Identifying Bivalve Shellfish

Manila Littleneck Clam

Size and Shape: Up to 2.5 inches. Oblong (similar to Native Littlenecks but more oval).

Shell: Concentric rings with radiating line ridges. Siphon tips are split. Color can vary – typically grey, brown, or mottled. Some purple on the inside.

Depth: 2-4 inches.

Habitat: Gravel, mud, sand. Above the half-tide level.

Manila littleneck clam.
Manila littleneck clams.

Native Littleneck Clam

Size and Shape: Up to 3.5 inches. Rounded (similar to Manila Clams but more round).

Shell: Concentric rings with radiating ridge lines. Siphon tips are fused. Color can vary – typically cream, grey, brown, or mottled. White inside shell.

Depth: 6-10 inches.

Habitat: Gravel, mud. Normally mid-tide level, sometimes lower intertidal or subtidal zone (up to 60 feet).

Native littleneck clams.
Native littleneck clam.

Butter Clam

Size and Shape: Up to 5 inches. Heavy for size. Oval to square.

Shell: Concentric rings. No radiating ridge lines. Yellow to grey/white colored.

Depth: 12-18 inches.

Habitat: Sand, gravel, cobble. Lower intertidal or shallow subtidal zone (up to 60 feet).

Warning! Butter clams retain marine biotoxins longer than other clams.

Butter clam.

Varnish Clam

Size and Shape: Up to 3 inches. Oval, flat.

Shell: Concentric rings and shiny brown coating. Purple inside the shell.

Depth: 1-2 inches.

Habitat: Gravel, cobble, mud, sand. Upper one-third of the intertidal zone but can range into lower tidal level. Most abundant near freshwater inputs.

Warning! Varnish clams retain marine biotoxins longer and at higher levels than other clams. They are often found near fresh water sources, which makes them more susceptible to runoff pollution. They are also bi-modal feeders (they filter feed and bottom feed) which may account for the higher toxin levels typically found in this species.

Varnish clams.
Inside and outside of varnish clam.

Cockle Clam

Size and Shape: Up to 5 inches. Round, somewhat triangular (pointed hinge).

Shell: Prominent evenly spaced radiating ridges which fan out from the hinge. Mottled, light brown colored.

Depth: 1-2 inches.

Habitat: Sand, mud. Intertidal or subtidal (up to 50-60 feet).

Cockle clam.

Macoma Clam

Size and Shape: Up to 4 inches. Oval to square, to somewhat triangular.

Shell: Wafer-thin, chalky-white shell may be bent at siphon end.

Depth: 4-6 inches.

Habitat: Sand, mud. Middle intertidal zone.

Macoma clams.

Horse Clam

Size and Shape: Up to 8 inches. Oval.

Shell: Chalky-white with yellow/brown patches of "skin" on the shell. Shell flares around siphon. Siphon can't be fully pulled into shell and has a leather-like flap on the tip.

Depth: 1-2 feet.

Habitat: Sand, mud, gravel. Lower intertidal zone (up to 50-60 feet).

Horse clam.
Horse clam.

Eastern Softshell Clam

Size and Shape: Up to 6 inches. Oval to square.

Shell: Brittle, thin shells with rough irregular surface and uneven concentric rings. Chalky-white to grey with brown/yellow skin on the edges. Rounded at foot, pointed at siphon end. Siphon doesn't have leather-like flap on the tip like the Horse Clam.

Depth: 8-18 inches.

Habitat: Sand, mud. Upper half-tide level near river mouths (low salinity).

Eastern softshell clam.


Size and Shape: Shell up to 10 inches. Weighs an average of 2.5 pounds and up to 10 pounds. Oblong (appears rounded at one end and cut-off at the other end).

Shell: Gaping oblong shell with concentric rings. White shell with flaky brown skin. Siphon and mantle are too large to withdraw into shell.

Depth: 2-3 feet.

Habitat: Mud, sand, gravel. Subtidal zone (some intertidal, accessible only on extreme low tides).


Razor Clam

Size Shape: Up to 6 inches. Oblong.

Shell: Gaping oblong shell with concentric rings. Brown shell. Siphon is too large to withdraw into shell.

Depth: 6 inches or more (moves rapidly downward when dug).

Habitat: Sand. Intertidal coastal zone (ocean beaches).

Razor clams.
Razor clam.


Size and Shape: Blue mussel up to 3 inches. California mussel up to 6 inches. Oblong shape.

Shell: Blue-black or brown shell.

Habitat: Attaches to rocks, pilings, boats, gravel, or other hard surfaces in the intertidal zone.



Size and Shape: Up to 12 inches for some species. Oblong with irregular, wavy edges.

Shell: Chalky white or grey.

Habitat: Rocky beaches in the intertidal zone.


Fresh Water Clams and Mussels

Freshwater clams and mussels, like their marine counterparts, are filter feeders. There are no water quality standards that apply to freshwater clams and mussels, and they are not tested. Because they can live close to sources of contamination and can concentrate toxins, chemicals, pathogenic bacteria, and viruses from their environment, they are not considered safe to eat. The Department of Fish and Wildlife prohibits the harvest of freshwater clams and mussels from all Washington fresh water sources – see Shellfish/Seaweed Species Rules in WDFW's Sportfishing Rules Pamphlet.

More Resources

Bivalve Shellfish Identification Handout (PDF)

Recreational Shellfishing, WDFW

Shellfish Illness Prevention