Biology and Culture of the Hard Clam (Mercenaria Mercenaria)

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. Hard clams are ment. In most southern states, and by providing adequate food. known to live for 30 years or farm-raised clams may be harvest- Broodstock are manipulated by longer. ed year round, which offers a mar- alternately exposing them to keting advantage over wild-har- Predation chilled (18 to 24 ºC) and warmed vested clams in some states. (28 to 30 ºC) seawater containing When clams are disturbed, they Check with your state natural a suspension of hard clam sperm. burrow deeper to avoid predators. resources agency about regulations After several cycles, the clams Common predators are blue for harvesting cultured clams. will spawn, with the male usually crabs, stone crabs, mud crabs, spawning first. The eggs are conch, sting rays, horseshoe crabs Hard clam aquaculture sieved, collected and placed in and snails, which feed mostly on Hard clams are the most com- growing tanks where they develop juvenile or small clams. monly cultured of the bivalve into larvae. Females average 1 Producers use rafts, trays, cages, species. Clam culture in the U.S. million eggs per spawn. bags and nets to exclude predators. began in the early 1920s. Brood- The next phase of the hatchery Biological control of crabs has stock management is a vital step process is larval culture, which been attempted by stocking toad- in fulfilling the potential of hard lasts through day 7. Larvae are fish in culture trays. Toadfish do clam aquaculture. Efforts to pro- raised in various sizes and types of

Figure 2. Algal production in hard clam hatchery. Figure 3. Broodstock tempering. containers. The larval tanks are Permitting Nursery supplied with filtered seawater (20 The nursery and growout phases of to 30 ppt) at a temperature of 20 to The nursery is a critical link in clam culture require permits from 30 ºC. The concentration of larvae the hard clam grow-out process. state regulatory agencies if clams in the tanks varies, but 20 to 30 Placing seed clams from the are grown and harvested from larvae per ml is recommended. hatchery directly into the field for state-owned waters or bottoms. In During this stage, they are fed a grow out can cause a high level of addition, any hard structures will diet of cultured algae. mortality. The nursery provides a require permits from the state reg- controlled, intermediate step, A major requirement of hatchery ulatory agencies. Hatcheries may whereby the hatchery-reared seed production is producing algae to need permits for water intakes, clams are nurtured to a size less feed young clams. For the first 7 docks and outfalls if structures are vulnerable to the stress and pre- to 10 days clams are fed small fla- in public waters. Recent EPA regu- dation found in the field grow-out gellates (Isochrysis), followed by lations do not require National phase. It would not be cost effec- diatoms such as Chaetoceros and Pollutant Discharge Elimination tive to grow seed to the size Skeletonema. The initial feeding System Permits for shellfish aqua- required for the grow-out stage rate is 25,000 algal cells per ml or culture. A prospective grower within an intensive hatchery envi- 1,000 cells per clam larva. The should begin site selection and per- ronment using cultured algae. So, larval stocking density and algal mitting as early as possible so that natural seawater is generally used feeding rate fluctuate through the good sites are selected and regula- in these systems. Natural feed is larval grow-out cycle. Generally, tory holdups are avoided. provided by seawater as it moves the clam larval density is reduced through the nursery system. and the feeding rate per clam larva is increased as the clams grow larger. Between day 8 and day 12, the larval clams develop into the pediveliger stage. They are kept in post-set tanks and fed cultured algae. Filtered seawater at 26 ºC circulates through the system to ensure optimum survival and growth. Post-set clam production is the next phase of production; it generally lasts 13 to 35 days. As in earlier stages, water temperature is maintained at 26 ºC, water is filtered, and the post-set clams are fed cultured algae. Newly set clams are placed in shallow raceways or in cylinders with up- or down-welling water flows. The clam seeds are kept in the hatchery until they reach about 1 mm. At this point, the seed is graded and separated by size and maintained in the nursery until ready for planting. One of the long-standing prob- lems in bivalve aquaculture is the difficulty of culturing massive quantities of suitable algal species economically. Algae are needed to grow seed to the proper size for field planting. The cost of producing this algal biomass is relatively high compared to the cost of seed clam production or the projected annual gross revenue of this aquaculture venture. Figure 4. Land-based raceway nursery system. Replacement and maintenance costs are higher for the floating upweller systems because of their location in the water and the greater risk of damage by predators, fouling and wave action. Survival rates may be higher for the land-based systems because there is greater control over water quality and a lack of predators, but the production cost per clam seed may be less with the in-water systems, particularly if waterfront property is not available. Grow-out The time needed for grow-out will largely depend on water quality, food availability and temperature. Grow-out to 50-mm Figure 5. Floating upweller for clam seed production. (2-inch) market clams from 7- to 15-mm nursery clams may require 12 to 24 months. The oldest method of nursery cul- tom setting. Seed clams smaller There are various grow-out culture is called the land-based race- than 3 to 4 mm should be used. ture systems for raising hard way method. The raceway system Traditional designs employ subti- clams for the market. Grow-out typically uses long, shallow, wood- dal and intertidal trays made of systems are stocked at a density en trays lined with plastic or cov- wood that contain a layer of grav- ered with epoxy resin or other pro- el or sand and have a protective tective coating. A thin layer of sand cover to discourage predators. covers the bottom of each tray, and More recent innovations include the seed clams are distributed over bottom bags and systems of bags it. Raw seawater is pumped into held together in long belts. The one end of the tray at a prescribed series of bags significantly rate to establish a horizontal flow reduces maintenance and labor across the seed clams. expenses. All of these field-based Another method of nursery cul- nursery techniques are carried ture is the land-based upweller out in protected, shallow water method. The upweller system areas so the threat of poaching pumps seawater to reservoir tanks can be minimized. and upflow cylinders, which pro- These nursery systems vary con- vide vertical flow for the seed siderably in terms of investment Figure 6. Clam grow-out covers in clams. The flow of water can be cost, operational expenses and Virginia. (photo courtesy of Virginia forced up through the seed clams required management skill. The Sea Grant Consortium) or pulled down through the seed land-based system requires water- clams. The seed clams rest on a front land and an fine-mesh screen. The movement investment in pumps, of water removes waste and pre- whereas the field-based vents the seed from being suffo- system is located on cated by any accumulated silt. leased bottom without Floating upwellers are a recent the need for controlled innovation. These are similar in water movement. The design to land-based upwellers, floating upweller sys- but can be powered (pumped) by tems are intermediate airflow, submerged pumps or to land-based and true tidal flow. field bottom plants. The fourth method of nursery cul- Energy requirements ture is the field-based bottom plant are much higher for the system, which involves placing land-based systems. seed clams directly from the Maintenance costs are hatchery into the submerged bot- lower for the upflow systems than for raceways. Figure 7. Market-size clams in a grow-out bag. of 50 to 70 seed per square foot Clam farmers in certain counties for final grow out. Although land- in four states—Florida, South based grow-out methods such as Carolina, Virginia and raceways and tanks have been Massachusetts—may participate developed, the field-based grow- in the first USDA Pilot Crop out methods are more economical Insurance Program in aquacul- and better suited to hard clam ture. Clams 10 mm and larger are production. Grow-out operations insurable against natural losses in use some form of hard structures excess of state average mortali- (pens, trays) or soft structures ties, and farmers may purchase (bags, net covers) to reduce preda- crop insurance with differing tion and ease harvesting. Pens are deductibles to cover insurable harvested by hand rakes or with losses and reduce their financial mechanical harvesters, where risk. legal. A lifting apparatus is Figure 8. Nursery (right) and grow- out (left) bags. Clam culture is a risky invest- required to lift trays from the ment, as is any aquaculture or water. Covers (bottom plants) are farming operation. Call on the placed over the seed clam planted Extension aquaculture specialist area and staked down to discour- in your state for additional infor- age predators. To harvest clams, mation. the net is rolled from the planted areas and the clams exposed. Soft For more detailed information, bags resemble 4-foot by 4-foot obtain a copy of: laundry bags; usually a number of AManual for the Culture of the bags are connected together to Hard Clam Mercenaria spp. in form a line that is staked to the South Carolina. Nancy H. bottom. Harvesting soft bags is a Hadley, John J. Manzi, Arnold matter of pulling up the bags G. Eversole, Robert T. Dillon, because the clams are confined in Figure 9. Harvesting clam bags. Colden E. Battey and Nancy the bags. To increase the survival (photo courtesy of Leslie Sturmer, M. Peacock. South Carolina rate, seed clams longer than 6 mm University of Florida) Sea Grant Consortium. should be used, even though larger seed are more costly and are in limited supply.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have not been revised are considered to reflect the current state of knowledge.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 2003-38500-12997 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.