Marine Animal Encyclopedia
Common Piddock Pholas dactylus
The common piddock is a mollusk that has a pronounced “beak” covered in toothlike projections at the front end of its shell. It uses this feature for boring holes into relatively soft substrates, such as mud, chalk, peat, and shale. Like the shipworm, the common piddock relies on its burrows for protection from predation, because the shell does not encase all of its body—its two fused siphons (tubes for eating, breathing, and excretion) trail out behind it. The shell is fragile, elliptical, and covered in a pattern of concentric ridges and radiating lines. If disturbed, the common piddock has an unusual defense strategy: it squirts a luminous blue secretion from its outgoing, or exhalant, siphon. Such bioluminescence is very rare in bivalve mollusks and is seen in only one other of the 14,000 species.