Food and Agriculture Organization of the United Nations Fisheries and for a world without hunger Aquaculture Department Cultured Aquatic Species Information Programme Mercenaria mercenaria (Linnaeus, 1758) I. Identity V. Status And Trends a. Biological Features VI. Main Issues b. Images Gallery a. Responsible Aquaculture Practices II. Profile VII. References a. Historical Background a. Related Links b. Main Producer Countries c. Habitat And Biology III. Production a. Production Cycle b. Production Systems c. Diseases And Control Measures IV. Statistics a. Production Statistics b. Market And Trade Identity Mercenaria mercenaria Linnaeus, 1758 [Veneridae] FAO Names: En - Northern quahog(=Hard clam), Fr - Praire, Es - Chirla mercenaria Biological features Shell solid, equivalve; inequilateral, beaks in the front half of the shell; broadly oval in outline. Ligament a deeply inset, dark brown elliptical band, behind the beaks reaching half-way to the posterior margin. Lunule well defined, broad, heart-shaped. Escutcheon indistinct. Sculpture of concentric lines, raised here and there into ridges, and fine radiating lines. In young specimens the ridges are present all over the shell but in the adult they persist, after wear and tear, only near the anterior and posterior margins. Growth stages prominent. Both valves with three cardinal teeth; in addition there is present in each valve a rough tooth-like area behind the beaks and immediately below the ligament; this area has the appearance of a supplementary posterior cardinal tooth which has been broken off. No laterals. Pallial sinus not deep, triangular. Margin grenulate. Colour a dirty white, light varnish-brown, dull grey or grey-brown. Inside of shell white, sometimes deep violet about the adductor muscle scars. View FAO FishFinder Species fact sheet FAO Fisheries and Aquaculture Department Images gallery Photo: LeRoy Creswell Photo: Randy Newman Profile Historical background There are two important factors that make the hard clam relatively unique as a farmed bivalve mollusc. The first is that it is more valuable at a small size; it stays in this size range for a relatively short period of time, after which the value decreases. The second aspect is related to hard clam biology, and makes it distinct from most other bivalves currently in large scale aquaculture; the hard clam does not typically produce concentrated sources of seed that can be collected from wild sets. The lack of seed necessitated the development of hatchery and nursery technology before critical experiments on field grow-out could take place. The development of relatively low-cost unicellular algal production techniques made production of large numbers of seed economically viable. Experiments started in the late 1940s and the hatchery technology was developed in the early 1950s. By the end of that decade a number of individuals were attempting to grow the species commercially. Most of these early attempts were failures because predators consumed the seed, but limited numbers of clams were sold. In the 1970s a series of size-specific plantings of clam seed with a variety of protection techniques were instituted. The resulting data provided the science-based criteria for the further development of the nursery systems that needed to produce the size of seed for field operations. Once large quantities of seed at the critical size of about 10 mm could be produced, several entrepreneurs learned the protection techniques (in the late 1970s and early 1980s) and, by the late 1970s, significant hard clam production through aquaculture had begun in the states of Massachusetts, New Jersey and North Carolina, United States of America. These pioneers spread the technology throughout the US east coast and modifications were made to the protection devices. By the late 1980s, hard clam aquaculture was being practiced from Massachusetts through Florida, but the bulk of production was in Massachusetts, Connecticut, New York, New Jersey, Virginia, South Carolina and Florida. These states have continued to be the largest producers, with Florida and Virginia becoming increasingly important in recent years. Main producer countries FAO Fisheries and Aquaculture Department Main producer countries of Mercenaria mercenaria (FAO Fishery Statistics, 2006) Habitat and biology The native range of the species is from the Gulf of St. Lawrence in Canada through the northern Gulf of Mexico to Texas. It has been introduced to the west coast of the United States (Washington and California), the island of Puerto Rico in the Caribbean region, the United Kingdom, France, Holland and Belgium in Europe, and Taiwan Province of China in Asia. Some individuals are also being reared in Wenzhou and Yantai in China. Hard clams can be found in coastal lagoons and estuaries from about 12-30‰ but most large populations occur at >15‰. They grow from 10-30 °C, but most growth occurs from 18-25 °C; below 5 °C clams become dormant. Clams burrow from just below the surface to about 15 cm in all sediment types, near oyster reefs, and in sea grass beds, but prefer sediments that are a mixture of sand and mud with some coarse material. They range from the intertidal zone to 15 m. Hard clams filter the water to obtain phytoplankton and other suspended particles that they use as food. The hard clam has a typical bivalve life history. The sexes are separate, sperm and eggs are released into the water, and fertilization is external. The free swimming larvae feed on phytoplankton and other organic materials in the water for 7-21 days and then metamorphose into benthic organisms. These newly settled organisms attach to sediment with a byssal thread and develop a calcified shell within a day or so. The seed reach approximately 10-15 mm in year 0, and approximately 25-30 mm shell length (SL) by the end of year 1, but there is a large variability in growth. Individuals may develop sperm in year 0, but sexual maturation typically occurs by the end of year 2 or 3. Mature females typically spawn 1- 5 million eggs per spawning event, but may produce 40 million or more eggs per year. Commercial size is reached at the end of the second year of life, or earlier in southern waters, but may take four or five years in the northern parts of the range. During the first year of life the animals are subject to high losses from invertebrate and vertebrate predators, chiefly crabs. They become less susceptible to predators when they reach 20-25 mm SL, and only large crustaceans (lobsters), large snails, a few fish and birds can consume mature hard clams. The average life span has been estimated to be between 12-20 years, but clams older than 50 years have been reported. FAO Fisheries and Aquaculture Department The major difference in the life history of hard clams compared to most bivalves is that large quantities of seed are not typically found in nature. This has caused hard clam aquaculture to rely entirely on hatcheries for seed. Much of the farming of this species is conducted in habitats with a significant mud component, both because of the availability of such sites and because this habitat eliminates some predators. Production Production cycle FAO Fisheries and Aquaculture Department Production cycle of Mercenaria mercenaria Production systems FAO Fisheries and Aquaculture Department Hard clam production systems range from intensive (hatchery, nursery and some grow-out) to extensive (some grow-out). Seed supply All hard clam seed are produced in hatcheries. The production cycle begins with adult organisms with well- developed gonads. The conditioning of these adults relies on a number of techniques. For early spawning, adults may be conditioned in the hatchery facility by increasing water temperature while providing sufficient food in the form of unicellular algae. Conditioning takes 4 to 10 weeks depending on the starting point. Clams may also be placed in high growth areas and allowed to condition naturally for spawning later in the season. Typically, conditioned adults are mass spawned by placing them in a seawater bath in which temperature can be adjusted. Sperm, microalgae, or serotonin may be used to provide a stimulus for gamete release. Fertilized eggs are retained on a screen, then counted and distributed into tanks for development. After two days the larvae have reached the straight hinge stage and are removed from the tank, redistributed into another tank, and supplied with unicellular algal food. This process is repeated for 7 to 14 days until settling (setting), when clams are typically held at about 1/ml. Clams are removed from the larval tanks and placed in containers with mesh bottoms in which water with unicellular algae enters from the top (downwellers) or placed directly on raceways and supplied with very low flow. The clams remain in downwellers or with limited flow for one to two weeks. Clams in downwellers are then placed in mesh bottom containers with the water flowing up through the seed (upwellers). Nursery There is no definitive moment at which the hatchery system ceases and the nursery ensues. However, most people consider the point at which unicellular algae are no longer supplied as the end of the hatchery part of the cycle. At this point the clams feed exclusively on materials in the natural water being pumped from a nearby estuary. Clams usually remain in upwellers until they reach from 2-5 mm SL and then they are distributed into raceways, mesh bags in the field, or kept in upwellers where they grow to planting size (8-15 mm SL). Ongrowing techniques Clams are planted in plots in the intertidal or shallow subtidal zones. In most areas these are covered with a plastic mesh (6.4 or 12.7 mm square mesh) to prevent predation losses. In some locations clams are planted in mesh bags which are staked to the bottom.