Invasive Alien Species in Northern Ireland

Crepidula fornicata, Slipper limpet

Crepidula fornicata (Linnaeus, 1758)
The slipper limpet was introduced to the south-east coast of Britain between 1887 and 1890. The species was imported attached to American oysters Crassostrea virginica originating from Long Island Sound in North America. It forms high densities in semi-exposed and sheltered bays and estuaries and on the leeside of islands. It is found on a wide range of sediments.
The slipper limpet is a snail with a reduced apical spire and large aperture with a distinctive inner shell shelf. It can attain 5cm maximum dimension. The red to brown, often blotched, outer surface is smooth apart from some fine disturbance marks. The inner shell surface including the shelf is normally cream or white. They may be found consecutively attached to each other in ‘chains’ of up to twelve individuals, becoming progressively larger to the bottom of each ‘chain’. The edge of the shell can vary according to the shape of the substrate to which it is attached.
Country of origin
The snail is native to the eastern coast of North America ranging from the St. Lawrence Estuary to the Gulf of Mexico. It is commonly associated with oysters in shallow water.
Current distribution
The species now occurs on the North American Pacific coast, Japan, northern Europe, southern France, Sicily, Uruguay. It has been reported from Italy in the Mediterranean Sea. In Europe it is known from the Atlantic coasts of France and Spain, both sides of the English Channel and coasts of the southern North Sea ranging from the east coast of Britain to Norway. It is also known from Welsh coasts.
Location in Ireland
There are no known existing populations in Ireland. Individuals were imported with oysters at different times and were removed. Two populations may have existed in Ireland, in Killmakilloge Harbour, County Kerry and in Clew Bay, County Mayo but are thought to have died out at the time of the cold winter of 1962/63. There is a record from Strangford Lough but this record cannot be verified and none are presently known from this bay. Shells were found in Tralee Bay but these were from imports of autoclaved mussel shell; no living animals were imported. Their shells were used for increasing the settlement surface for native oysters so as to increase production. Since 1993 imports of half-grown Pacific oysters from France, to be ongrown in Dungarvan Bay and Waterford Harbour, have occasionally been found with low numbers of small male slipper limpets. Most of these had been crushed during transport.
Life cycle
The slipper limpet is a suspension feeder often occurring at high densities. Females of c.24mm may annually produce c.200,000 eggs. In northern Europe it may have more than one brood each year. Capsules containing eggs are usually laid in May and June. These hatch as free-swimming larvae and remain in the plankton for about three weeks. Following settlement individuals will crawl seeking to attach to the shell of a female where it remains for about two years acting as a male once it attains c.4mm. It gradually transforms to a female. Further males normally become attached to form chains with the oldest and female limpets at the base. Usually one individual is added to the ‘chain’ each year. Its shell can attain 50mm x 25mm after four to five years. Solitary males may be self-fertile. It can survive aerial exposure under cool damp conditions for several days. It can survive light frosts and in temperatures up to c.30°C, and can endure turbid water, reduced salinities and ranges from the low tide level to depths of c.20m.
Wildlife and habitat impacts
The slipper limpet, where it is abundant, is thought to result in a change to the food web in coastal regions. In some bays they may be especially abundant: in one French bay they exceed 500,000 tonnes and under these conditions can occur at densities of more than 1700m-2 and 10kgm-2. It affects the environment by filtering out planktonic species and by changing the conditions on the bottom by modifying the texture and nature of the sediment with extensive deposits of shells and accumulations of fine particulates from their wastes. The resulting shelly-muds are high in nutrients but the sediments often have low oxygen levels. Free-living coralline algae are affected by such deposits and the organisms living within their spaces are smothered with these fine sediments and this reduces biodiversity and plant abundance. Species that live close to the bottom may also be affected, some mysids and fish are known to have suffered population declines.
Human impacts
The presence of slipper limpets can increase the costs in shellfish aquaculture. In some enclosed bays in France they are dredged because they are thought to reduce the growth of commercial bivalves by competing with them for food. Mortality of important fishery and aquaculture species may be increased because of the greater opportunities for predators, such as crabs and some predatory snails, that shelter among the shell accumulations. Slipper limpets create added labour by attaching to oysters. These are removed before they are marketed. They may also foul equipment and boat hulls.
Key vectors
The principal modes of spread are well known and involve:
  1. Distribution with half-grown oysters relaid on shores.
  2. As hull fouling on vessels and other structures that are moved.
  3. Other likely modes of dispersal and localized spread include:
    1. In discharges of ships’ ballast water
    2. Release of imported live foods intended for direct consumption
    3. Relaying of mussels imported from infested areas
    4. Release by anglers using them as bait
    5. Disposal of trawling or dredged by-catch
    6. Attached to the carapace of migrating crabs.

      What you can do as an individual
      Do not import any slipper limpets even if it is intended to place them in a marine aquarium as importation can pose an unquantifiable risk of creating a new population. Any sightings should be reported directly to Fisheries Division of the Department of Agriculture and Rural Development, the Natural Heritage Division of the Environment and Heritage Service of Northern Ireland and/ or CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email[at]
      Advice to specific groups relevant to this species
      Imports of half-grown oysters from Britain or from continental Europe are likely to have some slipper limpets associated with them. These should be carefully checked. Imports of hatchery produced spat are unlikely to contain slipper limpets.
      Management measures
      Oysters are generally cultivated within meshed bags held on trestles. Should slipper limpets occur in Ireland at some future time they may result in competition in some bays. The dredging of slipper limpets reduces their biomass and is a method that has been employed in some French bays. The collected shells have been used to lime land. Bivalves with attached slipper limpets should not be imported when stocking bays. If removed from ships or other floating structures in dry-dock, all removed fouling biota should be destroyed and not returned to the water. New records should be reported immediately as it may be possible to control the species soon after arrival if confined to a small area. However, bounties were used in an attempt to control their establishment in the south-west coast of England, but this measure failed.
      Management groups
      Undertaking inspections of imported shellfish consignments is a wise precaution. Should there be slipper limpets present it may be necessary to destroy the entire consignment. Fishery inspectors should be made aware of the possibilities of such introductions. Monitoring of shellfish grounds and of mollusc culture sites on a periodic basis is advised.
      Further information
      Attempts were made during the 1940s to utilize slipper limpets as a food but this project did not succeed.
      Blanchard, M. (1996). Spread of the slipper limpet Crepidula fornicata (L., 1758) in Europe. Current state and consequences. Scientia Marina 61(Suppl. 2): 109-118.
      Minchin, D., McGrath, D., Duggan, C.B. (1995). The slipper limpet Crepidula fornicata (L.) in Irish waters with a review of its occurrence in the north-east Atlantic. Journal of Conchcology, Londo 35(5): 297-301.
      Sauriau, P.G., Pichocki-Seyfried, C., Walker, P., De Montauduin, A., Pascual, A., Heral, M. (1998). Crepidula fornicata L. (Mollusca, Gastropoda) in the Marennes-Oleron Bay : side-scan sonar mapping of subtidal and stock assessment. Oceanologica Acta 21: 353-362.
      Walne, P.R. (1956). The biology and distribution of the slipper limpet Crepidula fornicata in Essex rivers with notes on the distribution of larger epi-benthic invertebrates. Fisheries Investigations London (2) 20 No 6: 1-52.
      See the Invasive Species Ireland web site for further information -
      Text written by:
      Dan Minchin, MOI, Ballina, Killaloe, Co Clare