Piddocks are group of marine molluscs that can bore down into soft rocks like limestone, clay and chalk. They are in a group called bivalves as their shell is made up of two halves, other molluscs in this grouping are mussels and clams and piddocks have been referred to as clams in the past. The Piddock was once considered a delicacy and served all over Europe.
The coastline off Sussex has some areas of substrate that are just perfect for these molluscs to live. The Mixon Hole located just over a mile of shore at Selsey, is not really a hole but does contain a cliff that drops away to about 25 metres. From the beach it is possible to see the Mixon Hole Marker that warns boats of the top, which can be exposed on big tide. The cliff is made of blue clay and whilst other life struggles to maintain a purchase on this changeable substrate the piddocks love it. It is likely that their boring contributes to erosion on the Mixon Hole, but storms also have a large part to play.
There are five species of piddock recorded in Sussex and as they like to live buried in the bottom it is not easy to know which you are looking at. Judging by the evidence washed up on shore we have a lot of Common Piddocks – Pholas dactylus, by evidence I means the remains of their shells. Under the water they are not much to look at – you will swim over a single or double hole in the bottom and might catch a glimpse of their siphons sticking out the hole when feeding. The common piddock can be sizable with shells reaching about 15cms in length and several hundred can be found per square meter of bedrock – that is a lot of holes! How to they make the holes they live in? They twist in their shells, moving back and forward, working deeper until they are happy with their location.
The shells we find are not brightly coloured but white or grey, occasionally with a blue tinge to the centre. They have clear ridges and are not a smooth shell and often provide a home to tube worms and bryozoans once the piddock has died.
We do know the Common Piddock when alive is phosphorescent, producing a green blue light in the dark, however we do not know advantage this offers the animal. History records that the mouths of people that fed on them would glow and Hippolytus of Rome (Refutation of All Heresies Book IV, Chapter XXXI) 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”
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