Phylum Mollusca : Molluscs
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.
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.
Prosobranchs are usually characterised
by a coiled shell, sealed by a horny operculum attached
to the top of the animals foot. However, others
have internal shells, or cone shaped shells (e.g.
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.
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.
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
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
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.
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.
(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
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
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
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.
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
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
A potentially important commercial
family of bivalves. Includes several species commonly
eaten on the Continent and present in the intertidal.
Genera include Angulus, Fabulina,
Two genera in British waters, Scrobicularia
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.