Phylum Mollusca : Molluscs

Class Polyplacophora: Chitons

Class Gastropoda

Order Archaeogastropoda

Order Mesogastropoda

Order Neogastropoda

Class Bivalvia

One of the largest and most widely distributed groups of marine organisms. Includes some extremely valuable commercial species. Many species are also valued for their shells, and all species may be collected in small numbers anywhere on the coast by amateur and professional conchologists. They are widely taken for human consumption (for personal use and to supply commercial markets) and may also be used for bait. All species of mollusca are classified as shellfish, or seafish, under fisheries statute. Collection of any of these species is therefore governed by statutory fisheries legislation, regardless of intended end use.

Most species of marine mollusca are dioecious (with separate sexes). Primitive species (e.g. archeogastropods and most mesogastropods) exhibit external fertilisation, with planktonic eggs and larvae that may be dispersed widely. Neogastropods produce smaller numbers of eggs that are fixed to the seabed (limiting their dispersal and ability to recover from over-exploitation). Simultaneous or consecutive hermaphroditism occurs within a few marine molluscs, and all Ophisthobranchia are hermaphrodites.

The taxonomy of the Mollusca is in a considerable state of flux. The following classification broadly follows that in Hayward and Ryland (1995); other publications (e.g. Hepple et al. in Howson and Picton 1997) vary this.

Class Polyplacophora: Chitons

Herbivorous grazing molluscs living on rock surfaces. Characterised by a limpet-like body made up of a shell of eight interlocking plates attached to a tough mantle skirt around the edge of the animal. Larger species are doubtless taken for human consumption in other parts of Europe and elsewhere, but most British species are too small to be valued for this purpose.

Class Gastropoda

Subclass Prosobranchia

Prosobranchs are usually characterised by a coiled shell, sealed by a horny operculum attached to the top of the animal’s foot. However, others have internal shells, or cone shaped shells (e.g. the limpets).

Order Archaeogastropoda

Family Haliotidacea : Abalone

Haliotis tuberculata Linnaeus. Ormer. Found from the extreme low water mark of rocky shores to the shallow sublittoral, from the Mediterranean north to the Channel Islands. Not present on British and Irish mainland. Commercially important and hand collected. Slow-growing, possibly taking five to eight years to reach market size, and sometimes subject to pronounced fluctuations in recruitment. The ormer is under severe fishing pressure, and often the subject of strict management to prevent population depletion.

Family Patellidae: Limpets

Patella vulgata Linnaeus. Common limpet. Shell up to 60 x 50 x 30 mm in dimension. Found on all suitable rocky shores from the mean high water mark (highest in shaded and wave exposed sites) to the extreme low water mark. Distribution extends from the Lofoten Islands in Norway to the Mediterranean. This limpet has been an important component of human food since prehistoric times, and occurs commonly in shell middens around the coast. This and related species (see below) are, however, no longer widely collected around the British Isles, and elsewhere. Limpets may, however, be taken from the shore for use as bait (e.g. for crabbing), or on heavily used beaches, simply detached for no apparent reason. An educational programme and ‘limpet reserve’ is being used to discourage removal of limpets in the Kimmeridge Marine Reserve, Dorset.

Patella ulyssiponensis Gmelin, the China limpet. Slightly smaller (50 x 40 x 20 mm), and restricted to the lower shore (mean low water mark of neap tides) and below. Favours wet areas and exposed shores. Distributed from the Mediterranean to the UK, where it reaches its northern limits in the UK, and absent from shores between the Isle of Wight and Humber, or on the Continent east of Barfleur (near Cherbourg).

Patella depressa Pennant, the black-footed limpet. Slightly smaller than the common limpet and a southern species. Prefers vertical surfaces between the mean high water and mean low water marks of neap tides, on exposed rocky shores. Distributed from the Mediterranean to south-west England and Wales.

Family Trochidae : Top shells

Several small topshells occur in the intertidal in the UK. These are not known to be targeted for bait or for human consumption. However, some of the larger species (e.g. Gibbula umbilicalis (da Costa), flat top shell, which reaches a size of 20 x 22 mm, and Monodonta lineata (da Costa), thick top shell, maximum 30 x 25 mm) may be collected as ‘winkles’ along with other small gastropod molluscs.

Order Mesogastropoda

Family Littorinidae: Winkles

The taxonomic status of several of the smaller species and some species complexes within this family is still to be resolved. However, the status of the common periwinkle, the largest species (and most important in the context of this report) is undisputed.

Littorina littorea (Linnaeus). Common periwinkle. The largest littorinid, reaching a size of 32 x 25 mm. It is found in the intertidal of almost all rocky shores (except in conditions of extreme wave exposure) and extending into the sublittoral in northern areas. Most abundant on the mid and lower shore. Tolerant of low salinities, and also found in saltmarsh pools. Distributed from northern Spain to the White Sea, but uncommon in some isolated island groups (e.g. the Isles of Scilly and Channel Islands), possibly because its planktonic egg capsules only rarely reach their shores. This species appears in prehistoric shellfish middens throughout Europe, and is therefore known to have been an important source of food since at least 7,500 BC in Scotland (Ashmore, quoted in McKay and Fowler 1997 b). It is still collected in huge quantities in Scotland, mostly for export to the Continent, and also consumed locally. The official landings figures for Scotland indicate that over 2,000 tonnes of winkles are exported annually. This makes winkles the sixth most important shellfish harvested in Scotland in terms of tonnage, and seventh most important in terms of value. However, since actual harvests are probably twice reported levels, the species may actually be the fourth and sixth most important, respectively (McKay and Fowler 1997 b). The extent of collection activity in England, Wales, and Northern Ireland is unknown, but likely similarly under-reported in official figures.

All winkle collection is completely unregulated, but some buyers set a minimum size of 14-15 mm for marketing reasons. This is fortunately adequate to ensure recruitment to the population. Although reproductive capacity in many species of mollusca is proportional to the size of the female, this is, unusually, not the case for L. littorea, which is an intermediate host for a number of parasitic trematodes that can reduce egg production. Rate of infection grows exponentially with age, such that in some populations most egg production may come from the smallest, first time spawners which are generally only some 11-12 mm in shell height (Robson and Williams 1971, McKay and Fowler 1997 b).

Quigley and Frid (1998) report that a previously popular shore for winkle collection in Northumberland has apparently been over-harvested in the past and has a low abundance of winkles, presumably the result of over-collection by commercial collectors. Other popular collection sites have a high relative abundance of small individuals.

The South Wales Sea Fisheries Committee has introduced a byelaw prohibiting the collection of winkles using a vacuum pump. This indiscriminate method hoovers up all sizes of winkles and other molluscs and has the potential to seriously damage local stocks.

Order Neogastropoda

Family Muricidae

Nucella lapillus (Linnaeus). Dog whelk. Reaches up to 42 x 22 mm in size. Very common intertidal species, abundant on virtually all rocky shores from the mean high water mark of neap tides to mean low water springs. Also occurs in the sublittoral, but less commonly. Widely distributed from the Straits of Gibraltar to the Arctic. Not a particularly popular species for human consumption, but may still collected as food together with winkles in some areas. (Some populations were seriously depleted as a result of pollution from tri-butyl tin anti-fouling paints in the 1980s and early 1990s, which prevented reproduction in females.)

Family Buccinidae: Whelks

Sixteen species recorded in British waters, but almost all are sublittoral.

Buccinum undatum Linnaeus. Common whelk. Large gastropod, up to 110 x 68 mm. Occasionally found at the low water mark of spring tides, where it might be collected, but usually sublittoral on hard and soft substrata. Fished commercially in some parts of the country using baited pots.

Class Bivalvia

Predominantly sedentary animals when adult, living attached to fixed substrata, in crevices, or burrowing in bottom sediments after settlement of the planktonic larvae. Traditionally, only a limited number of species have been collected from British shores. However, over the past two decades markets for a wider range of species have been opening up, mainly for export to the Continent, and to a lesser extent, for consumption in the UK. Many of the smaller edible bivalves, such as the Veneridae and Tellinidae, are very popular on the Continent, but are still neglected target species in the UK. Only a few examples of these families are included below.

Family Mytilidae : Mussels

Thirteen species are found in British waters. Two are commonly collected on the shore (but are not easily distinguishable) and a third important species may occur on the lower shore. The others are small, or restricted to the sublittoral. Intertidal mussels have been an important source of human food for at least 300,000 to 400,000 years in Europe (Siegfried 1994), and are still taken in commercial fisheries and for personal use for food and fishing bait. However, the collection of intertidal mussels for fishing bait is now only at a fraction of the level one hundred years ago (McKay and Fowler 1997 a).

Mytilus edulis Linnaeus. Common mussel. Length usually 50-100 mm but some populations are unlikely to reach more than 30 mm, while others can exceed 150 mm. This species can occur in small groups or as dense beds from the upper shore down into the sublittoral, and is widespread from the Arctic south to the Mediterranean. One hundred years ago, huge quantities were being collected by hand for food and for bait for inshore line fisheries. Today, common mussel harvests are a fraction of these historic levels, even though the species is still the subject of important commercial fisheries (usually operated by dredgers working during high tide), and large quantities are picked by hand from the shore in some areas. Market demand is increasing and there is some farmed mussel production, but most landings are taken from wild stocks, these predominantly in England and Wales. The minimum (non-statutory) required size for sale in the UK is about 55 mm.

Mytilus galloprovincialis (Lamark). Mediterranean mussel. Very similar in appearance to the common mussel, and also collected from the shore and in commercial fisheries. This species reaches the northern limits of its intertidal distribution around south-west England, south Wales, and south and west Ireland.

Modiolus modiolus (Linnaeus). Horse mussel. A large species, reaching lengths of over 100 mm. It sometimes occurs on the lower shore, but is most abundant on coarse sediments in the sublittoral, out to depths of 150 m offshore. Occurs around all British shores and south to the Bay of Biscay. Still sometimes collected for food or fishing bait, particularly in Scotland.

Family Ostreidae : Oysters

Formerly an extremely important source of food for coastal and inland communities, native wild oysters have virtually disappeared from the intertidal and shallow sublittoral in the UK. There is a small amount of natural settlement onto the lower shore of introduced species of oyster in some areas. Where populations are present, these are usually protected from public collection by Several Order.

Ostrea edulis Linnaeus. Flat oyster. The native British oyster, found from the lower shore into water depths of about 80 m. This species occurs naturally from Norway to the Mediterranean. It is now very scarce in the wild, as a result of disease, habitat damage and over-exploitation.

Crassostrea gigas (Thunberg). Pacific oyster. Introduced for cultivation in the south-west and south-east, and breeding and settling sporadically in the wild.

Crassostrea virginica (Gmelin). American oyster. An unsuccessful introduction, virtually absent from the UK.

Family Pectinidae: Scallops

Only rarely encountered on the shore, and therefore not targeted by collectors, although large individuals of Aequipecten opercularis (Linnaeus), queen scallop, and Chlamys varia (Linnaeus), variegated scallop, are likely collected when encountered in some areas. The largest and most valuable species, Pecten maximus (Linnaeus), great scallop, is restricted to the sublittoral.

Family Cardiidae: Cockles

Eleven species are recorded from the British Isles. The common cockle is one of the most important intertidal bivalves taken commercially.

Cerastoderma edule (Linnaeus). Common cockle. Up to 50 mm in length, and found in sandy muds, sands and fine gravels from the mid tide level to just below the extreme low water mark of spring tides, sometimes in extremely dense beds, and often in association with bait worms. Common on all UK coasts (including estuaries), and its range in the Northeast Atlantic extends from north-east Norway to west Africa. Collected by hand and mechanically for the commercial market, and by hand for personal consumption. Conflicts have been reported between cockle gatherers, and bait worm diggers (bait digging can smother cockles and cause serious damage to the cockle bed habitat, Jackson and James 1979, Shackley et al. 1995).

Cerastoderma glaucum (Poiret). Lagoon cockle. Broadly similar in appearance and size to the common cockle, and overlapping with part of its range, but restricted in distribution to brackish water habitats. Recorded on the UK coast from East Anglia to South Wales (but likely to occur elsewhere in suitable habitats), and elsewhere in Europe, the Mediterranean and Black Sea. Reported to be less palatable than the common cockle (R. Mitchell pers. comm.).

Family Veneridae: Venus or carpet shells

An important commercial family of bivalves, particularly in Continental Europe, and collected from the wild for sale (often for export) and for personal consumption. Some species are entering cultivation. Genera include Venus, Venerupis, Tapes, Dosinia and Mercenaria. A few examples of important species are given below.

Mercenaria mercenaria (Linnaeus). Quahog. A large (to 120 mm) and valuable edible bivalve, introduced into the UK from the USA on occasions since the mid 19th Century. Self-sustaining populations still present in Southampton Water and the Solent (where fisheries have conflicted with nature conservation interests) and at Burnham-on-Crouch. Found in lower shore and shallow sublittoral muddy habitats.

Tapes rhomboides (Pennant). Banded carpet shell. Solid shell, up to 60 mm long. Burrows in coarse sands and gravels from the lower shore and into deep water offshore. Found on all British coasts and from Norway to the Mediterranean and north-west Africa.

Tapes decussatus (Linnaeus). Chequered carpet shell. Solid shell, up to 75 mm long. Shallow burrower in sand, muddy gravel and clay on the lower shore and in the shallow sublittoral. A southern species, occurring mainly on southern and western UK coasts and south to the Mediterranean and West Africa.

Family Mactridae

Another potentially important commercial family of bivalves, although under-exploited in the UK compared with Continental Europe and the USA. One fisherman has started a Spisula fishery in south-west England, taking surf clams from shallow water using a dredge and air lift. A significant and saleable bycatch of lugworms Arenicola sp. is reported (P. Coates pers. comm.). An example is:

Spisula solida (Linnaeus). Thick trough shell. Very solid shell, up to 50 mm long. Burrows in sand on the lower shore and in the sublittoral. Occurs from south Iceland and Norway to Spain and Morocco, and widespread in British waters.

Family Tellinidae

A potentially important commercial family of bivalves. Includes several species commonly eaten on the Continent and present in the intertidal. Genera include Angulus, Fabulina, and Macoma.

Family Scrobiculariidae

Two genera in British waters, Scrobicularia and Abra.

Scrobicularia plana (da Costa). Peppery furrow shell. Up to 65 mm in length, and restricted to deep burrows in soft estuarine tidal flat and muds. Ranges from Norway to the Mediterranean and West Africa, and is widely distributed (often abundant) around the British Isles. Traditionally collected for food (as indicated by its common name).

Family Solenidae: Razor shells

Large, actively burrowing bivalves with shells which gape at each end and have an external ligament. Most species are restricted to the lower shore and shallow sublittoral, where they burrow in sandy sediments, and all are widely distributed around the UK and from Norway to southern Europe or northern Africa. Razor shells have traditionally been hand collected for food (or possibly bait), both for personal consumption and for resale (usually for export to the Continent). They have recently been the target of mechanical commercial harvesting using sublittoral suction-dredgers. Hand and mechanical collection are both controversial, potentially conflicting with nature conservation interests. An example is:

Ensis ensis (Linnaeus). Common razor shell. Large bivalve, up to 130 mm long (E. siliqua and E. arcuatus are larger). Found burrowing in fine sand on the lower shore and shallow sublittoral around all British coasts, and distributed from Norway to the Mediterranean and North Africa.

Family Myacidae : Gaper shells

Three species in British waters. Of considerable commercial importance in other parts of the world (these are among the clams used in American clam chowder).

Mya arenaria Linnaeus. Sand gaper, or soft shell clam. Oval shell with pronounced posterior gape, up to 150 mm in length. Found in sand, often mixed with mud or gravel, from the lower shore to a depth of 20 m. May be extremely common in estuaries, where extensive beds are sometimes found. M. arenaria is a circumboreal species in the North Atlantic, not reaching the Mediterranean.

Mya truncata Linnaeus. Blunt gaper. Similar to M. arenaria, but with abruptly truncated posterior, only reaching 70 mm in length. Found in mixed sandy sediments from the lower shore to a depth of 70 m around all British coasts and circumboreal, extending south to the Bay of Biscay in the NE Atlantic.

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