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Biology/Natural History: To bore their hole, pholads extend the foot from the gape in the anterior end of the shell (see photo) and apply it to the base of the burrow as a sucking disk. The anterior part of the anterior adductor, as well as the ventral adductor muscles contract, squeezing the valves into a narrow profile. The foot contracts, pulling the anterior end of the shell down to the bottom of the burrow. The posterior adductor and the posterior part of the anterior adductor muscles contract, pulling the anterior ends of the valves apart from one another (rocking on the hinge) and pressing them against the walls and base of the burrow. The shell rocks backward and upward, scraping the burrow with the rough anterior portion of the shell. The valves rock back and forth with the hinge as a fulcrum. Unlike most other clams, piddocks have no hinge ligament. After the clam is fully grown, the foot degenerates and the anterior-ventral gape between the valves, through which the foot formerly projected, is covered over by a callum, or calcareous plate.

The united, flat-topped, white to reddish-brown siphons of this species are frequently seen by divers. The inhalant siphon is of larger diameter than the exhalant. They don't burrow more than about 30 cm into the rock but are important agents for rock erosion.


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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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