Pebbles and beach stones riddled with small holes are a common beachcomber’s find. These small burrows and borings in the stone are frequently made by marine worms. The worms themselves, and the mud and sand tubes in which they live within the burrows, are usually absent. However, on the water’s edge in many coastal locations, if you know where to look, it is possible to spot the burrows still occupied by the worms; this is usually in large and mainly immovable boulders, or in the bedrock of the beach platform, or the base of a cliff face. The worm itself is almost impossible to see because at low tide, when the rock is exposed to the air, it retreats into the tube and burrow. Though sometimes, apparently, its two palps or feelers can be seen protruding from the hole and waving around vigorously. I haven’t observed that myself so far.
Without microscopically examining the actual worms, it isn’t possible to say with a 100 per cent certainty what these worms are. Nevertheless, there are enough characters available to say that these are most likely to be marine polychaetes of the Spionidae, and probably one of the Polydora group, maybe Polydora ciliata (Johnston).
All the Polydora species make a U-shaped tube from small particles of mud, or whitish calcareous matter if they have been burrowing into calcareous algae, shell, or limey stone; all this is stuck together with secreted mucus. The tube is normally embedded in the burrow that it has excavated. There are two holes in the mud tube, one at the front and one at the back end – but they lie side by side because the tube and burrow are U-shaped. In the examples photographed here, many worm tubes are packed together, and there are instances where the chalk burrows have joined together and broadened out into deeper, less well-defined, depressions.
The method by which the worms create the burrows is thought to be an almost incidental process. The worms initially settle and manufacture their mud tubes in the shelter of slight cracks and crevices in rock or shell surfaces, or between sessile barnacles, or amongst soft algae in rock depressions, and other such places on the seashore where it always remains damp at low tide. The metabolism of the living worm leads to the production of slightly acidic waste. Over time, the seepage of these waste products gradually eats into and dissolves the rock or shell on which the worm tube lies, enabling the worm to retreat further and further into the safety of the substrate. The burrow formed like this reflects the shape of the U-shaped mud tube.
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