Biology and Culture of the Hard Clam (Mercenaria Mercenaria)
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SRAC Publication No. 433 VI August 2005 PR Revision Biology and Culture of the Hard Clam (Mercenaria mercenaria) Jack M. Whetstone1, Leslie N. Sturmer2 and Michael J. Oesterling3 Hard clam aquaculture is the largest Life history expand and may contain a few and most valuable of the shellfish mature gametes. Mature gametes aquaculture industries on the East Reproduction cycle are released and fertilization takes Coast. It accounts for more than $50 Hard clams usually spawn in the place externally. The ripe stage is million in economic value annually. spring, summer or fall. The opti- followed by the resting and/or Hard clams are bivalve mollusks that mal range of water temperatures spent stage to complete the repro- live in saline (>25 parts per thou- (79 ºF or 26 ºC) occurs at differ- ductive cycle. sand) waters and cannot tolerate low ent times of the year at different The development of the veliger lar- salinities or freshwater for an extend- latitudes. Clam reproduction vae is complete 24 hours after fer- ed period. Hard clams occur natural- occurs earlier in the year at lower tilization. These larvae swim, but ly all along the Atlantic coast from latitudes. Dimodal (two peaks) or are moved primarily by tidal cur- Nova Scotia to Florida. They have polymodal (multiple peaks) rents. The larvae grow to a maxi- been introduced along the shore of spawning takes place in southern mum size of 200 to 275 microme- the Gulf of Mexico from Florida to populations, and spawning may ters. By the sixth to tenth day, the Yucatan, as well as along the West occur more than once per spawn- skin-like outside tissue, called the Coast of the United States, in the ing season. mantle, starts to form two shells British Isles, and in parts of France. There are several stages in the and the umbo. The umbo is the The distribution of hard clams is reproductive cycle, which for dis- rounded area of the shell just above determined by hydrodynamic factors cussion purposes may begin with the hinge. With the extra weight of and, possibly, sediment types and the resting and/or spent stage. In the shell, larvae no longer swim depth. Sediment characteristics affect this stage the clam completely or freely and settle to the bottom. the number and types of both inver- almost completely lacks gametes Only 10 percent of fertilized eggs tebrates and their fish predators. (eggs and sperm). The resting survive to this stage. During meta- Hard clams support a major com- stage is followed by the early morphosis, the clam “seed” bur- mercial fishery along the entire East development stage, during which rows into a suitable substrate Coast. New York, Rhode Island, the follicle walls thicken and where it remains mostly immobile. New Jersey and Virginia are the immature gametes develop. In the Clams prefer a combination of mud leading states for hard clam commer- late development stage, the folli- and sand as substrate but other cial landings. The southeastern states cles rapidly expand to accommo- suitable substrates are pure sand, are the leading aquaculture produc- date the larger and more numer- gravel and mud. ers of hard clams. ous gametes. In the ripe stage, the Larval setting follicles are fully expanded and 1 Clemson University thin-walled. The lumen of female Many bivalve species attach to 2 University of Florida 3 Virginia Sea Grant Advisory Services follicles contains mature ova sand grains or other debris by one (eggs), while mature sperm domi- or several strong byssus threads. This fact sheet was revised from the original nate the lumen of male follicles. Byssus threads are thin strands 1995 version written by Wendell J. Lorio and Sandra Malone. The germinal ducts have begun to secreted by a gland in the middle Figure 1. Internal anatomy of the hard clam. (drawing courtesy of John Norton, South Carolina Sea Grant Consortium) line of the surface of the foot. shell, and around it are the con- The rate of shell deposition is a Hard clams lose their ability to centric lines of shell growth. major factor affecting growth. make byssus as they grow older. Each umbo may point slightly to Clam shells are formed by the Clams may release from this the anterior so that it is usually deposition of calcium carbonate attachment and may crawl or be possible to determine right and crystals on an organic material. moved by currents to another left valves. Shells grow in rings and/or layers location where they attach again. deposited on this organic matrix. As clams mature, they use the Respiration and growth Growth is not continuous, but muscular foot to burrow into the Most clams remain close enough incremental, with periods of shell bottom sediments. By alternately to the surface that the tips of dissolution. Shell growth occurs extending, swelling and contract- their siphons are exposed. only during aerobic respiration ing the foot, the body is pulled Siphons, sometimes called the when the valve is open. The man- down into the substrate. The neck, are specialized tubes of the tle, which covers the inner surface whole body usually is below the mantle. One siphon brings in of the shell, is responsible for shell substrate, except for the incoming water that carries oxygen for res- formation because it secretes and outgoing siphons. piration and food for growth. organic material that reacts with The body of the clam is com- Incoming water passes over the calcium carbonate to form shell. pletely enclosed in a mantle, gills where the oxygen is New shell is deposited between which is subdivided into lateral absorbed and where algae and the inner shell surface and the lobes that secrete a calcium car- other food particles are filtered mantle epithelium. Shells grow lat- bonate shell. The mantle covers out. Food particles are drawn erally (in thickness) when growth the foot and visceral mass and is over the gills by the action of is slow. Faster growing individuals connected to the shell by pallial cilia that cover the gills and man- usually have thinner shells. muscles close to the edge of the tle. Food is trapped by mucus on All hard clams, whether natural shell. This line of mantle attach- the gills and carried to the food or cultured, grow at different ment to the shell is seen on the groove along the ventral edge of rates. Hard clams with maximum inner surface of the shell as the the gill, and then to the palos growth rates attain market size pallial scar line. around the mouth. Filtered water in 12 to 24 months. In 10 to The two valves of the shell are and waste are then expelled by 16 months, fast growers may be joined dorsally by an elastic the other siphon. twice the size of slow growers. hinge ligament, which acts as a The growth of hard clams is affect- The relatively long period of spring that forces the valves ed by tidal movement and by algal time needed to produce market apart when the adductor muscles concentrations along the substrate- size clams is an important limit- relax. water interface. Ideal conditions ing factor in their aquaculture. Growth rate may be influenced The dorsal margin of each valve are moderate tidal movement with dense algal concentrations and ade- by genotype or genetics, because bears a prominent point near the a percentage of any clam popula- hinge ligament called the umbo. quate dissolved oxygen levels— above 4 ppm. tion originating from the same This is the oldest part of the spawning matures at a faster rate. Therefore, scientists have reduce the number of crabs and duce genetically improved bivalve been developing selective breed- increase clam survival significant- broodstock are probably as old as ing programs to establish faster ly, but the economic feasibility of bivalve mariculture itself. growing genetic lines, which stocking them is questionable. The hard clam production process would have economic benefit. It Marketing consists of three consecutive has become apparent that remov- stages: hatchery, nursery and ing slow-growing clams from a Hard clams are marketed whole. grow out. Each stage is designed production system reduces pro- About 30 percent are shucked to produce a specific size clam. duction costs. When clams grow and the remainder are marketed The ultimate objective is to pro- at varying rates they must be han- for raw or steamed consumption. duce market size clams. Farmers dled more, which increases stress Unshucked clams command the may develop any or all the produc- to the animals and decreases the highest prices on a per clam tion stages into viable businesses. predictability of cash flow. basis. Higher prices are paid for Environmental factors also influ- clams designated in the market as Hatchery ence growth rates. These include littleneck and topneck clams, 1 The hatchery is where broodstock water temperature, food availabil- which range from 1 to 1 ⁄4 inches clams spawn and larval clams are ity, salinity, water quality and thick. Size class designations may raised through the post-set stage to tidal currents. vary from state to state. 1 mm juveniles or seed clams. Generally, hard clams are 1 to Hard clam prices fluctuate with Broodstock are held in condition- 2 inches long at harvest (at about the supply, which is dependent on ing tanks at 19 ºC and fed a diet of 2 to 3 years of age), with a meat the stocks of clams, access to pro- cultured algae. Hard clams can be weight of 18 to 20 grams. Growth duction areas, weather conditions, stimulated into spawning in colder slows with increasing age. In 7 to harvest seasons, and the bacterial months by conditioning them grad- 8 years hard clams may be only content of the growing environ- ually to increasing temperatures 3 inches long.