened. Many fine ridges radiate out from the hinge. These ridges are crossed by growth rings. White to tan; sometimes stained with geometric patterns. Can live up to 14 years and grows to 4 in (10 cm).   
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom. This clam grows and repopulates slowly, taking 4 to 6 years to reach commercial size. An over-harvested bed may take 25 years to re-establish itself.  
 Fun Facts: These clams are often predated upon by moon snails, which leave a characteristic hole drilled near the hinge as evidence of their appetite.  
   

 

Manila Clam

 

(Venerupis philippinarum)

 
 Manila Clam Description: Also known as the "Japanese littleneck." The shells (valves) are more oblong than that of the native littleneck. Abundant just below the surface in sheltered bays and tidal flats. Varies from gray to brown, with streaking pattern radiating from the hinge common. Interior often touched with purple. Can live up to 14 years, and grows up to 3 in (8 cm).  
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom. Grows to commercial size in just 2 years.  
 Fun Facts: This clam was accidentally introduced into Puget Sound in the 1920's in shipments of pacific oyster seed stock. Species has out-competed native littleneck in many places.  
   

 

Butter Clam

 

(Saxidomus gigantea)

 
 Butter Clam Description: A large thick-shelled clam. Gray to white. Shells can be stained dark by iron sulfate. Prominent growth rings. Valves close together tightly (unlike the horse clam). Found in the low intertidal zone 8 to 14 in (20 to 36 cm) below the surface. Can live up to 20 years, and reaches 6 in (15 cm) in size.  
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: Considered the best clam for chowders.  
   

 

Sand Clam

 

(Macoma secta)

 
 Sand Clam Description: A thin-shelled, white clam found buried 8 to 16 in (20 to 41 cm) in the sands of sheltered bays and tidal flats. Faint growth rings. Twin long, white siphons. Shell angled sharply downward at posterior end. Grows up to 4 in (10 cm) in diameter.  
 Food: Filter feeder specializing in detritus.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: Valves have a sharp bend line along one edge.  
   

 

Bent-Nosed Clam

 

(Macoma nasuta)

 
 Bent-Nosed Clam Description: A thin-shelled, white clam found buried 4 to 6 in (10 to 15 cm) in the mud of sheltered bays and tidal flats. Lies on its side. Faint growth rings. Twin long, orange siphons. Grows up to 2 in (5 cm) in diameter.  
 Food: Filter feeder specializing in detritus.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: When viewed edge-on, valves are bent at an angle.  
   

 

Soft-Shell Clam

 

(Mya arenaria)

 
 Soft-Shell Clam Description: Thin, very brittle, elongated shells which are white to gray in color. Found buried 4 to 8 in (10 to 20 cm) in mud and sand in sheltered bays and tidal flats. Long siphon does not retract fully. Large spoon-shaped projection (chondrophore) at hinge of one shell. Prefers areas of low salinity such as the mouths of rivers. Grows up to 4 in (10 cm).  
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: This species is believed to have been introduced from the Atlantic Ocean sometime around the 1920's.  
   

 

Horse Clam

 

(Tresus capax)

 
 Horse Clam Description: Also known as the "fat gaper." Shells chalky white edged with dark flaky covering called periostracum. Shells sometimes stained dark by iron sulfates in the mud. Valves gape apart around siphon which cannot retract fully. Found up to 20 in (51 cm) below surface in intertidal zone. When exposed at low tide this clam can be seen spitting jets of water into the air when disturbed. Can grow up to 10 in (25 cm) locally.  
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: This clam is often home to two tiny pea crabs which live inside its shell.  
   

 

Geoduck

 

(Panope abrupta)

 
 Geoduck Description: Pronounced "gooey-duck." Found mostly subtidally. Valves gape at end like the horse clam, but their shape is more rectangular, with prominent concentric growth rings. Neck and siphon my be 3 ft (1 m) long. The largest intertidal clam in the world, shells may exceed 8 in (20 cm), and total weight can reach 20 pounds (9 kg). Harvested only on the lowest tides of the year.  
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals; fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: Among the oldest of animals in the world. Can live as long as 146 years.  
   

 

Rough Piddock

 

(Zirfaea pilsbryii )

 
 Rough Piddock Description: White shells with brown periostracum covering. Shells are roughly oval with teeth development at anterior end. Siphon end of shell gapes. The siphon can extend up to 12 in (30 cm). This clam can live up to 8 years, and its shell grows up to 4 in (10 cm).  
 Food: Filter feeder.  
 Reproduction: Bivalves are typically gonochoristic (having separate male and female individuals); fertilization is external, and the developing larva (veliger) eventually settle to the bottom.  
 Fun Facts: This clam rasps away at clay or soft rock, digging a circular burrow in which it lives until it dies.