One of the frequent shell finds on Rhossili Beach is the large Spiny Cockle Acanthocardia aculeata (Linnaeus) – at least that is what I think it is! (If you know better, please do let me know). There are several similar large species of cockle found on UK coasts. I have identified the specimens I found on the shore this week as A. aculeata, rather than A. echinata (Prickly Cockle) or A. tuberculata , first because of the size, second because of the characteristic outline shape of the valves, and third because of the sculpturing and spines on the ribs.
It seems to me that the species exhibits quite a bit of variability possibly as the result of age and wear. The greater the age of the living specimen, or the longer the shells have been rolling around on the sea bed, then the more likely it is that the spines are broken or worn down. However, the outline shape of all the specimens I looked at has a typical straight(-ish) posterior margin compared with the curved anterior edge. The spines, even when worn, are all joined by a continuous ridge down the centre of each rib.
My hesitation over the identification relates to the hinge area of the shell and the relative size of the two cardinal teeth in the left valve. There is always a possibility that different specimens I have been looking at are in fact different species – accounting for the fact that the teeth in the hinge don’t seem to conform to the descriptions and illustrations for A. aculeata in the reference works. Or it could be that the shape of the teeth is variable within the species. Mostly, it is just the empty shells, often still attached in pairs by the ligament, that end up on the strand-line. Occasionally, after very stormy weather for example, live specimens are washed ashore too.
I include in this post photographs (taken from various angles to show the typical diagnostic features) of the paired valves of a Spiny Cockle left on the sand after high tide this week. I have included two shots taken of live specimens stranded on the same beach on an earlier occasion. Finally, I have added some further views of a pair of empty valves to show details, in particular of the outline shape, the details of the inner hinge area, and the grooved margins on the edge of the shell.
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