From the Davidson, Alan. The Oxford Companion to Food (Oxford University Press, 1999), page 602.
Piddock (pholas dactulus), a european atlantic specifies.
The creature is luminous, to an extent that one observer found that the immersion of a single piddock in half a pint of milk created a luminous glow sufficient to make faces in the dark; and that someone who chewed a piddock and kept it in his mouth would have luminous breath, as though breathing fire.
Mentioned in the BBC's Excess Baggage