& THE SLIPPER LIMPET
by Jane Lilley
Slipper Limpets (left) with European Oyster (right)
Oysters used to be abundant in shallow water off the south coast of England. They have been eaten for thousands of years; the Romans believed them to be an aphrodisiac, and not only sent them all over England, they exported them to Rome. They were an extremely valuable crop in many coastal areas from Essex to Cornwall until the mid nineteenth century. But the numbers of oysters have dropped dramatically, and production in most areas is very low.
Slipper Limpets, Crepidula fornicata, on the other hand, were unknown in this country until the late nineteenth century, but they are now so abundant that their distinctive boat-like shells, alive or empty, are often the commonest shells seen along the south coast. In places the seabed is a shell gravel composed largely of slipper limpet shells. They are innocuous filter-feeders which take no interest in oysters, but their success is very closely linked with the decline of the oyster. And although it was unintentional, man was largely responsible for the decline of the oysters he prized.
The only species of oyster native to Britain is the European Oyster Ostrea edulis, although several other species are found elsewhere. 'Edulis' means 'edible', and it has always been much prized as a food; the flavour is considered finer than that of the other oysters. The adults live a dull life, cemented to the seabed, pumping water endlessly through a filter in the gill-chamber to collect tiny algae for food. They grow slowly, taking about four years to reach a marketable size.
If conditions are good the oysters will start to breed in the summer of their third year. A biologist would describe oysters as protandrous alternating hermaphrodites, which is a shorthand way of saying that each oyster is both male and female, but not at the same time - it changes sex regularly, producing first sperm ('protandrous' means that it starts off as a male), then eggs, then sperm again. When one male discharges its sperm into the water, this triggers other ripe males to shed their sperm as well; the females draw it into their shells as they pump in water for feeding and respiration, and it fertilises their eggs. These develop inside the female's shell for a while, until they hatch into minute swimming larvae; an oyster with eggs is not considered edible, which is why oysters should not be eaten in summer (I don't know whether they are actually inedible; more likely they are less meaty and more likely to die and go bad in hot weather, and a ban on eating oysters in summer would allow them to breed and so produce a new crop of baby oysters).
The larvae are released when they are about 0.22 mm long. They spend about two weeks swimming and feeding in the plankton, then settle to the bottom to search for a suitable place to live. If possible they settle close to other oysters, because that gives their eggs a much greater chance of being fertilised when they come to breed; clean empty shells close to other oysters are their ideal settlement site, so in favourable places great beds of oysters may develop, built up on layers of old oyster shells. The larvae, known as spat, cement themselves to their chosen shell and change into tiny oysters barely the size of a pinhead, which feed and grow until they can breed in their turn. Those that survive the various hazards of life may live and breed for many years.
Although oysters grow well in the open sea, many oyster beds were in estuaries and inlets and creeks, and these beds were carefully tended to encourage the oysters. Predators such as starfish and the carnivorous snail known as the Sting Winkle were removed, as were large sea-squirts and other filter-feeders which would compete with the oysters for food; clean shells were made available for the spat to settle on, and harvesting was often controlled to some extent in an attempt to ensure that enough mature oysters survived to breed and replenish the stocks. (Most of these operations involved dredging or harrowing the beds, so one wonders what proportion of the oysters were damaged or smothered in the process, but the system obviously worked reasonably well for some hundreds of years). There were always some problems: unfavourable conditions could sweep the larvae away or kill them and prevent a spatfall in some years, and severe winters could kill much of the stock - oysters were dredged up in autumn and moved from vulnerable sites to deeper water in some places to avoid this; - but in general oyster fishing remained very important in many places until the mid nineteenth century.
Oyster Industry's Demise
With hindsight, the oyster industry was at least partly a victim of its own success. Demand for oysters grew rapidly, and the beds were over-exploited. Too few mature oysters were left to breed, and populations fell catastrophically. In an attempt to increase the supply, Portuguese Oysters and American Blue-Point Oysters were imported as small 'seed oysters' which were 'set' in the beds and left to grow to marketable size. These species are less well-flavoured than our native oyster, and rarely breed in English waters, which are usually too cold to trigger spawning, but they grow well here. Unfortunately it was not only oysters that were imported. The American east coast has its own range of predatory species, and the American Oyster Drill hitched a lift across the Atlantic with the Blue-Point Oyster and established itself here. So did the Slipper Limpet. And unlike the foreign oysters, Oyster Drills and Slipper Limpets bred and flourished here, with disastrous results.
Many animals besides humans find oysters tasty, but only a few have reliable ways of opening the shell. The Common Starfish, Asterias rubens, is one; it pulls the two halves of the shell apart until it can insert its stomach and digest the animal inside, and is an important predator on oysters and mussels. The Sting Winkle or Rough Tingle, Ocenebra erinacea, also eats oysters when it can get them; this is a snail which looks rather like a small and knobbly whelk. It drills a hole through the oyster's shell, softening it with a chemical and then rasping away, sometimes taking several days to drill through a shell. When it finally succeeds, it injects a poison which kills the oyster; the shell opens, and it begins to feed. Unfortunately the smell of dead oyster attracts crabs and other scavengers, so the oyster is soon eaten and the Sting Winkle moves on to another victim.
The Common Whelk, Buccinum undatum, attacks oysters in the same way and can be a serious pest on the beds, but it is less committed to oysters than the Sting Winkle, also eating a wide range of bivalves and some fresh carrion.
The American Oyster Drill, Urosalpinx cinerea, looks so similar to the Sting Winkle that it was overlooked for some years after it arrived in this country, and was well-established before it was recognised in 1928. It drills through shells in much the same way, and prefers oysters when it can find them. It lives on the oyster beds, eating enormous numbers of the smaller oysters whose thin shells can be penetrated rapidly; one Oyster Drill can apparently eat twenty or more spat oysters a day, so even a moderate population will rapidly reduce the productivity of a bed. It does not spread quickly from bed to bed - the eggs are attached and the larvae crawl instead of swimming and being dispersed by the currents - but this means that all the young remain on the same bed as their parents, so a few Oyster Drills can become major infestation very rapidly.
The Slipper Limpet is also a snail, but you would probably not realise that from looking at it. The shell opening has been enlarged and widened until the shell looks more like an overturned boat than a snail, with a flat shelf projecting halfway across the open section; the only trace of coiling is a rounded knob at one end. Like a true limpet, the Slipper Limpet lives attached to a rock or pebble, or to another shell; it does not move around, and feeds in the same way as the oyster, filtering edible material out of the water. It does not eat oysters itself, but simply by competing for the same food it reduces the amount available for the oysters, slowing their growth. And where it likes the conditions - and oyster beds are ideal for it - it breeds rapidly.
The breeding behaviour of Slipper Limpets is peculiar. Most snails release eggs and sperm into the water in vast numbers and hope that a proportion of the eggs will be fertilised; but Slipper Limpets are among those which mate, so that the eggs are fertilised internally. Adult Slipper Limpets cannot move around, so they must live very close together if mating is to be possible: on each other's shells. Larvae settling to the bottom will attach themselves to the shells of other Slipper Limpets if they can; 'chains' of limpets rapidly form, each attached to the one below, with the largest and oldest at the bottom of the 'chain' clinging to a stone or empty shell. The bottom limpet or limpets are always female, the top ones always male, so the male simply extends his long penis down the side of the chain and inserts its tip into the female's shell to fertilise her eggs. The eggs are laid underneath the female, attached to whatever she is clinging to, and protected until they hatch into swimming larvae.
Slipper Limpets live for several years, but eventually the oldest female at the base of the chain dies; the next oldest continues to hold onto her empty shell, and, if it is not already a female, promptly changes sex and becomes one. So once a chain has formed, it can continue indefinitely; baby limpets join the top of the chain and mature as males, changing in due course into females as they grow older and the females below them die, while more larvae settle on the topmost limpet to extend the chain. Any larva that cannot find a chain to join will attach itself to a pebble, mature as a female, and secrete a chemical which attracts other larvae, so starting another chain. There can be up to 25 or 30 limpets in a large colony, and I have seen fist-sized colonies of Slipper Limpets covering the seabed like large pebbles.
These habits mean that Slipper Limpets can become exceedingly numerous on an oyster bed, forming masses several inches thick which entirely cover the bottom, so that they both starve the oysters and smother them by sheer weight of numbers. Even more disastrously, mud accumulates around the Slipper Limpets, covering the clean shells that the oyster spat need to settle on, and so preventing the beds from regenerating: they change the habitat permanently, making it unsuitable for oysters. The larvae are carried to new sites by tides and currents, so they spread easily along the coast, and even if a bed is laboriously cleared by dredging, more Slipper Limpets will rapidly move in. Up to twenty tons of Slipper Limpets per acre have been dredged from derelict oyster beds.
Unfortunately Slipper Limpets appear to be economically useless for food or anything else; they were eaten by the near-starving during the war, but are apparently not recommended. If any marine animal preys on them, I have not heard of it. Their shells are sometimes bored by the Boring Sponge Cliona, and make very unsatisfactory homes for hermit crabs unable to find anything better; tiny crabs, worms and other creatures probably shelter in them, but they do not appear to be very useful to the marine communities. The shells are remarkably tough and slow to break up, and in many places they are a major component of pebble banks and mixed sediments on the seabed and the shore.
Attempts at Revival
Meanwhile the inshore oyster
beds, already over-exploited and in decline when the Slipper Limpet began
to spread, were generally neglected during the First World War and then
attacked by disease, Bonamia
ostrea; in the past, a few years' work would have made them productive
again, but it proved very difficult to reclaim them from the continual
invasion of Slipper Limpets. Various attempts were made to revive the oyster
industry, but it had been reduced to a fraction of its former productivity,
and has not recovered.
Oyster with Chiton
Photograph by Andy Horton
Oyster dredging is still carried out in places, but yields are small; I do not know which of the old oyster beds are still productive, but it cannot be many. In not much more than a century the flourishing oyster industry has virtually died, not through lack of demand of oysters, nor from the simple over-exploitation that has decimated the fisheries for herring, cod, and many other fish, although this has played its part, but because those two unintended imports, the Oyster Drill and Slipper Limpet, have upset the ecological balance and made it virtually impossible for the oyster-dominated communities that we call oyster beds to survive.
Commercial Oyster Fishing recommenced out of Shoreham early in the 21st century. Four boats were fishing for oysters at the beginning of 2004.
The collective noun or description of a chain of slipper limpets is a 'bungalow'.
Slipper Limpets originated from the eastern coast of North America.
Wikipedia entry link
British Marine Life Study Society: Mollusca