Slipper Limpet shells abound on the beach at Studland Bay in Dorset. They are well named because the empty shells can be seen to have a large internal shelf, making them look like small slippers about 40 mm long and 25 mm broad. Living individuals are attached one on top of another in a long curving chain with younger ones settling on the older ones. Usually there are up to a dozen limpets in a chain but side chains can form if there are more than thirty.
The Latin name of this gastropod mollusc is Crepidula fornicata (Linnaeus). The specific name alludes to a very peculiar breeding habit which is demonstrated by a close consideration of the chains of individuals that make up the large fist-sized clumps – like the ones illustrated by the photographs. Unlike the more usual varieties of limpet (Patella spp.), Slipper Limpets are immobile for their entire life, except for a very short planktonic stage just after hatching.
A larva will settle once and forever on any clean surface – but it is chemically attracted to other Slipper Limpets in particular. When a young individual settles on another Slipper Limpet, it develops into a male with a long tapering penis with which it fertilises female individuals living beneath it in the chain, from its now permanent position.
If the larva settles on any other object than another living Slipper Limpet, it does not develop into a male because there are no females to fertilise; it grows into a female. Any larva subsequently settling on this female will develop into a male that fertilises the female on which it has settled. That newly settled male will in turn transform into a female when another larva settles on its shell.
Sounds a bit complicated doesn’t it?
Basically, in a long chain of individuals, the uppermost ones are always functional males. In the middle of the chain is a group that is changing sex from male to female. At the base of the chain the individuals are always female.
In the clumps of Slipper Limpets photographed here, the older basal females are larger and tend to have a dull matt shell surface tinted green. The lowest shell in the pile is frequently an unoccupied one. In one of the aggregations, the limpets are attached to an old Common Whelk shell showing infestation damage of small regular holes caused by a boring sponge.
The smallest shells, with a glossy surface and red stripes on a pale ground colour, are the most recently settled individuals, developing into male specimens. In addition to the larval settlement of Slipper Limpets on the shell chain, other organisms also attach themselves. For example, calcareous tubes from marine bristle worms encrust one shell; and red seaweeds are attached to another.
Revision of a post first published 9 March 2010
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