We picked up these two mollusc shells, which have an interesting history, on the Norfolk coast last Tuesday. Both are alien species that were introduced with American oysters sometime around 1880 and have since colonised the south-eastern and southern coasts of England. The animal above is the slipper limpet Crepidula fornicata which, despite appearances is more closely related to periwinkles than to limpets and has a rather exotic sex life (which is why the specific name is fornicata). The inverted shell, which has a shelf inside that gives it a resemblance to a slipper, contains the mollusc that has a powerfully muscular foot that it uses to attach itself to the back of another slipper limpet. So they pile up, one on top of another, youngest at the top, oldest at the bottom. As the older animals die, younger ones attach to the top of the pile. The longest chain of living animals that I ever found had seven individuals. The most amazing aspect of their reproduction is that they change sex as they age – the females are at the bottom, the males (youngest animals) are at the top and the middle ones are in the process of transition. This seems to be a very successful reproductive arrangement because slipper limpets colonised the English coast very rapidly once they arrived here, aided by the production of vast numbers of planktonic larvae that you can see here.
The animal above is the American paddock Petricola pholadiformis, commonly called 'false angel wings', which was also an introduction that arrived at about the same time as the slipper limpet and has been equally successful. It bores into soft substrates like chalk, limestone, shale and compacted mud using its rough shell for a drill, broad end first. It seems extraordinary that a relatively delicate seashell can be used for boring in this way, but by using its muscular foot to endlessly fidget the shell from side-to-side the animal drills down into the rock, growing larger as it burrows deeper – so once it’s in, it can never leave. It’s armoured by being encased in rock, but the rock also becomes its tomb.
You can read more about both of these at http://www.marlin.ac.uk/speciesinformation.php?speciesID=4077
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