Boring piddocks

They're not boring, in fact they're rather interesting. But they do bore through things. I found some of their vacated shells when we went out fieldtripping the other week to the beach at Exmouth. The shells are a rather distinctive sinuous shape and the honeycombed structure caught my eye.

Pholas dactylus, the boring piddock. (Blue tac extra).

They live on shores with soft rocks like limestone or chalk or slate, and bore down into those rocks to make conical burrows with a narrow entrance and a larger chamber below.  Apparently this improves the diversity of other species present, because other creatures can use their burrows when the piddocks die. From the safety of their burrows they filter food through a siphon.

The guidance for collecting bivalves urges you to collect good quality specimens with both shells. But it's not easy to find a pair because the valves separate soon after the piddock's death. Sometimes the shells you see have been eroded from rocks where the creatures lived hundreds or thousands of years ago.

You might be lucky though and see live ones in their burrows. Jessica Winder has many pictures on her blog.  The honeycombed area on the right of the shell above would be much spinier in life and help the mollusc grind away at the rock - the piddock can twist its shells in several directions.

(Dead) piddocks in their holed rock. CC image by Jerry Kirkhart.

An even more curious piddock fact is that they produce light. The Roman writer Pliny knew about this:
"It is the nature of these fish to shine in darkness with a bright light when other light is removed, and in proportion to their amount of moisture to glitter both in the mouth of persons masticating them and in their hands, and even on the floor and on their clothes when drops fall from them, making it clear beyond all doubt that their juice possesses a property that we should marvel at even in a solid object."

The biochemical mechanism of this bioluminescence was being investigated as long ago as the 1880s. I imagine the light has evolved to attract snacks? (There's a nice summary about bioluminescence here).  Today one of the proteins involved has been isolated and is being used to indicate the early signs of infections in athletes. Scientific research.... who knows where a similarly strange observation might take you.

BHL image. Another strange mollusc which Science has only recently believed in and investigated.