Pholadidae – Piddocks

Description: Piddocks have rather uniquely evolved shells. Rather than containing their body fully inside a bivalve shell enclosure, they burrow a cavity into wood, rock, and depend on other materials as well (including other shells) for their protection. To accomplish this, the main shell halves each have formed into separable, movable, grinding plates, which are usually too small to completely encircle the mollusc. These rounded plates have stubby external spikes on the anterior sides, for grating much like a nutmeg grater. Each half-shell also has a unique spoon shaped apophysis on the inside surface. This serves as a muscle attachment to allow dorsal/ventral movement in addition to anteror/posterior movement (See bottom figure on this page). Several additional points of attachment mark other muscle bundles that facilitate rotational movement. Ciliary currents of water flush out the debris while the shell assembly operates against a rock surface much as a pestil and mortar function to grind hard materials.; The two principal shells leave large gaps over the bivalve’s soft body parts. In the adult, the gaps may later become covered by a number of flexible supplementary plates (protoplax, metaplax, mesoplax, and siphonoplax). The siphonoplax, for example, consists of two half round collars that fully encircle the large siphon of the clam, providing some degree of flexible protection.

Classification

Class: Bivalvia

Subclass: Heterodonta

Order: Myoida

Superfamily: Pholadacea

Major Genera