Estuarine and Marine Bivalve Mollusk Culture

Winston Menzel

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351088817
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Biología marina

This book presents the biology, culture techniques, research and development, and future of the fishery of some of the most important bivalve mollusks cultured throughout the world. The book emphasizes those species that are truly cultured during some part of their life cycle rather than those that are harvested from natural populations. Graphs and figures summarize fisheries information and provide quick access to important production figures. Species covered include oysters, soft-shell and hard-shell clams, scallops, mussels, pearl oysters, razor clams, cockles and giant clams. Geographic areas featured include United States, Mexico, South and Central America, Europe, India, Japan, China, Philippines, Australia, New Zealand, and the coral atolls of the Pacific Ocean. Estuarine and Marine Bivalve Mollusk Culture brings together the lifetime efforts of the late Dr. Winston Menzel to characterize and improve bivalve mollusk culture worldwide. Aquaculturalists, private oyster and bivalve culturalists, and fisheries scientists will find this book to be an invaluable guide to bivalve mollusk culture.

Chapter 1

THE SOFT-SHELL CLAM MYA ARENARIA (LINNAEUS) IN NORTH AMERICA

Carter R. Newell

TABLE OF CONTENTS

I. Biology

A. Morphology

B. Range

C. Environmental Tolerances

D. Sexuality

E. Larval and Juvenile Biology

F. Growth

II. Fishery

A. Commercial Fishery

B. Management

III. Aquaculture

IV. Constraints

References

I Biology

A Morphology

The soft-shell clam Mya arenaria (Linnaeus) (family Myidae; common names: soft-shell clam, long-necked clam, nannynose, sand gaper, steamer clam) has an elongate, elliptical shell (Figure 1), with a large siphonal gape at the elongate posterior end and a small gape anteriorly. The left valve has a distinct chondrophore, which encloses the hinge ligament attached to the right valve. Distinct growth lines are apparent on the shell surface and in cross-section. In living position, a long contractile siphon extends from the posterior end of the clam to the sediment surface. The incurrent siphon has a ring of tentacles. The foot is small and muscular and extends from an opening in the anterior end of the shell. The mantle lobes are fused except at the siphon and pedal gape.

B Range

The species occurs on the east and west coasts of the North Atlantic and has been successfully introduced into Pacific waters from California to Alaska. It is intertidal and subtidal to a depth of 200 m along the Atlantic coast from Labrador to South Carolina and extends, in lesser abundance, south to Florida;¹ the fishery extends from Chesapeake Bay northward. It is most abundant intertidally along New England and Canadian Maritime coasts and subtidally in Chesapeake and Delaware bays.

C Environmental Tolerances

Larvae are more sensitive to salinity than adults. The salinity stress point is 5‰ for Chesapeake Bay clams, 15‰ for Massachusetts clams, and 22 to 24‰ for Medomak River, ME, clams. Mortalities were as high as 90% in Chesapeake Bay when salinities dropped to 2‰ after hurricane Agnes.² Clams located in runoff streams can suffer salinity stress in some intertidal environments. Clam survival at low salinity is greater at lower temperatures;³ however, salinity acclimation, which is regulated by amino acids in the hemolymph, occurs more rapidly in warmer waters.⁴

Oxygen utilization is governed by body size, water temperature, rate of metabolism, and oxygen concentrations in the water. Oxygen intake is dependent on oxygen concentration down to 2.8 mg/l.⁵ Clams living in the intertidal zone can switch to anaerobic metabolic pathways at low tide, functioning as facultative anaerobes. When the clams are reimmersed, they oxidize the end products of anaerobic respiration by pumping rapidly.

At the northern end of its range, the soft-shell clam is limited by temperatures too low (12 to 15°C) for spawning, and in the south, its range is limited by high temperatures.⁶ Temperature influences the timing of spawning and the length of larval life. There is a single spawning north of Cape Cod with two spawns prevalent to the south. In Maryland, a spawning takes place when water temperatures reach 10°C in April and another when later summer temperatures fall to 25°C.⁷ Clams in Chesapeake Bay rarely survive temperature above 28°C.⁸ Optimal temperature for larval development is about 20°C.⁹ The pumping rates of soft-shell clams increase up to 16°C.¹⁰ Temperature also influences the rate of burrowing; rates are highest at 18°C in Chesapeake Bay and lower at temperatures above 21°C or below 9°C.¹¹ Clams tend to grow faster in warmer waters, and greater shell growth occurs during the warmer temperatures in clams from Maine.¹² Oxygen intake is greatest at about 20°C.¹³ Appeldoorn¹⁴ found that temperature accounted for 68% of the growth rate differences in clams collected along the east coast of North America. At low temperatures, clams continue to filter, but food assimilation rates may be low.¹⁵

D Sexuality

Sexes are separate; the sex ratio is 50:50 and there is very low incidence of hermaphrodites in most populations. Clams longer than 20 mm in shell length are usually capable of spawning.¹⁶ Clams from cold Maine waters spawn at a smaller size than those from Massachusetts or Long Island Sound. In the North Atlantic, gametogenesis begins in late winter and early spring, and spawning peaks from June to September, depending on location.¹⁷ In Maryland, clams begin to spawn in April when water temperature reaches 10°C, and larval abundance peaks when water temperatures are 20°C.¹⁸ Several studies have elucidated regional differences in timing of reproduction. In Ipswich, MA, clams had an early spawn in March and April, followed by a large spawn in June and July. Clams from Long Island Sound (Connecticut) spawned twice annually in one location and just once a year in two other locations.¹⁶ On a Maine mudflat, spawning was observed in June when water temperatures fluctuated between 13°C at high tide and a seasonal maximum of 21°C on a sunny day in shallow water at low tide.¹⁹ There is an increase in fecundity with size; a female clam 60 to 70 mm long produces as many as 3 million eggs a year.²⁰ However, fecundity also varies with location and food availability. In the middle intertidal zone, females only produced 120,000 eggs a year at Cape Ann, MA.¹⁹

FIGURE 1 Soft-shelled clam shell.

E Larval and Juvenile Biology

Fertilization is external, and the spherical egg (66 μm in diameter) is white and gelatinous.²¹ The embryo forms a top-shaped trochophore larvae after about 12 h. At about 80 μm in size, a shell is formed, and the larva becomes a veliger, which passes through the prodissoconch I (straight hinge) and prodissoconch II (shell with umbo) stages. Locomotion is provided by a ciliated velum. Clams reared in a hatchery until settlement have been measured and photographed,²² but identification of clam larvae from plankton samples is still difficult because the shape of the Mya shell is easily confused with that of Hiatella sp. However, the larvae can be identified on the basis of hinge structure.²³

The larval period lasts from about 14 to 21 d, depending upon water temperature, and is longer at temperatures below 20°C. In culture, larval temperature optima were between 17 and 23°C and salinity optima between 16 and 32‰; highest salinities and lowest temperatures depressed the Maryland stocks.²⁴ Along the New Hampshire coast from June to October, veliger abundance peaked in late summer with numbers as high as 1000/m³. Numbers were higher in inshore waters than in offshore waters, and higher at depths from 5 to 9 m than near the surface or at 13 m.²⁵

At settlement, the velum is lost, and the clam attaches to suitable substrate with a byssal thread. Larvae are thought to attach first to filamentous algae, eelgrass and its associated epiphytes, and other projections in the subtidal zone.²⁶ Addition of adult pallial fluid stimulates mature larvae (256 μm) to attach the byssal thread, suggesting a gregarious component to clam settlement.²⁷ Initial byssal attachment may be ephemeral as the clam searches for a suitable substrate. After settlement, the spat (0.25 to 1 mm long) undergoes a swimming-crawling stage that lasts for 2 to 5 weeks.²⁰ During this stage the foot and byssal thread are active in the migration, which is thought to be partially mediated by resuspension by wind-induced waves and tidal currents.²⁸ It is unclear whether recently set spat have a separate drifting thread such as is found in the blue mussel Mytilus edulis L. When the clam burrows into the sediment and reaches 5 mm in size, it is referred to as a clam seed until it reaches market size (50 mm).

Juvenile clams have a complex behavior in the first year; substantial repositioning occurs until they reach about 15 mm in shell length. In New England, several authors have attributed a shoreward migration of seed clams to physical factors. Clam flat sediments are often dynamic, with erosion and deposition of up to 10 cm occurring over the course of a year,²⁹ indicating that shallow-burrowing seed clams could well be moved along with the sediments. Clams at a Massachusetts beach that set in the summer and grew to over 5 mm the following spring were moved shoreward along with coarse sediment particles during spring storms.³⁰ In Maine, movement of first-year juveniles peaked in the fall during early growth and again in May after growth resumed in the spring.³¹ Clams migrated up to several hundred yards toward the shore.

Clam distribution is aggregated in intertidal and shallow subtidal areas,³² possibly because of gregarious settlement, predation effects, or concentration by hydrodynamic factors. In some environments...