Pebbles with holes in them are a common occurrence on the beaches along the Jurassic Coast in Dorset – and lots of other U.K. locations as well. Many of these tunnels, burrows or borings have been made by small marine invertebrate animals such as certain species of bivalved molluscs, polychaete worms, and even sponges. The arrangement of pebbles shown in the photograph above illustrates a selection of the different kinds of holes created by these animals in soft rocks.
In this post I am just going to talk briefly about the larger tunnels which tend to be about a centimeter or more in diameter. These have been excavated by rock-boring bivalve molluscs. There are several types whose habits result in these borings and it is not easy to say which species has made which tunnel from the shape and size of the holes alone. Fortunately, the shells of these creatures often remain in the burrows. The appearance of the shells is diagnostic for each species. Unfortunately, the shells of some of these bivalves are both surprisingly fragile and wedged securely in the burrow so that they are difficult to extract.
The most commonly occurring rock-boring molluscs are the Piddocks and some other related species. From the Pholadidae family, for example, these include the Common Piddock Pholas dactylus Linnaeus and the White Piddock Barnea candida (Linnaeus). From the Hiatellidae family, these include the Wrinkled Rock-borer Hiatella arctica (Linnaeus). And from the Gastrochaenidae, the Flask Shell Gastrochaena dubia (Pennant).
The softer rocks into which these bivalves bore are generally low on the shore and under water most of the time. When pieces of this rock break away, the stone becomes more rounded and worn and ends up as a pebble on the beach – like the one above which was seen at Charmouth in Dorset.
The photograph below shows a flat platform of soft Blue Lias shale extending seawards at Charmouth. If you look closely at the near-vertical edge of the rock where it is lapped by the water, you may be able to see that the shale has many perforations caused by these rock-boring molluscs.
Below is a closer view of the seaweed free edge of the rock platform with the piddock holes.
The next picture shows empty shells still in situ in the burrows. In future posts I will illustrate the actual shells of these rock-boring molluscs and will describe something of their life in such a unique habitat. I will also discuss at a later date the other smaller types of rock-borings made by worms and sponges; and show how they also colonise and leave physical evidence of their presence on mollusc shells. When this evidence is found in shells recovered from archaeological excavations, it provides clues to the environment that was being exploited for its marine resources by people in the past.
Revision of a post first published 4 October 2009
COPYRIGHT JESSICA WINDER 2011
All Rights Reserved