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with at least 250 species, are vermiform animals

which have calcareous spicules, scales, or plates em-

bedded in the mantle and which are separable into

two principal groups, sometimes considered as inde-

pendent classes. The more or less cylindrical, gono-

choristic Caudofoveata (Figure 1a), having the body

somewhat separated into anterior, medial, and pos-

terior sections and possessing an anteroventral pedal

shield, attain lengths of 140 mm and live mostly as

infaunal burrowers. Fewer than 100 species are

known, and they feed mainly on microorganisms

and detritus. The free-living, more or less uniformly

worm-like, elongate and laterally compressed Soleno-

gastres (Figure 1b, c), with over 200 recognized

species, are 1–300 mm in length. Hermaphroditic

and predacious, they have a distinct pedal groove,

posterior gill folds and live either epibiotically on

sediments or epizoically on cnidarians, which usually

constitute their principal food.

000 7 The Monoplacophora (Figure 1d, e), with fewer

than 20 living species, have a controversial fossil

history dating from the Early Cambrian; they are

benthic, mainly deep-sea animals having a single

cap-shaped or limpet-like shell and variously paired

organ systems such as the pedal retractor muscles,

gills or ctenidia, nephridia or kidneys and gonads,

reflecting a primitive metamerism. In monoplaco-

phorans, the foot is large with a flattened, creeping

sole, and the pallial mantle cavity is peripheral along

the sides of the animal.

0008 The chitons or so-called coat-of-mail shells, separ-

ated as the class Polyplacophora (Figure 1f, g), also

have a broadened, flat muscular foot and a mantle

cavity containing numerous pairs of ctenidia. Con-

sisting of about 600 species of marine benthic epi-

faunal animals with dorsoventrally depressed bodies,

they are unique in having eight calcareous shell plates

held together by a strong, peripheral girdle. The body

outline is elongate to ovate, and adult individuals

range in size from 3 to almost 400 mm in length.

Although some chitons are known to occur at depths

beyond 7000 m, most live in shallower waters and are

mainly grazing herbivores, with a radula having 17

teeth in transverse rows and cusps reinforced with

magnetite. The anterior portions of the alimentary

canal have pairs of glands for the digestion of carbo-

hydrates, and the broad, large foot is used to creep

over, or to attach by suction to, the substrate. Special

photosensory structures, the esthetes, are found on the

dorsal surface of the valves and may, along with the

iron-containing radula, facilitate homing behavior.

000 9 The Gastropoda or snails (Figure 2a–c), the largest

and most diverse class with as many as 40 000

species, have representatives in most of the world’s

biotopes and are known from the Early Cambrian.

Although most species live in the marine environ-

ment, many kinds occur both on land and in fresh

water; a few taxa have become specialized internal

parasites. Principally, gastropods are univalves,

usually with a coiled, apically closed calcareous

shell, which also exhibits a great many different shapes

and forms, being sometimes merely cap-shaped,

greatly reduced, or even altogether lost. The very

largest of living snails has a shell over 500 mm in

length, the smallest less than 1 mm.

0010Anatomically, snails and their relatives are basic-

ally divisible into head, foot, and visceral mass; they

are all characterized by a unique process called tor-

sion which occurs during ontogeny. Primitively, the

mantle cavity, into which the alimentary canal, excre-

tory structures, and gonads of the visceral mass

empty, contains a pair of laterally disposed respira-

tory ctenidia or gills and is located posteriorly in the

early larval gastropod (Figure 2a). During develop-

ment, the entire mantle cavity and its contents are

twisted or torted through an arc of 180



to the right,

bringing the cavity itself into an anterior position

above and behind the neck and head of the snail

(Figure 2b). Internally, torsion causes a peculiar cross-

ing (or streptoneury) of the laterally disposed, paired

nervous connections between the anteriorly placed

cerebral ganglia and the posteriorly located visceral

ganglia.

0011The head of a gastropod usually has at least one

pair of cephalic tentacles, and eyes are often associ-

ated with them; however, both structures can be lost.

The foot has a creeping sole, and primitively, there are

lateral grooves between it and the mantle, forming a

so-called epipodium with sensory tentaculate and

integumentary organs. The sole of the foot may be

subdivided and variously modified for swimming

and, in the case of internal parasites, it is entirely

lost. The distinctive visceral mass, a dorsal hump, is

covered by the mantle which secretes the shell, and

contains the internal organs or viscera, including the

heart, kidneys, gonads, and alimentary canal. Gener-

ally speaking, the jaws and radula assist the mouth in

bringing food into the digestive tract; one or more

pairs of salivary glands or specialized pouches may

contribute enzymes to the buccal cavity or esophagus;

the stomach leads through the intestine to the

terminal anus and may have a crystalline style, style

pouch, typhlosole, digestive diverticula, and a tritur-

ating gizzard associated with it. All these structures –

the head, foot, visceral mass, mantle and shell – are

different and variously altered in the major sub-

divisions of the Gastropoda.

0012Traditionally, the snails are subdivided into three

major subclasses: the Prosobranchia, Opisthobran-

chia, and Pulmonata. Recently, with the advent of

SHELLFISH/Characteristics of Molluscs 5217

Mouth

Pedal

groove

of foot

Gill

folds

(a) (b) (c)

Shell

Mouth

Gills

Foot

Mantle

Anus

Mouth

Head

Mantle

cavity

Gills

Foot

Anus

Nephridipore

Gonopore

Posterior value

Girdle

(mantle)

Intermediate shell valve

Anterior valve

(f) (g)

(d) (e)

fig0001 Figure 1 (a) Aplacophoran caudofoveate showing tripartite body form and spicules. (b) Aplacophoran solenogastre showing the

ventral pedal groove. (c) Aplacophoran solenogastre on a cnidarian. (d) Dorsal view of monoplacophoran shell. (e) Ventral view of

monoplacophoran showing gross anatomical detail. (f) Dorsal view of polyplacophoran with the shell plates and girdle. (g) Ventral

view of polyplacophoran showing gross anatomical detail. Reproduced from Shellfish: Characteristics of Molluscs, Encyclopaedia of

Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

5218 SHELLFISH/Characteristics of Molluscs

cladistic analysis and the discovery of unusual forms

in the deep sea, authorities have begun to dismantle

the more conservative classification system long in

use by elaborating complex split taxonomic systems

incorporating numerous new higher-level taxa; but

at least 350 families constitute the diversity of

gastropods, about 150 in the prosobranchs and 100

each in pulmonates and opisthobranchs, and if one

(a)

(d)

(b) (c)

(e) (f)

Pedal

retractor

muscle

Pericardial

cavity

Heart

Digestive gland

Intestine

Stomach

Umbo

Hinge

adductor

muscle

Labial palp

Byssal retractor

muscles

Foot

Byssal

threads

Posterior

pedal

retractor

muscles

Ctenidium

Mantle

edge

Incurrent

area

Excurrent

siphon

Anus

Posterior

adductor

muscle

Rectum

Foot

Shell

Mantle cavity

Ciliated

mantle tissue

Anus

Nephridium

Gonad

Digestive

gland

Stomach

Mouth

Captacula

Sediment

surface

Water

movement

Water

fig0002 Figure 2 (a) Pretorsional gastropod showing ctenidium (A) in posterior mantle cavity and uncrossed cerebrovisceral connective

nerves (B). (b) Posttorsional gastropod showing ctenidium (A) in anterior mantle cavity and crossed cerebrovisceral connective

nerves (B), or streptoneury. (c) Primitive prosobranch gastropod showing: g, ctenidium; o, eye; r, rectum; s, stomach; t, tentacle; v,

ventricle of heart traversed by rectum. (d) External view of a bivalve mussel with its byssal attachment. (e) Internal view of a bivalve

mussel with gross anatomical detail. (f) Internal view of a scaphopod with gross anatomical detail.

SHELLFISH/Characteristics of Molluscs 5219

includes the fossil taxa, there are about 8000 different

genera though many more than this have been named.

0013 Prosobranchs (Figure 2c), with the gills anterior to

the heart, form the most primitive and most numer-

ous of gastropods with more than 20 000 species; the

shell is almost always present and usually spirally

coiled, although it may sometimes be patelliform,

tubular, or lost in the adult; usually the foot with its

creeping plantar sole bears a calcareous or corneous

operculum, a structure capable of covering the aper-

ture of the shell when the animal is retracted. As a

rule, the mantle cavity is anteriorly directed and it

contains, in the most primitive prosobranchs, a pair

of respiratory structures (the ctenidia), a pair of

chemosensory organs (the osphradia), and a pair

of mucus-secreting structures (the hypobranchial

glands – sometimes also a source of purple pigments

that are a problem in commercially important

species); the alimentary canal as well as the excretory

and reproductive tracts empty into the mantle cavity.

The ventricle of the heart is, in the most primitive

forms, traversed by the rectum; there are also paired

lateral auricles. In more advanced prosobranchs, for

example, the so-called neogastropods, the mantle

cavity becomes somewhat detorted and shifted

toward the right; the paired structures, e.g., the gills,

osphradia, hypobranchial glands, auricles, and

kidneys, are reduced to single ones; siphonal struc-

tures, elaborated by folds of the mantle, direct the

flow of water into and out of the mantle cavity and

thus facilitate respiration as well as the removal of

excretory and alimentary waste products. The ner-

vous system is streptoneurous, with the cerebrovisc-

eral connectives crossed, and the animals are usually

gonochoristic. Prosobranchs primitively are herbivor-

ous grazers or detritus feeders, using the radula to

scrape edible material off the substrate, but different

evolutionary lines have culminated in many highly

specialized carnivores, frequently accompanied by

changes in, or even loss of, the radula. According to

the kind of preferred diet, great differences obtain in

the nature of the alimentary canal with its radula and

various associated digestive glands.

0014 On more than one occasion, prosobranchs have

given rise to lineages which have invaded fresh-water

and terrestrial habitats, and, of course, concomitant

alterations, especially in the respiratory and repro-

ductive structures have resulted. Land prosobranchs

tend to be more tropical or subtropical in their distri-

bution while the fresh-water groups are widely suc-

cessful in both the tropics and temperate zones. In the

marine realm, prosobranchs are the predominant

snails from the intertidal areas over the continental

shelf and slope down to the greatest depths.

0015The opisthobranchs, with the gills, if present, usu-

ally behind the heart, are a numerically much smaller

group than either prosobranchs or pulmonates, with

fewer than 2000 species; their hallmarks include the

progressive loss of the shell and operculum in adults,

further detorsion and loss of streptoneury, wherein

the cerebrovisceral connectives are not crossed

(euthyneury), and finally, hermaphroditism.

0016With only a few fresh-water representatives, this

subclass of snails is virtually entirely marine and

includes herbivores, vigorous, rapacious carnivores,

and floating mucus-net suspension feeders. Probably

their most spectacular representatives are the nudi-

branches, which, as adults, lack the shell and mantle

cavity with its ctenidium; instead, they have some-

times elaborated separate respiratory structures, such

as cerata and anal or lateral gills, which, along with

the mantle, may be brilliantly colored.

0017The third subdivision of gastropods, the Pulmo-

nata, with about 15 000 species, are, with a few

exceptions, entirely fresh-water and terrestrial; these

animals are hermaphroditic and detorted, euthyneur-

ous, lack an operculum, ctenidium, or osphradium,

and have transformed the pallial cavity into a special,

highly vascularized pulmonary or lung-like cavity

which facilitates gaseous respiration and opens to

the exterior by means of a pneumostome, a narrowed,

contractile aperture. Generally, a shell is present,

spirally coiled, and variously shaped or reduced to a

patelliform or flattened disk; sometimes, the shell is

wholly enveloped by the mantle or totally lost, an

evolutionary phenomenon adopted by several differ-

ent lines of descent and leading to a slug-like con-

dition as an adult. In contrast to the prosobranchs,

the pulmonate radula shows a remarkable degree of

uniformity, mostly being adaptations for herbivory,

although some lineages of these snails have become

carnivores.

0018Fresh-water pulmonates, though occurring in trop-

ical and subtropical waters, are particularly success-

ful in exploiting marginal, sometimes ephemeral

habitats; they are widespread but more dominating

in the cool and temperate regions of the Paleoarctic.

Terrestrial lunged snails are cosmopolitan in distribu-

tion, with some taxa found on the most remote

islands or in the most rigorous deserts. Although

largely living on the ground or in leaf-litter, some

representatives of these snails have become arboreal,

adapting to conditions on the bark, branches, or leafy

canopy of trees. In these taxa, often the shells are

brilliantly colored and exhibit great polymorphism.

0019A sister-group of the Gastropods, the Cephalo-

poda, including squids and octopuses, with some

600 living species are also entirely marine organisms

5220 SHELLFISH/Characteristics of Molluscs

which have almost exclusively reduced, lost, or

internalized the shell, reorganized the body, and

adopted a predatory mode of feeding either in the

open ocean or along the bottom of the sea; these

animals are discussed elsewhere. (See Marine Foods:

Edible Animals Found in the Sea.)

002 0 The Bivalvia (Figure 2d, e), comprising familiar

organisms such as clams and mussels, with fewer

than 8000 species and over 100 families, are aquatic,

marine and fresh-water, epifaunal or infaunal, pre-

dominantly deposit or filter-feeding animals charac-

terized by having bilaterally disposed shells or valves

which can be tightly closed by adductor muscles. In

size, living bivalves are known to range from 1–2mm

up to over 1000 mm such as the giant clams of the

reefs of the Pacific Ocean. Formed by specialized

distal portions of the flap-like mantle, the valves are

connected dorsally by a hinge which may consist of

a ligament and so-called dental elements. The body

is generally laterally compressed; the spatulate or

lanceolate foot is variously developed, adapted for

creeping, burrowing, or cleansing, and, in attached

or cemented forms such as oysters, it may be greatly

reduced or lost. A pedally secreted byssus normally

occurs in bivalves, though sometimes only in the

young embryonic stages. In the reduction of a dis-

tinct cephalic region, these animals lost the radula

and evolved specialized palps on either side of the

mouth which assist in sorting the filtered, particu-

late food. Their nervous system is decentralized

and more specialized posteriorly in conjunction with

the posterior development of mantle openings or

siphons.

0021 In bivalves, the mantle cavity shows marked ex-

pansion and deepening, occupying much of the space

between the shells, and the ctenidia exhibit a signifi-

cant elaboration in most taxa. These form greatly

enlarged structures consisting of lateral pairs of plates

or sheets of filaments, in which there are blood vessels

for respiration and on the surfaces of which are

elaborated a complex ciliation, facilitating the

filtering of edible materials from plankton suspended

in the water column or microorganisms from the

detritus or bottom sediments. Mainly gonochoristic

with external fertilization, bivalves may also be

hermaphroditic and sometimes protandrous, with

free-swimming or parasitic larva; retention of the

young in the mantle cavity or in special incubatory

marsupia in the gills occurs in many species.

0022 Living benthically both in fresh and saline waters,

clams and their relatives have exploited many of the

biomes of aquatic habitats; they occur from shallow

intertidal waters to the greatest abyssal depths of the

oceans; they utilize both soft sediments and hard

substrates, attaching, burrowing, and even boring

into limestones, other shells, or wood. Fresh-water

bivalves include the tiny so-called fingernail clams

which almost become terrestrial, sometimes living in

moist litter, and the pearly mussels, widely distributed

on continental areas and even oceanic islands. One

group of small marine epizoic and commensal clams

has evolved as internal parasites in echinoderms.

0023Closely related to the bivalves, the Scaphopoda

(Figure 2f), commonly known as tusk or tooth shells,

with about 350 species, first occur in the Middle

Ordovician over 450 million years ago. With a

bilaterally symmetrical body occupying an elongate,

gently curved, tubular shell which is open at both

ends, scaphopods have fused their lateral mantle

flaps, reduced their cephalic region, and lost their

ctenidia, replacing these respiratory structures with

crenulations or folds in the mantle cavity. Calcareous

and composed of three layers, the shell is attenuated

into a narrow posterior opening; the external shell

surface is variously smooth or sculpted, and in size

the animals range from 2 to 150 mm in length in

adults, though one fossil species at the end of the

Paleozic attained nearly 300 mm. Located on a pro-

trusive proboscis, the mouth is surrounded by long

filiform, tentacular organs called captacula which

probe the sediment and collect, by adhesive glands

and ciliary tracts, tiny interstitial microorganisms

such as benthic foraminiferans, which constitute

their food.

0024Entirely marine and relatively conservative in their

radiation, scaphopods are shallow infaunal burrow-

ers found throughout the world’s oceans from the

sublittoral to deep abyssal zones below 6000 m. Pre-

ferring soft substrates, they flourish mainly in muddy,

sand bottoms with the posterior portion of the shell

with its aperture projecting from the substrate.

See also: Marine Foods: Edible Animals Found in the Sea;

Shellfish: Commercially Important Molluscs;

Aquaculture of Commercially Important Molluscs and

Crustaceans

Further Reading

Boss KJ (1982) Mollusca and classification of Mollusca. In:

Parker SP (ed.) Synopsis and Classification of Living

Organisms, vol. 1, pp. 945–1166; vol. 2, pp.

1092–1096. New York: McGraw-Hill.

Pojeta J, Runnegar B, Peel JS and Mackenzie G Jr (1987)

Phylum Mollusca. In: Boardman RS, Cheetham AH and

Rowell AJ (eds) Fossil Invertebrates, pp. 270–435.

Oxford: Blackwell Scientific.

Wilbur KM (ed.-in-chief) (1983–88) The Mollusca, 12 vols.

New York: Academic Press.

SHELLFISH/Characteristics of Molluscs 5221

Commercially Important

Molluscs

P F Duncan, The University of the Sunshine Coast,

Maroochydore DC, Queensland, Australia

Background

0001 The commercially important molluscs belong to the

classes Bivalvia, Gastropoda, and Cephalopoda, with

this article covering the first two groups (for cephalo-

pods, see Marine Foods: Edible Animals Found in the

Sea). Bivalves and gastropods constitute a huge fish-

ery resource, amounting to over 12 million tonnes

from marine harvest fisheries and aquaculture in

1999, and over 550 000 tonnes from freshwater. The

production methods are as diverse as the species, and

products range from high value, such as abalone, to

low value, such as periwinkles. This article provides

details on the size and diversity of the commercially

important species and includes information on major

production methods and countries, product types,

and current issues.

0002 Vernacular terms to describe edible molluscs vary

around the world and may cause confusion, e.g.,

‘clam’ may refer to at least three distinct taxonomic

groupings. Vernacular names will be used only for

important species, with scientific taxonomy used

generally.

Bivalves

0003 There are approximately 20 000 bivalve species

worldwide, with the principal commercial species

belonging to three taxonomic orders: Mytiloida

(including mussels, scallops, and oysters), Veneroida

(true clams and cockles), and Myoida (soft-shell

clams and geoducks).

0004 Bivalves are generally characterized by paired shell

valves joined at a dorsal hinge and a sessile, filter-

feeding way of life. Their internal body form consists

of adductor muscles (usually paired) for shell closure,

the mantle for shell secretion, gills for feeding and

respiration, a digestive gland, and seasonally variable

gonad tissue.

0005These features influence their commercial import-

ance and use as food products. Generally, bivalves are

consumed whole, e.g., oysters and mussels, or pro-

cessed to adductor muscles (meat) and/or gonad (roe),

e.g., scallops. Muscle tissue is rich in carbohydrate

(glycogen) and protein, and mature gonad tissue is

high in lipid.

Mussels

0006Production Commercially important mussels be-

long to the family Mytilidae and are predominantly

marine (see Table 1). Freshwater mussels involved in

pearl production belong to the Order Unionida. Com-

mercial Mytilid species come from wild harvest fish-

eries (global production of 237 823 tonnes in 1999)

and a greater proportion from aquaculture (1 451 032

tonnes in 1999). Global mussel aquaculture at first

sale was worth US$0.52 billion in 1999.

0007Harvest fisheries Harvest fisheries target three main

species, Mytilus edulis (Blue) (121 964 tonnes in

1999), M. galloprovincialis (Mediterranean) (55 819

tonnes) and Perna viridis (Green) (21 520 tonnes).

The main countries harvesting M. edulis were Den-

mark (96 215 tonnes), Canada (11 565 tonnes), UK

(7972 tonnes), USA (4086 tonnes), and France (2098

tonnes). Production of M. galloprovincialis comes

from Italy (37 876 tonnes), Greece (15 860 tonnes),

and Turkey (1800 tonnes). Thailand dominates pro-

duction of P. viridis (21 500 tonnes). Mytilid mussels

are principally caught by dredging flat seabed areas

where the animals occur in dense aggregations,

connected by byssus, which are proteinaceous threads

used for attachment in many bivalves.

0008Aquaculture Aquaculture production is also dom-

inated by M. edulis at 498 461 tonnes, worth

US$0.27 billion, and ranked as the 15th species

by quantity under cultivation. The principal produ-

cing countries are Spain (261 969 tonnes), The

tbl0001 Table 1 Principal commercial mussels, including main production sources and methods

Species Common name Mainproduction sources and method

Mytilus edulis Blue Europe/North America/China (F/A)

Mytilus galloprovincialis Mediterranean Europe, South Africa (F/A)

Mytilus chilensis Chilean Chile (F/A)

Aulacomya ater Cholga Chile, Peru (F)

Perna viridis Green South-east Asia (F/A)

P. p e r n a South American rock mussel Brazil, Venezuela (A)

P. canaliculus New Zealand or green lipped New Zealand (A)

A, aquaculture; F, fisheries.

5222 SHELLFISH/Commercially Important Molluscs

Netherlands (100 800 tonnes), France (51 600

tonnes) and Germany (37 912 tonnes).

0009 Also important is M. galloprovincialis at

162 179 tonnes, valued at US$100 million. Italy

(130 000 tonnes), Greece (16 912 tonnes), and France

(10 900 tonnes) dominate production. The green-

lipped mussel (Perna canaliculus) (not to be confused

with the green mussel, P. viridis) is also a significant

aquaculture species at 71 000 tonnes, worth US$21.3

million, with production dominated by New Zealand.

The other main species, P. viridis, provides 68 509

tonnes, is worth US$8.3 m, and is cultivated in Thai-

land (40 300 tonnes) and the Philippines (15 478

tonnes).

0010 It should be noted that China is the dominant

aquaculture producer of mussels at 608 115 tonnes,

worth US$90 million. International statistics do not

recognize individual species composition in Chinese

production, but other sources indicate that M. edu-

lis dominates, with additional supply from M. cor-

uscus, Musculus senhousei,andP. viridis. All are

local species. Mussel aquaculture is principally

based on suspended longline or raft culture,

although other methods exist. (See Shellfish: Aqua-

culture of Commercially Important Molluscs and

Crustaceans.)

0011 Product types Mussels are traditionally a chilled,

live product, particularly in Europe, where a post-

harvest shelf-life of around 5 days is expected. Re-

cently, new packaging and processing methods,

greater acceptance of convenience foods, and value

adding have led to new products. Mussels are now

frequently sold as individually quick frozen (IQF)

cooked in the whole or half shell, precooked ready-

to-eat meals and high value, luxury products such as

smoked mussels. Traditional pickled or brined pre-

served products are common.

0012Issues As coastal filter feeders, where the whole

animal is consumed, food safety is important for

mussels and other bivalves. Accumulation of bacteria,

viruses, toxic chemicals, elements, and biotoxins is

associated with bivalves. Most edible bivalves, in-

cluding mussels, are generally subject to purification,

or depuration, which places the shellfish in a clean

water flow for 12 h or more. This enables expulsion

of many potentially harmful contaminants. Health

certification of live mussels is now common, as are

monitoring and certification of growout waters.

0013However, depuration does not remove biotoxins,

produced by microalgae (more accurately phycotox-

ins), and their control and impact on mussel produc-

tion are generally managed by long-term monitoring

programs. These toxins may result in illnesses such as

paralytic, diarrhoetic and amnesic shellfish poison-

ing. In severe biotoxin outbreaks, area closures may

be necessary, and these have been well reported in

recent years. The protozoan parasites Marteilia spp.

affect Mytilus spp., although generally, mussels

appear to little affected by disease in culture systems.

Scallops

0014Production Commercial scallops belong to the

family Pectinidae, and the principal species are listed

in Table 2. Global harvest fishery production in 1999

was 567 507 tonnes and from aquaculture 951 866

tonnes. Aquaculture production has increased from

340 807 tonnes in 1990, with an equivalent increase

in value from US$0.55 billion in 1990 to US$1.35

billion in 1999.

0015Harvest fisheries Fisheries are dominated by Pati-

nopecten yessoensis (305 510 tonnes), Placopecten

magellanicus (131 962 tonnes), Pecten maximus

(35 411 tonnes), and Argopecten purpuratus (30 141

tonnes). Japan (299 628 tonnes, 98%) is the main

tbl0002 Table 2 Principal commercial scallops, including main production sources and methods

Species Commonname Mainproduction sourcesand method

Pecten maximus Great/king scallop Europe (F/A)

Pecten novaezelandiae New Zealand scallop New Zealand (F)

Pecten fumatus/meridionalis Commercial scallop South-eastern Australia (F)

Placopecten magellanicus Sea scallop USA, Canada (F)

Chlamys islandica Iceland scallop Iceland, Canada, USA (F)

Chlamys farreri Zhikong scallop China (A)

Patinopecten yessoensis Yesso scallop Japan, Russia, China (F/A)

Patinopecten caurinus Weathervane scallop Canada, USA (F)

Argopecten irradians Bay scallop China, USA (F/A)

Argopecten purpuratus Peruvian Calico scallop Chile, Peru (F/A)

Argopecten ventricosus Pacific Calico scallop Mexico (F/A)

Aequipecten opercularis Queen scallop Europe (F/A)

Amusium balloti Saucer scallop Western and eastern Australia, Japan (F)

A, aquaculture; F, fisheries.

SHELLFISH/Commercially Important Molluscs 5223

producer of P. yessoensis, with the Russian Feder-

ation (5764 tonnes, < 2%), providing the remainder.

P. magellanicus production is from the USA (77 179

tonnes, 59%) and Canada (54 756, 41%), and

P. maximus fishery production derives mainly from

the UK (19 108 tonnes, 54%) and France (12 745

tonnes, 36%). Peru provided all production of

A. purpuratus in 1999.

0016 Wild harvest of all the main commercial scallop

species usually involves a toothed (e.g., Pecten

spp.) or smooth-framed dredge (e.g., Placopecten

spp.), or occasionally conventional trawl gear (e.g.,

Amusium spp., Argopecten gibbus). There are several

large-scale diver fisheries for some species, e.g.,

Chile, China, and Mexico, and some small fisheries

that supply high-value, live product markets, e.g.,

Scotland.

0017 Scallop fisheries are noted for catch fluctuations, as

a result of exploitation and recruitment variability.

Several fisheries have consequently declined or

closed, although notably, the successful application

of stock enhancement and management programs

helped to revive Japanese and New Zealand produc-

tion during the 1970s and 1990s.

0018 Aquaculture Three countries and two species dom-

inate scallop aquaculture production. In 1999, China

was the largest producer of P. yessoensis at 712 330

tonnes, followed by Japan at 216 017 tonnes. Chile

produced 20 668 tonnes of A. purpuratus, with the

remainder from Peru (1585 tonnes). On a much

smaller scale, and using various species, other coun-

tries are developing pectinid aquaculture. For

example, in 1999, the UK produced 114 tonnes of

Aequipecten opercularis, whereas France, Spain,

Ireland and the UK produced over 700 tonnes of

P. maximus.

0019 Scallop production from aquaculture is based on

suspended longline culture or on some form of seabed

culture.

0020 Product types Scallops are sold in processed form;

fresh or frozen, meat (muscle) only or meat with

gonad (roe) attached, although there is a small, valu-

able market for live scallops, particularly in Western

Europe. Processing removes the meat (and gonad)

from the remaining viscera and shell, a procedure

known as shucking. This is usually a manual oper-

ation, although machines are available. Usable prod-

uct may be block frozen in multi-kilogram amounts

or IQF if the product is of a higher value and quality.

0021 In scallop marketing, the term Coquille St Jacques,

refers to the Genus Pecten and is specified under

French law. This category is important because of

France’s position as one of the principal scallop

markets. True Pecten species are sold as adductor

muscle and gonad together (roe-on) and command

higher prices. This premium relates partly to selling

gonad also, providing a greater return per scallop.

0022China is unique in predominantly producing a

dried scallop meat product, although small quantities

are locally sold live.

0023Issues Because of the type of product (meat and/or

gonad only), scallops are less affected by contamin-

ation and biotoxin problems, as the contaminated

portions (mainly mantle and digestive gland) are

discarded during processing. However, microalgae

biotoxins can concentrate in meat and gonad, and

have resulted in production closures in some coun-

tries (e.g., UK) and the widespread establishment

of precautionary monitoring programs (e.g., New

Zealand, UK, various states in the USA).

0024Diseases affecting scallops appear to be rare com-

pared with other bivalves, with Rickettsia-like and

Chlamydia-like organisms occasionally associated

with mass mortalities of wild scallops and hatchery

mortalities. Outbreaks may be associated with scal-

lop density. The protozoan Marteilia spp. has been

implicated in mass mortalities of wild scallops in the

USA, although such events are rare.

Oysters

0025Production Commercial oysters belong to the

family Ostreidae, and principally to the genera

Ostrea and Crassostrea (see Table 3). The global

wild harvest in 1999 was 157 538 tonnes, and

aquaculture production 3 711 606 tonnes, valued at

US$3.4 billion. Aquaculture production has increased

from 1 251 660 tonnes since 1990 (value US$1.57

billion).

0026Harvest fisheries The dominant fishery species in

1999 was the American oyster, Crassostrea virginica

(132 207 tonnes, 84% of total), followed by the Pa-

cific (cupped) oyster, C. gigas, at 12 271 tonnes (8%).

The genus Ostrea contributed an additional 5951

tonnes, the majority (2456 tonnes, 41%) from

O. edulis, the European flat oyster, the remainder

from O. lutaria and other minor species.

0027Fishery production of C. virginica in 1999 was from

three countries: the USA (89 714 tonnes), Mexico

(39 268 tonnes), and Canada (3225 tonnes). Almost

all (11 609 tonnes) of C. gigas production comes from

China, Taiwan, and South Korea. Fishery production

of O. edulis derives from Turkey (840 tonnes), Ireland

(680 tonnes), Spain (443 tonnes), and the UK (407

tonnes). New Zealand produced 994 tonnes of the

dredge oyster, O. lutaria, and Mexico 2489 tonnes of

unnamed Ostrea spp. European Ostrea fisheries were

5224 SHELLFISH/Commercially Important Molluscs

formerly more significant, but the protozoan, Bona-

mia ostreae, introduced from the USA, devastated

production in the 1980s. Commercial fishery

methods for oysters are predominantly dredge-

based, although hand picking occurs in some coun-

tries, e.g., USA, Australia.

0028 Aquaculture The dominant aquaculture oyster is

the Pacific (cupped) oyster, Crassostrea gigas, provid-

ing the greatest quantity of any aquaculture species in

1999 at 3 600 459 tonnes. This species accounted for

97% of all oyster aquaculture production with a

value of US$3.3 billion in 1999. China accounted

for 2 988 613 tonnes, with other major producers

including Japan 205 345 tonnes, Korea 177 259

tonnes, France 134 800 tonnes, and the USA 33 259

tonnes. Other aquaculture species are C. virginica

57 522 tonnes (54 037 tonnes, USA), C. iredalei

13 698 tonnes (Philippines), Ostrea edulis 6496

tonnes (Spain 3348 tonnes, France 2300 tonnes),

and Saccostrea glomerata (commercialis) 5104

tonnes (Australia). Production methods vary, al-

though most are based on the oyster’s natural attach-

ment to artificial structures, or seabed sowing.

0029 Product types Oysters are best known as live sea-

food, and most production is live, in-shell, or fresh

chilled in half-shell. Traditionally, oyster culture has

produced a bottled product, with the whole shucked

animal, washed and preserved in salt water. They

survive low-temperature storage for several weeks,

but half-shell and bottled oysters have a limited

shelf-life. Maintenance of quality and rapid transport

and sale are essential components of the industry.

Oysters are now sold as a frozen product, either in

whole or half shell. This maintains the traditional

appearance while limiting spoilage and health risk.

Other value-added products such as smoked oysters

and pre-cooked meals are becoming common.

0030 Issues Oysters are subject to many significant dis-

eases affecting fishery and culture operations. These

include the protozoans Bonamia spp. (particularly

affecting Ostrea spp.), Marteilia spp. (affecting

Ostrea and Saccostrea spp. (causing QX (Queensland

unknown) disease in the latter), Mikrocytos spp.

(affecting several Crassostrea, Saccostrea,andOstrea

spp.), Haplosporidium spp. (affecting Crassostrea

spp.), and Perkinsus spp. (affecting Crassostrea and

Haliotis spp. (abalone)). An iridovirus, causing gill

necrosis, is also associated with hatchery-reared

C. gigas.

0031From a human health perspective, oysters are often

associated with food poisoning events, because of

their intake of bacteria and viruses and subsequent

consumption as a whole, raw product. Such events

focus attention on the production environment and

require management of local sources of pathogenic

effluent. Consequently, water-quality monitoring

and health certification of production areas are

common, along with a certification requirement

from importing countries.

Other Bivalves

0032Production International fisheries statistics com-

bine true clams and cockles (order Veneroida) with

arks (order Arcoida, Anadara spp.) and indicate total

world production from fisheries at 812 501 tonnes,

with a further 2 744 846 tonnes from aquaculture,

alone worth US$3.15 billion. This constitutes a

commercially significant group, despite including

numerous species. They are treated under broad

groupings, with taxonomic relationships indicated

where appropriate.

0033Order Veneroida This includes, amongst many

others, the families Veneridae (venus clams), Cardii-

dae, (cockles), and Tridacnidae (giant clams). Several

Ruditapes spp. (carpet shells), support a modest

harvest fishery, mainly Japanese, producing around

58 874 tonnes in 1999. The most important carpet

shell is Ruditapes philippinarum (Manila clam),

which also provided 1 820 413 tonnes (worth

US$2.19b) from aquaculture, mainly from China.

Other cultivated Ruditapes spp. added 54 421 tonnes

and US$0.13 billion. The fishery for Chamelea

tbl0003 Table 3 Principal commercial oysters, including main production sources and methods

Species Common name Main production sourcesandmethod

Crassostrea gigas Pacific (cupped) oyster China, Japan, Korea, France, USA, New Zealand (F/A)

Crassostrea virginica American oyster USA, Canada (F/A)

Crassostrea iredalei Slipper oyster Philippines (F/A)

Ostrea edulis European flat oyster Europe (F/A)

Ostrea lutaria Dredge oyster New Zealand (F)

Ostrea chilensis Chilean flat oyster Chile (F/A)

Saccostrea glomerata Sydney rock oyster Australia (A)

A, aquaculture; F, fisheries.

SHELLFISH/Commercially Important Molluscs 5225

gallina (striped venus) was 45 012 tonnes in 1999,

and mainly from Italy (36 462 tonnes).

0034 Meretrix spp., known as hard clams, are fished in

South-east Asia, and provided 22 100 tonnes in 1999.

Indonesia is the largest supplier with 17 990 tonnes.

This genus is also cultivated in South Korea (Meretrix

lusoria, 17 tonnes), but more widely in Vietnam,

where 80 000 tonnes of M. meretrix and M. lyrata

are produced per annum. This product is sold into

South-east Asia, Japan, and Europe, although EU

regulations regarding biotoxin monitoring have

limited the European market.

0035 Arctica islandica (Ocean or black Quahog) is an

important fishery species in North America, with the

USA producing 147 933 tonnes in 1999. Iceland also

produces a small quantity (3501 tonnes) through har-

vest fisheries, most of which is exported to the USA.

0036 Mercenaria mercenaria (also known as the north-

ern Quahog or hard-shell clam) is also principally a

North American species, with Canada and the USA

traditional producers. In recent years, only Canada

has provided a harvest fishery product of 2536 tonnes

(1999), with a further 53 050 tonnes, worth US$63.4

million, from aquaculture, equally from China and

the USA. As with most clam culture, spat are sown on

to preselected sand/mud bottoms at densities appro-

priate for good growth and survival and with well-

exchanged phytoplankton rich water. Harvesting is

by dredge. To reduce growout periods, some clams

are cultured on trays, either suspended or stacked off

the bottom. This improves growth through increased

exposure to nutrients and higher temperatures.

0037 Fishery production of Paphia spp. (short-necked

clams) was 42 282 tonnes in 1999, from Thailand

and the Philippines. The North Atlantic surf clams,

Spisula solidissima and S. polynyma, are produced

from harvest fisheries in the USA (142 370 tonnes in

1999) and Canada (26 722 tonnes), respectively.

Razor clam (Solen spp.) aquaculture in China is sig-

nificant with 479 252 tonnes, worth US$383 million,

produced in 1999. Additional sources indicate that

this species may also be reported as Sinonovacula

constricta.

0038 The European edible cockle, Cerastoderma edule,

is dredge-fished in The Netherlands (50 888 tonnes)

and the UK (14 123 tonnes), for a total production of

70 401 tonnes in 1999. Recent aquaculture, based in

Spain (3713 tonnes) and France (1300 tonnes), also

contributed 5169 tonnes to production, valued at

US$8.9 million. Aquaculture produced very small

quantities (2 tonnes, in 1998) of cockles (Family

Cardiidae) in the USA.

0039 Order: Myoida The soft-shell clam, Mya arenaria,

and Geoducks, Panopea abrupta are produced and

consumed in North America. In 1999, the USA

and Canada produced 5113 and 2643 tonnes of

M. arenaria, and 2451 and 1800 tonnes of P. abrupta,

respectively.

0040Order: Arcoida There are three main genera of Ark

‘clams’ of commercial importance, Arca spp., Sca-

pharca spp., and Anadara spp. Fishery production

of Arca spp. and Scapharca subcrenata totalled

52 972 tonnes in 1999. Aquaculture of S. broughtonii

in South Korea produced 8550 tonnes, worth

US$49.8 million.

0041Dredge fisheries also produced 42 627 tonnes of

Anadara spp. in 1999, with A. granosa (misleadingly

called the blood cockle) the main species. Aquacul-

ture of A. granosa was 315 811 tonnes, worth

US$277.4 million, mainly from China (188 355

tonnes) and Malaysia (79 912 tonnes).

Gastropods

0042The class Gastropoda is commercially less important,

although the marine abalone, periwinkles, conchs

and whelks, and terrestrial snails are significant as

food products. Other species of gastropod are valued

as shell products, such as mother of pearl and speci-

men shells. Gastropods are generally characterized by

a single spiral shell and large muscular foot, enabling

greater motility than bivalves and forming the princi-

pal edible portion. Fishery production of gastropods,

as abalone, winkles, and conchs was 105 047 tonnes

in 1999. Equivalent aquaculture production was

2694 tonnes, worth US$32.4 million.

0043Abalone

Production Abalone (family Haliotidae) is mainly a

temperate marine group, although they extend into

tropical areas. Approximately 50 species are recog-

nized, associated with rocky habitats off Australia,

New Zealand, Japan, Pacific North America, and

South Africa. The main commercial species are

shown in Table 4. Abalone production is from

divercollected harvest fishery or aquaculture.

0044Harvest fisheries The total fishery production was

10 453 tonnes in 1999, the majority from Australia

(5593 tonnes of Haliotis rubra), followed by Japan

(2109 tonnes of H. gigantea). The value of combined

fisheries production has not been estimated, but, as

an indicator, the Australian fisheries production in

1999–2000 was worth approximately US$118 mil-

lion (A$235.6 million) (Australian Bureau of Agricul-

tural Resource Economics).

0045Commercial harvest is by diver using hookah or

scuba equipment. Abalone attach to rocky substrates

5226 SHELLFISH/Commercially Important Molluscs