Under cover of a merchant consignment of American oyster, the American piddock arrived in Britain by sea some time before 1890. A few short years later, this well-travelled piddock was boring away on Britain's beaches from Dorset and west, then north, and all the way around to the Humber; all without evidence of disturbing the indigenous piddock population. Within twenty years, its young had drifted its way across the Channel1 and evidence of the boring activities of this boring shellfish could soon be found all the way up the West coast of Europe from France to Norway, as well as south and east through the Mediterranean to the Black Sea.
The Life of an American Piddock
Some boring shellfish bore with chemistry, but the American piddock is a mechanical borer – repetitively grinding its shell with a rotating movement backward and forward to create a burrow for itself in softer substrates in the shallows2 of the intertidal zones. In the protection of its burrow, which it will never leave, it then spends a spectacularly dull lifespan of as much as ten years sucking food from the seawater through its syphon. Ok, its double syphon. That adds all the extra spice to life that drinking through two straws, rather than one, does.
The Impact of the American Piddock
An obscured and boring lifestyle doesn't have to be wholly lacking in impact, however. An ancient mariner or two may well have been sent to moulder on the sea-bottom by the enthusiastic boring of his ship's timbers. And a few current archaeologists and palaeontologists are not too happy about piddock holes in their excavated wrecks and fossil seabeds. Further, although the American piddock, indigenous to coasts stretching from Canada to Uruguay, arrived at a time when immigrants were more liable to be regarded as interesting additions, times change even in the world of shellfish. It's not yet a matter for tabloid headlines, but the impact of the immigrant piddock on indigenous piddock has become an issue.
It was in Belgium and the Netherlands that the invading piddock started to cause disquiet, displacing, according to ICES3 1972, the indigenous white piddock – a piddock of similar size, appearance and lifestyle. There is later evidence4 that in Belgium if not the Netherlands, as many white piddock were now to be found as American piddock. There is also a theory that white piddock have the advantage (of a longer siphocanal) as beaches silt up. There's no evidence that they've had any effect on white piddock numbers in Britain and to add to the confusion, Germany currently have it on their red list of threatened species of the Wadden Sea. However, all that not withstanding, American piddock were placed on the draft list of 'the worst invasive species threatening biodiversity in Europe' in the Belgrade 2007 EECCA report5. 'Worst invasive species threatening biodiversity' is quite a title for a shellfish but even though it might attract attention, a life spent making holes in timber, limestone and EEC budgets doesn't exactly set the imagination alight.
Their procreation proclivities might cause an askance look. They do have separate sexes but there any humanoid parallels cease: the female produces over three million eggs per year. Spawning happens over six weeks ending in August. The young piddock is plankton for about up to two weeks, during which they drift where the current takes them. If they a) survive uneaten, and b) end up somewhere suitable, they settle on the seabed and after nine months start growing shells, putting down growth rings at a rate of one each year6.
It might help to give them their full scientific, italicised, titles: American piddock are the Petricolaria pholadiformis, of the phylum or family; mollusc (Mollusca), belonging to the class; bivalve (Pelecypoda). Pretty grand sounding; but it doesn't alter the fact that they're still small, rubbery, creamy-greyish nondescript lumps of sea-creature hiding in a shell.
But What a Shell
When these piddocks' shells are cast up onto the beach they look like a discarded pair of angel wings. Classic angel wings. They are long, white, tapered ovals, heavily ribbed towards the bottom end and striated across concentric rings to look like layer upon layer of feathers. Although not as large7 or as stunning as others in the group, they are beautiful, and they give this piddock its other name; when it shuffles off its mortal siphocanal it becomes one of the angel wings.
The Invasion of the Angel Wings doesn't sound quite the same as the Invasion of the Piddock.
Other Piddock, Angel Wings and Facts
Not all piddocks are angel wings, and not all angel wings are piddocks. The American piddock is the false angel wing, the title angel wing itself going to the rock-eater clam (Cyrtopleura costata). There's also the fallen angel wing, the Atlantic mud-piddock (Barnea truncata) and the Campeche angel wing (Pholas campechiensis). False and fallen angels - if you were of a mind to see worlds in Blake's grains of sand, these piddocks and angel wings could set you wondering for a while about living safely in burrows or risking high-flying. Not that the piddock has any choice of course, his hidden lifestyle is laid out for him and his wings really are just shells.
Europe's indigenous piddock, none of which are even called angel wings, include common piddock (Pholas dactylus), little piddock (Barnea parva), oval piddock (Zirfaea crispata), paper piddock (Pholadidea loscombiana) and white piddock (Barnea candida). There is almost a piddock for everyone - there is even a rough piddock (Zirfaea pilsbryii), although that is another American one.
Why give a piddock? There is an estimate, based on observations at Lyme Regis, that over twelve years piddocks can remove 41% of the material lying beneath the shore, materially undermining its stability. So giving a piddock might depend on your proximity to the coast. But they are also part of the ecosystem. Their burrowing changes the habitat; letting other species in. They are part of the food chain - break that link in the chain and there are consequences all the way up. They are part of the diversity of life.
And... although the American piddock seems to have gone largely unnoticed in Europe except by some marine biologists, the French on the Normandy coast, (bless their onion strings), tried eating them. Successfully it seems; they pickle them in vinegar or cook them in herbs and breadcrumbs.
I Want One!
They don't make good pets, they never come out their holes. Never. However...they need a tidal flow of turbid water full of suspended organic matter and zooplankton on the sort of scale found in the Thames estuary, the correct salinity levels, correctly oxygenated water of the right temperature ranges, and a firm, stable but reasonably soft sediment to burrow in such as clay, sandstone, softer chalk, soft limestone, compact mud and peaty substrate. Unusually hard clay, loose mud, loose sand, hard chalk won't do. For specifics: try London Clay or Thanet Sandstone, but avoid Lower Greensand Gault clay and Blue Lower Lias. Having got all that right, keeping up the turbidity so that sediment never settles deeply enough to suffocate them, and keeping up a regular tidal flow so that they don't desiccate from too long a period out of water - they are said to settle pretty readily.