Hard Clam Hatchery and Nursery Production

SRAC Publication No. 4301 VI September 2007 PR Hard Clam Hatchery and Nursery Production Nancy H. Hadley1 and Jack M. Whetstone2 Biology of the hard clam remove food particles, primarily small phytoplankton, bacteria and DOM. phytoplankton (single-celled algae), The ciliated velum, which gives the Hard clams of the genus Mercenaria from the water. An adult clam filters veliger its name, is used for both loco- are found from the Gulf of St. an average of 7 to 8 liters (about 2 gal- motion and feeding. The length of the Lawrence to the Gulf of Mexico and lons) per hour. larval period depends largely on tem- have been introduced to other areas of perature and food supply. After 7 to the United States, notably the coasts Hard clams usually reach sexual maturity at a shell length (SL) of 21 days the veliger larva develops a of California and Washington. They foot and is called a pediveliger (Fig. 1). also have been introduced to Puerto about 35 mm (about 1.4 inch) (Fig. 1). The sexes are separate but are exter- This stage is brief. The clam soon Rico and Great Britain. There are two loses its swimming organ (velum) and species, Mercenaria mercenaria and M. nally indistinguishable. Clams are protandric, maturing as males at an develops siphons. This is referred to as campechiensis. M. mercenaria is distrib- settlement, and newly settled clams uted primarily in the more northerly early age and changing sex in subse- quent years to spawn as females. may be called “set” or “post-set.” Post- latitudes, while M. campechiensis is the set clams assume the sedentary more southerly species. Both species When stimulated by appropriate envi- ronmental conditions (normally high lifestyle of the adult. The term post-set are found on the west coast of Florida is informal and usually refers to clams and in Georgia, South Carolina and water temperatures), clams release gametes (sperm or eggs) into the that have completed the larval stage North Carolina, and some hybridiza- but are still housed in a hatchery. tion between the two may occur. A water. The presence of gametes in the subspecies, M. c. texana, is found in water stimulates other clams in the Juvenile clams less than 35 mm (about the western Gulf of Mexico. This fact immediate vicinity to begin spawning. 1.4 inches) SL are called seed. As seed sheet refers specifically to M. merce- Fertilization occurs in the water col- grow to market size they are classified naria because it is the major cultured umn. Clams usually spawn intermit- into commercial categories, which are species, although some of the general- tently from May through October. A arbitrary classifications that may vary izations may apply to all hard clams. female clam can release several mil- from state to state and dealer to deal- lion eggs in a single spawning, but rel- er. In most states the smallest legal The hard clam is rarely found where atively few survive the larval period size for wild-caught clams is a little- salinities average less than 20 parts to become juvenile clams in the wild. neck, usually defined as 1 inch (25 per thousand (ppt). Hard clams occu- mm) thick. This generally corresponds py intertidal and subtidal habitats, Fertilized eggs (zygotes) undergo rapid cell division and within 12 hours to a shell length of 45 to 50 mm (1 to where they burrow into the substrate 2 inches). Slightly larger clams (1.25 to various depths (normally less than develop into free-swimming tro- chophore larvae. Within another 12 inches or 30 mm thick, 50 to 60 mm 20 cm or 8 inches). They are found in or 2 to 2.5 inches SL) are referred to a variety of substrates, including sand, hours, bivalve shells have formed and the larvae are in the veliger, or as topnecks. Cherrystones are approxi- mud, shell and mixtures of these mately 1.25 to 1.5 inches (32 to 38 materials. Clams are filter feeders and straight-hinge, stage. Veligers are often referred to as “D” larvae because their mm) thick and 65 to 79 mm (2.5 to 3 shape resembles a capital letter “D.” inches) long. Anything larger than that is a chowder clam. Littlenecks com- 1 Marine Resources Research Institute, South Before the veliger stage the larva is Carolina Department of Natural Resources, sustained by lipids stored in the egg mand the highest price of the com- Charleston, South Carolina mercial categories. In some states mar- 2 and by dissolved organic matter Baruch Institute of Coastal Ecology and Forest icultured clams may be sold at smaller Science, South Carolina Sea Grant Extension (DOM) absorbed from the surround- Program, Clemson University, Georgetown, ing water. The veliger feeds on small sizes and some companies have creat- South Carolina ed their own names for these size Straight hinge Veliger Maintenance and conditioning where fertilization takes place. Age range: 1-5 days of broodstock Spawning is induced by alternately Length range: 90-140 μm heating and cooling the clams in a Adult clams are brought into the water bath. Sperm or eggs from a sac- Length hatchery for “conditioning” several rificed clam also can be used to stimu- 110 µm weeks before spawning. Conditioning late spawning. If controlled breeding Straight is the process of inducing gametogene- is desired, the clams are spawned in hinge sis, or the ripening of gonads, to make individual beakers so fertilization can Height clams ready for spawning. In the be controlled. If the objective is sim- 87 µm southern U.S. clams become naturally ply to produce larvae and the parent- ripe in the spring and remain intermit- Velum age is unimportant, it is easier to tently ripe into the fall. To get them to “mass spawn” the clams in a common ripen at any other time of year, one Umboned veliger container. Sperm from spawning must simulate early spring conditions Age range: 3-15 days males will stimulate other clams to of cool temperature (18 to 20 °C) and Length range: 140-220 μm spawn also and the eggs will be fertil- ample food. Clams will usually condi- ized in the common spawning con- tion in 2 to 8 weeks, but this varies Length 175 µm tainer. Early with the time of year and the physio- umbo logical condition of the clams. A clam Larviculture that is already ripe can be maintained Height in this condition for a long period (up Zygotes (fertilized eggs) are main- 160 µm to 6 months) by keeping it in cool tained in clean, filtered seawater at water (18 to 20 °C) and providing relatively high densities (30 or more abundant food. Partially ripe clams per milliliter of culture water) for 24 will be ready to spawn in 2 to 4 hours or until the veliger stage is weeks under these same conditions. It reached. At that point the larvae are is much more difficult to condition thinned to 5 to 10 per ml and given Pediveliger clams that have recently spawned. So food (algae). The veliger stage lasts 7 Age range: 8-20 days for year-round operation it may be to 21 days, depending largely on tem- Length range: 170-230 μm advisable to collect naturally ripe perature and food quality (Table 2). During this time the larvae grow from Length 220 µm clams in early spring and keep them in conditioning tanks to maintain a 100 µm to 180 to 250 µm and are Umbo population of readily spawnable indi- gradually thinned to a final density of viduals. Naturally ripe clams also can 1 per ml. Larvae are kept in static cul- be collected in the fall, although fewer tures, the water is changed frequently Height wild clams are ripe at this time than (daily if possible), and food is added 188 µm in the spring. The general require- daily. Some hatcheries aerate larval ments for maintaining and condition- cultures, but this is not usually neces- ing broodstock are listed in Table 1. sary. Water for larval culture must be filtered (1 to 10 µm) to remove silt Spawning and native plankton. Depending on Foot the quality of the seawater, it may When clams spawn they release eggs also need to be treated with charcoal Figure 1. Veliger and pediveliger larvae. or sperm into the water column, Table 1. General culture requirements for maintaining and condition- classes. In the southern U.S. wild ing broodstock clams in closed system raceways. clams grow to littleneck size in 2 to 4 Variable Requirement years, while farm-raised clams may reach this size in about 1 to 2 years. Salinity 25-35 ppt Temperature 18-20 °C for gradual conditioning/maintenance Hatchery culture 22-23 °C for rapid conditioning/pre-spawn priming There are five main processes in the Water pretreatment 25 µm filtration hatchery culture of the hard clam: 1) UV and/or charcoal maintenance and conditioning of broodstock; 2) spawning; 3) larval cul- Water change at least 3 times per week ture; 4) post-set culture; and 5) food Stocking density 1 clam/4 liters water production (algal culture). These Feed mixed phytoplankton activities are supported by a water Feed rate 1-3 x 10 9/clam/day distribution and treatment system, air distribution system, freshwater wash- Feed method batch, continuous drip, discontinuous metering down facilities, supplemental lighting Duration 2-8 weeks for conditioning for algal culture, and test equipment up to 6 months to maintain conditioned clams and instrumentation. to remove dissolved organics and ster- veliger stage is fairly short; within a Downwellers (also known as “silos”) ilized (usually with ultraviolet light) to few days the velum disappears entire- are open-ended cylinders, usually con- kill bacteria.

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