We mostly think of sponges as being soft-bodied organisms. So it may be surprising to learn that some types of sponge can bore holes in soft stones and seashells. They do this not by drilling but by secreting acids that dissolve the calcium-based substrates on which they live.
Typically a pebble or shell that has been affected by this kind of sponge infestation will have a series of regular-shaped circular holes on the surface. These holes may cover the entire surface of the object in severe infestation. Sometimes there are only a few holes when the sponge is just starting to colonise the pebble. These may appear as a single row of holes with a range of increasing size. There may be inter-connected surface lines or grooves which look like the impressions of rows of beads.
Below the surface, deeper down inside the pebble, the sponge creates a fairly hap-hazard network of spaces that resembles bubbles or an irregular sort of honeycomb. This inner damage is revealed when the outer surface of a badly sponge-infested pebble is either broken or worn away.
Cliona celata (Grant) is one of the sponges that commonly makes these holes in pebbles and shells in the UK at the present time. There is more information about sponge borings in shells in the earlier post: Sponge borings in Flat Oyster shells.
Revision of a post first published 18 December 2009
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