Horse Clam

At a glance

Horse Clam (Tresus capax) is a bivalve mollusk regulated in 2 of 66 jurisdictions tracked by CatchRules across the U.S. and Canada.

The strictest bag limit is 6 (British Columbia); the most generous is 50 (California).

Confirmed by 541 research-grade iNaturalist observations, with British Columbia, Washington, and California the top jurisdictions by observation count.

Notable details

• One of the largest clams on the Pacific Coast, with shells reaching up to 8 inches.
• Burrows over a foot deep in sandy or muddy intertidal flats.
• Its siphon is too large to fully retract, leaving a visible gap in the shell.
• Ranges along the Pacific Coast from Alaska south to Baja California.
• Can live more than 20 years, growing slowly in cold coastal sediments.

Where Horse Clam are seen

• British Columbia294 iNat obs
• Washington136 iNat obs
• California49 iNat obs
• Alaska40 iNat obs
• Oregon22 iNat obs

Jurisdictions with rules for Horse Clam

• British ColumbiaCanada · official source
• CaliforniaUnited States · official source

Background

Tresus capax is a species of saltwater clam, marine bivalve mollusk, common name the fat gaper, in the family Mactridae. It also shares the common name horse clam with Tresus nuttallii a species which is similar in morphology and lifestyle. Both species are somewhat similar to the Geoduck (Panopea generosa which is in the family Hiatellidae), though smaller, with shells up to eight inches long…

Background excerpt adapted from Wikipedia's Horse Clam article (CC BY-SA). Visit Wikipedia for the full entry.

Other clams/oysters/mussels on CatchRules

• Asian ClamCorbicula fluminea
• California MusselMytilus californianus
• Eastern OysterCrassostrea virginica
• Blue MusselMytilus edulis