Pholas dactylus, or common piddock, is a bioluminescent clam-like species of marine mollusc in the family Pholadidae.

The piddock bores into the substrate for shelter, and lives in a tubular burrow formed by grinding the material away with hard parts of the shell by rotating on the longitudinal axis.

It is a filter feeder, using its siphons to reach the water outside the burrow.

Pliny spoke of luminescence in the mouths of people who ate Pholas, the rock-boring shell-fish, and of such importance is this phenomenon that it is even said to have gained the first king of Scotland his throne.

[4] Hippolytus of Rome tells us that it was a common pagan trick to use the luminescent property of this clam to create the illusion of burning, "And they accomplish the burning of a house, by daubing it over with the juice of a certain fish called dactylus.