Tresus capax

The Gaper Clam

Phylum Mollusca
Class Bivalvia
Order Veneroida
Family Mactridae
Genus Tresus
Species capax

Description: 20 cm long, usually one and a half times as long as it is high; chalky white except near the periostracum, unless the shells are colored black from iron sulfate in the sediment; shaggy brownish-yellow periostracum is worn off near the syphon, which is not fully retractable in larger specimens; large oval-oblong shells with fine concentric growth lines.

Habitat: Can be found buried as deep as 1 meter in sediment of mud, sand and gravel near low-tide lines.

Natural History: This is one of our largest clams. It lies buried deep in the sediment with its siphon extended the entire length. Growths of algae and sedentary invertebrates are usually found growing on the hard leathery plates on each side of the neck. Because of its size it is often confused with Panopea generosa, or the Geoduck, which is found much deeper, and only at the lowest of low tides. You can find the commensal Pea Crab, Pinnixa littoralis, in the mantle cavity of this clam , we found a few ourselves.