The black strips of dead eelgrass, impaled on the white recurved spikes of the shell, drew my attention to this cockle on the Studland strandline. So pretty, I thought, as I picked it up; and then discovered that the shell was still occupied. I put it on the wet sand to get a better photograph in the slanting light of the late afternoon sun. It was at that moment that it started to move.
The two ribbed valves slowly parted to expose the animal within the shell. Nestled among the soft pale tissue of the mantle that lines the shell, a bright orange-red structure could be just seen. This was the foot which is resposible for moving the cockle into and out of the wet sediments of the low shore. Gradually the foot unfolded itself by a combined effort of contracting and relaxing layers of circular, longitudinal and cross muscles that surround a blood-filled space.
Bit by bit, the foot lengthened and straightened in an attempt to make contact with the surface of the sand. Normally, the cockle engages in a “digging cycle” where the pointed tip of the extended foot first pushes down into the wet sediments before swelling and forming a pedal anchor. The shell valves would then close – at the same time that foot muscles contract and shorten - so that the cockle can be dragged downwards and below the surface. Next, the two shell valves open to this time create a shell anchor that holds the cockle in position, while the foot again lengthens and repeats the process.
In this instance, the cockle had been placed with the hinge lowermost so that, despite repeated extension attempts, twistings, and manoeuvrings, its foot was unable to make that important first contact with the sand to start digging itself in – until I returned the mollusc to the water.
Revision of a post first published 24 May 2010
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