Biology of the hard clamHard clams of the genus Mercenariaare found from the Gulf of St.Lawrence to the Gulf of Mexico andhave been introduced to other areas ofthe United States, notably the coastsof California and Washington. Theyalso have been introduced to PuertoRico and Great Britain. There are twospecies, Mercenaria mercenaria and M.campechiensis. M. mercenaria is distrib-uted primarily in the more northerlylatitudes, while M. campechiensis is themore southerly species. Both speciesare found on the west coast of Floridaand in Georgia, South Carolina andNorth Carolina, and some hybridiza-tion between the two may occur. Asubspecies, M. c. texana, is found inthe western Gulf of Mexico. This factsheet refers specifically to M. merce-naria because it is the major culturedspecies, although some of the general-izations may apply to all hard clams.The hard clam is rarely found wheresalinities average less than 20 partsper thousand (ppt). Hard clams occu-py intertidal and subtidal habitats,where they burrow into the substrateto various depths (normally less than20 cm or 8 inches). They are found ina variety of substrates, including sand,mud, shell and mixtures of thesematerials. Clams are filter feeders and

remove food particles, primarily smallphytoplankton (single-celled algae),from the water. An adult clam filtersan average of 7 to 8 liters (about 2 gal-lons) per hour.Hard clams usually reach sexualmaturity at a shell length (SL) ofabout 35 mm (about 1.4 inch) (Fig. 1).The sexes are separate but are exter-nally indistinguishable. Clams areprotandric, maturing as males at anearly age and changing sex in subse-quent years to spawn as females.When stimulated by appropriate envi-ronmental conditions (normally highwater temperatures), clams releasegametes (sperm or eggs) into thewater. The presence of gametes in thewater stimulates other clams in theimmediate vicinity to begin spawning.Fertilization occurs in the water col-umn. Clams usually spawn intermit-tently from May through October. Afemale clam can release several mil-lion eggs in a single spawning, but rel-atively few survive the larval periodto become juvenile clams in the wild.Fertilized eggs (zygotes) undergo rapidcell division and within 12 hoursdevelop into free-swimming tro-chophore larvae. Within another 12hours, bivalve shells have formed andthe larvae are in the veliger, orstraight-hinge, stage. Veligers are oftenreferred to as “D” larvae because theirshape resembles a capital letter “D.”Before the veliger stage the larva issustained by lipids stored in the eggand by dissolved organic matter(DOM) absorbed from the surround-ing water. The veliger feeds on small

phytoplankton, bacteria and DOM.The ciliated velum, which gives theveliger its name, is used for both loco-motion and feeding. The length of thelarval period depends largely on tem-perature and food supply. After 7 to21 days the veliger larva develops afoot and is called a pediveliger (Fig. 1).This stage is brief. The clam soonloses its swimming organ (velum) anddevelops siphons. This is referred to assettlement, and newly settled clamsmay be called “set” or “post-set.” Post-set clams assume the sedentarylifestyle of the adult. The term post-setis informal and usually refers to clamsthat have completed the larval stagebut are still housed in a hatchery.Juvenile clams less than 35 mm (about1.4 inches) SL are called seed. As seedgrow to market size they are classifiedinto commercial categories, which arearbitrary classifications that may varyfrom state to state and dealer to deal-er. In most states the smallest legalsize for wild-caught clams is a little-neck, usually defined as 1 inch (25mm) thick. This generally correspondsto a shell length of 45 to 50 mm (1 to2 inches). Slightly larger clams (1.25inches or 30 mm thick, 50 to 60 mmor 2 to 2.5 inches SL) are referred toas topnecks. Cherrystones are approxi-mately 1.25 to 1.5 inches (32 to 38mm) thick and 65 to 79 mm (2.5 to 3inches) long. Anything larger than thatis a chowder clam. Littlenecks com-mand the highest price of the com-mercial categories. In some states mar-icultured clams may be sold at smallersizes and some companies have creat-ed their own names for these size

VIPRSeptember 2007

SRAC Publication No. 4301

1 Marine Resources Research Institute, South Carolina Department of Natural Resources, Charleston, South Carolina

2 Baruch Institute of Coastal Ecology and Forest Science, South Carolina Sea Grant Extension Program, Clemson University, Georgetown, South Carolina

Hard Clam Hatcheryand Nursery Production

Nancy H. Hadley1 and Jack M. Whetstone2

classes. In the southern U.S. wildclams grow to littleneck size in 2 to 4years, while farm-raised clams mayreach this size in about 1 to 2 years.

Hatchery cultureThere are five main processes in thehatchery culture of the hard clam: 1)maintenance and conditioning ofbroodstock; 2) spawning; 3) larval cul-ture; 4) post-set culture; and 5) foodproduction (algal culture). Theseactivities are supported by a waterdistribution and treatment system, airdistribution system, freshwater wash-down facilities, supplemental lightingfor algal culture, and test equipmentand instrumentation.

Maintenance and conditioning of broodstock

Adult clams are brought into thehatchery for “conditioning” severalweeks before spawning. Conditioningis the process of inducing gametogene-sis, or the ripening of gonads, to makeclams ready for spawning. In thesouthern U.S. clams become naturallyripe in the spring and remain intermit-tently ripe into the fall. To get them toripen at any other time of year, onemust simulate early spring conditionsof cool temperature (18 to 20 °C) andample food. Clams will usually condi-tion in 2 to 8 weeks, but this varieswith the time of year and the physio-logical condition of the clams. A clamthat is already ripe can be maintainedin this condition for a long period (upto 6 months) by keeping it in coolwater (18 to 20 °C) and providingabundant food. Partially ripe clamswill be ready to spawn in 2 to 4weeks under these same conditions. Itis much more difficult to conditionclams that have recently spawned. Sofor year-round operation it may beadvisable to collect naturally ripeclams in early spring and keep themin conditioning tanks to maintain apopulation of readily spawnable indi-viduals. Naturally ripe clams also canbe collected in the fall, although fewerwild clams are ripe at this time thanin the spring. The general require-ments for maintaining and condition-ing broodstock are listed in Table 1.

Spawning

When clams spawn they release eggsor sperm into the water column,

where fertilization takes place.Spawning is induced by alternatelyheating and cooling the clams in awater bath. Sperm or eggs from a sac-rificed clam also can be used to stimu-late spawning. If controlled breedingis desired, the clams are spawned inindividual beakers so fertilization canbe controlled. If the objective is sim-ply to produce larvae and the parent-age is unimportant, it is easier to“mass spawn” the clams in a commoncontainer. Sperm from spawningmales will stimulate other clams tospawn also and the eggs will be fertil-ized in the common spawning con-tainer.

Larviculture

Zygotes (fertilized eggs) are main-tained in clean, filtered seawater atrelatively high densities (30 or moreper milliliter of culture water) for 24hours or until the veliger stage isreached. At that point the larvae arethinned to 5 to 10 per ml and givenfood (algae). The veliger stage lasts 7to 21 days, depending largely on tem-perature and food quality (Table 2).During this time the larvae grow from100 µm to 180 to 250 µm and aregradually thinned to a final density of1 per ml. Larvae are kept in static cul-tures, the water is changed frequently(daily if possible), and food is addeddaily. Some hatcheries aerate larvalcultures, but this is not usually neces-sary. Water for larval culture must befiltered (1 to 10 µm) to remove siltand native plankton. Depending onthe quality of the seawater, it mayalso need to be treated with charcoal

Table 1. General culture requirements for maintaining and condition-ing broodstock clams in closed system raceways.

Variable RequirementSalinity 25-35 pptTemperature 18-20 °C for gradual conditioning/maintenance

22-23 °C for rapid conditioning/pre-spawn primingWater pretreatment 25 µm filtration

UV and/or charcoalWater change at least 3 times per weekStocking density 1 clam/4 liters waterFeed mixed phytoplanktonFeed rate 1-3 x 10 9/clam/dayFeed method batch, continuous drip, discontinuous meteringDuration 2-8 weeks for conditioning

up to 6 months to maintain conditioned clams

PediveligerAge range: 8-20 days

Length range: 170-230 μm

Height188 µm

Length 220 µm

Umbo

Umboned veligerAge range: 3-15 days

Length range: 140-220 μm

Straight hinge VeligerAge range: 1-5 days

Length range: 90-140 μm

Length 175 µmEarly umbo

Height 160 µm

Straighthinge

Length110 µm

Height87 µm

Velum

Foot

Figure 1. Veliger and pediveliger larvae.

veliger stage is fairly short; within afew days the velum disappears entire-ly and a siphon is formed. This meta-morphosis is known as “setting” andthe young clams are referred to as“set” or “post-set.”

Post-set culture

Pediveligers are often removed fromthe larval culture system and kept in aseparate post-set culture system untilmetamorphosis is completed. Thisimproves survival because pedi-veligers, although still capable ofswimming, spend most of the timecrawling on the bottom of the tankswhere waste products and dead larvaeare concentrated. Post-set clams maybe cultured in a variety of systems.Many hatcheries use shallow trays in agentle flow of seawater, augmentedwith cultured algae. These trays can bestacked in tiers to save floor space. Analternative is a recirculating culturesystem with downwellers and/orupwellers. This system is easier toclean than a tray system, makes it easi-er to control water quality and feedingrations, and supports a large numberof clams in a small space (Table 3).

to remove dissolved organics and ster-ilized (usually with ultraviolet light) tokill bacteria.Larval culture containers may bemade of fiberglass or plastic and areusually no more than 1 meter deep (3feet). Container size is dictated by theamount of larval production desired.Plastic buckets (20-liter, 5-gallon) canbe used to grow small quantities oflarvae (20,000). Large hatcheries havefiberglass tanks that may hold severalthousand liters of water and millionsof larvae. Containers can be flat-bot-tomed or conical-shaped. Conical bot-toms facilitate draining and cleaning.Tanks may have valved drains at thebottom or may be drained with siphonhoses. When the water is drainedfrom the containers, the larvae arecaptured on a fine-mesh nylon orpolyester screen. Toward the end of the larval period,usually at a size of 200 to 250 µm, thelarvae begin the metamorphosis to thejuvenile form. The first indication ofthis metamorphosis is the appearanceof a foot. This stage is called “pedi-veliger” because the larva has both avelum and a foot (Fig. 1). The pedi-

Downwellers (also known as “silos”)are open-ended cylinders, usually con-structed of plastic pipe, suspended in areservoir. The bottom of the cylinder iscovered with a fine mesh that supportsthe clams (Fig. 2). Before they can bemoved to silos, the clams should belarge enough to stay on a 150-µmmesh. Smaller mesh will clog too readi-ly, obstructing the flow of water. Water(and food) is circulated through thesilos with airlifts (Fig. 3). In a down-weller, the airlift is positioned outsidethe cylinder and moves water from thereservoir into the silo. Downwellers areused for early post-set that might besucked up the airlift. When the post-setreach a size of about 0.5 mm SL, thesilos can be converted to upwellers bymoving the airlift to the inside of thecylinder, drawing water up through theclams. The silos are suspended in areservoir (a container accommodatingone or more silos). The reservoir vol-ume should provide at least 0.5 mlwater for each clam. Food is added tothe reservoir either in batches or bycontinuous delivery. All the water inthe reservoir is changed regularly—daily if possible, but at least threetimes a week.Post-set clams are usually maintained ina hatchery until they are at least 1 mmor can be retained on a 710-µm mesh.At that point, the clams may be trans-ferred to a nursery culture system.

Food production

Broodstock, larvae and post-set are fedphytoplankton produced in the hatch-ery. There are three basic methods ofalgal production: the “Wells-Glancy”method, the “brown water method,”and the “Milford” method. The Wells-Glancy method and the brown watermethod are relatively low-tech andinexpensive, but are not as reliable asthe Milford method for producing con-sistently high-quality food. They aremost often used in seasonally operated,low-budget hatcheries or as a supple-ment to the Milford method. The Wells-Glancy and brown watermethods both rely on the native phyto-plankton available in the seawater sup-ply. The water is filtered (5 to 15 µm) toremove zooplankton and used immedi-ately as culture water for larvae (thebrown water method) or allowed tobloom for 24 to 48 hours before beingused. Fertilizer can be added to encour-

Table 2. General culture requirements for larval hard clams in closedsystem tanks.Variable RequirementSalinity 25-35 pptTemperature 24-28 °C optimal, 20-30 °C acceptableWater treatment 1 µm filtration, UV and/or charcoal recommendedAeration optional gentle aeration, especially if tanks are deepWater change daily if possible, every other day at leastStocking density Stage Density (number/ml)

fertilized eggs 20-30early veligers 5-10late veligers 1-2

Food type small flagellates (e.g., Isochrysis); for older veligers add diatoms (e.g., Chaetoceros)

Feed rate 10,000-25,000 cells/ml of culture water as back-ground + amount belowStage Feed (cells/clam/day)

early veliger 1,000-5,000 mid-veliger 5,000-10,000 late veliger 15,000-30,000 pediveliger 30,000-50,000

Feed method batch, continuous drip, discontinuous meteringDuration 7-21 days

Table 3. General culture requirements for post-set hard clams raised in downwellers housed in reservoirtanks. The system is converted to upwellers as clams grow to 0.5 mm SL.

Variable RequirementWater quality 200-500 µm—1 µm filtered with UV or charcoal if needed

500 µm-1mm—supplement above with slow flow of ambient water Salinity >25 pptTemperature 24-28 °C optimal, 20 to 30 °C acceptableWater exchange 200-500 µm—every other day

>500 µm—add flow-through gradually to acclimateStocking density Size clams/cm2 ml/cm2

up to 600 µm 575 0.03700 µm 285 0.031 mm 140 0.04

Food type mixed diatoms (e.g., Chaetoceros) and flagellates (e.g., Isochrysis) at about a 50:50 ratioFeed rate 25,000-50,000 cells/ml of culture water as background + amount below

Stage Size range Feed (cells/clam/day)early post-set 300-400 µm 50,000-75,000 mid post-set 400-600 µm 75,000-150,000 late post-set 600 µm-1mm 150,000-500,000

Feeding methods batch, continuous drip, discontinuous meteringDuration 4-6 weeks

Overhead view

Distribution line Tap Inflow

Reservoirtank

Silo

Gutter

Supportrod

Drain pipe

Cleanout

Trough

Cross-section

Water level Elbow Silo

Tank wall

Couplings

Gutter

Mesh

Figure 3. Flow-through upwelling nursery system.

PVC cylinder(4 in-12 in diameter)

PVC coupling throughwall of cylinder

(1 in-2 in diameter)

Polyester mesh

Retaining ring

Water level

A. Cylinder construction

C. Downweller

Air line

Water level

Cylinderwall

Retaining ring

ClamsMesh

B. Upweller

Figure 2. Downwelling and upwellingpost/set culture cylinders.

age a denser bloom. Shallow tanks areusually used for this method so thatlight can penetrate. Tanks should beaerated gently. This type of culture isusually done in a greenhouse or solari-

um, although artificial lighting mayalso be used. The Milford method of algal culture isa sequential process in which a singlespecies of phytoplankton is grown in

batch cultures. Phytoplankton for cul-ture can be obtained by isolating nativespecies, but they are usually purchasedfrom another hatchery or from a labo-ratory that specializes in isolating andproducing phytoplankton clones. Mosthatcheries grow two or more differentspecies of phytoplankton to providethe varied rations required by clam lar-vae, post-set and broodstock.The quantity of phytoplankton need-ed depends on the seed productiongoals of the hatchery. Algal consump-tion rates for different size clams arepresented in Table 4. Fifty broodstock(a minimum for a spawning) willrequire about 1.5 x 1011 algal cellseach day; that will be 30 to 50 liters ofdense algal culture containing 5 x 106

cells per ml. One million veliger lar-vae will consume 1 x 109 cells eachday (1 liter); a similar number of set-ting size larvae will consume 50 timesas much. A million post-set clams willrequire three to four times as muchalgae as the broodstock (200 liters perday). Algal culture is probably themost time-consuming and challengingelement of the hatchery operation,but it is essential to its success.

Nursery cultureWhen post-set clams reach a size of 1mm SL (retained on a 710-µm screen)they can be transferred to a nurserysystem. The purpose of a nursery is toprovide a protected environment forsmall seed until they reach a size (8 to10 mm) suitable for field grow-out.Studies have shown that survival in

field grow-out increases with seedsize. However, it is difficult to keepclams larger than 10 mm in a nurserybecause of space and water flow limi-tations. Some Florida growers planttheir seed at smaller sizes (6 to 7 mm),but survival varies. There are many types of nursery sys-tems. These can be roughly dividedinto land-based and field-based sys-tems. Common land-based systemsare raceways and upwelling systems.Field-based systems use a variety ofon-bottom and off-bottom containers,floating rafts, and floating upwellers.Also included under field-based sys-tems are nurseries located in protect-ed areas such as impoundments andponds. Field-based systems are relativelyinexpensive to construct and operatebut have high maintenance require-ments, offer limited protection frompredators, may be subject to environ-mental damage, and have unreliableproduction. Land-based systems aremore expensive to construct and oper-ate but provide almost completepredator control, ease of access, andnear-optimal conditions for the growthand survival of seed clams.

Land-based systems

A land-based nursery is a semi-con-trolled environment for culturing juve-nile clams. Water is usually not treat-ed except for gross (200 µm) filtrationto reduce the influx of fouling organ-isms (e.g., barnacles and oysters), andthe natural food supply is not supple-

mented. However, predators areexcluded, fouling organisms are con-trolled, and conditions necessary forrapid growth are provided. Clams canbe introduced into a nursery at a sizeof 1 mm or larger and are usuallygrown to a size of at least 8 mm beforethey are transferred to field grow-out.Clams can grow as much as 2 mm permonth. Therefore, it will take 3 to 4months (at least) for a 1-mm clam toreach 8 mm, and another month foreach 2 mm of growth beyond that size The two basic types of onshore orland-based nursery systems are race-ways and upwelling systems. Both areenergy-intensive, requiring the continu-ous pumping of large volumes of high-quality estuarine water. Both requireconsiderable capital investment forwaterfront property and infrastructure.Both are labor-intensive, requiringdaily cleaning and frequent monitoringof the seed. Despite these drawbacks,onshore nurseries are popular becausethey promote rapid growth and havehigh seed survival rates.

Raceways

Raceways are shallow, rectangulartrays that may be stacked in tiers.Water is introduced at one end, flowsover the seed clams, and exits at theother end through a drain. Ten to 20liters of water per minute should besupplied for each liter of large seedclams (8 mm). Although raceways pro-duce rapid growth with good survival,they are not well-suited to areas wherethe water supply has a high load offine silt. Silt settles in the raceways andcreates labor-intensive maintenanceproblems. If not cleaned daily, smallseed clams suffocate in this fine silt.Cleaning is particularly difficult if theseed are very small (less than 4 mm).Another drawback with raceways isthat clams near the drain end receiveonly water that has already passedover all the other seed and is largelydevoid of useable food. Thus, the seedmust be rotated in the raceway toachieve uniform growth. Finally, race-ways cannot support large biomassesof seed. A typical raceway can supportabout 0.2 L of large seed per 0.1 m2 (1ft2). This is about 1,400 seed at 8 mmSL or 700 at 10 to 12 mm SL. Race-ways probably should not be used forseed smaller than 4 mm SL because ofthe cleaning difficulties. Seed needabout 2 months to grow from 4 to 8mm in raceways.

Table 4. Estimated consumption rates and recommended feeding ratesfor different life stages of hard clams fed a diet of Isochrysis galbana.

Feeding rate(concentration

Consumption rate in culture Life stage Age/Size cells/clam/day tank: cells/ml)

Veliger 1-2 days 1,000-5,000 10,000-75,000

3-5 days 5,000-10,000

5-8 days 10,000-15,000

8-14 days 15,000-30,000

Pediveliger 14-21 days 30,000-50,000

Early post-set 300-400 µm 50,000-75,000 25,000-100,000

Mid post-set 400-600 µm 75,000-150,000

Late post-set 600 µm-1mm 150,000-500,000

Broodstock >35 mm 1-3 x 109 50,000-200,000

Upwelling systems

Upwelling systems are probably themost widely used land-based nurserysystems. Although they, too, have highcapital costs and energy expense,upwellers promote the rapid, uniformgrowth of seed; they are easier tomaintain than raceways and makemore efficient use of space. Anupwelling system consists of a reser-voir (usually rectangular and about 2feet deep) that contains several cultureunits (Fig. 3). Each culture unit (“silo”)has a screen bottom that supports theseed clams. The silos are placed so thattheir sides project above the waterlevel and the bottom screen is severalcentimeters above the reservoir bot-tom. A drainpipe near the top of thesilo extends through the silo wall andthe reservoir wall and empties into anexternal common drain. Water ispumped into the reservoir, rises upthrough the screens and the seedclams, and exits out the drains near thetop. The vertical water flow supportsmuch larger biomasses of clams thanthe horizontal raceway flow. Each 0.1m2 of upweller screen can support 1.0to 1.5 liters of planting size seed (7,000to 10,000 seed at 8 mm SL). The waterflow requirements for upwellers aresimilar to those for raceways. Eachliter of large (7-mm) seed requires aflow of 10 to 20 liters per minute; verysmall seed (less than 3 mm) shouldreceive ten times that much (Table 5).

Field-based systems

Field-based nurseries are not as reli-able as land-based systems, but theyrequire less capital investment andare less expensive to operate. They donot protect clams from predators aswell as land-based systems and aremore susceptible to environmentaldamage (e.g., from storms). They areless accessible and consequently moredifficult to maintain, but the mainte-nance is done less frequently. Thewater flow (and thus food supply) tothe seed may be variable, whichresults in less uniform growth.However, some floating systems rivalland-based nurseries in productioncapability and are relatively inexpen-sive to build and operate.

Floating upwelling systems

Three floating upwelling systems havebeen used to some extent in the south-

ern U.S. Two of these are powered byairlifts and one is tidal-powered.Operation and maintenance is similarto that for a land-based system. Floatingupwellers have been used with verysmall seed (1 mm), but their successdepends on proper site selection.Both airlift systems are designed foruse in protected waters such as salt-water ponds or impoundments. Theycan also be moored to a dock in atidal creek if they will not be exposedto heavy wave action. One airlift sys-tem uses a raft of 4-inch PVC pipe tosupport the silos and an air manifold.The manifold is pressurized with asmall air blower. Silos are suspendedwithin the raft and water is circulatedthrough the silos with airlifts (Fig. 4).The second airlift system consists of areservoir (tank) that is floated in apond or adjacent to a dock. Water ispumped into the reservoir with alarge airlift or with a submersiblepump and flows out the silo drains asin a land-based system.A tidal-powered upwelling nurseryconsists of a raft with a scoop on oneend and an interior chamber whereupwellers are housed (Fig. 5). The raftis tethered from the scoop end so thatit swings to point the scoop into thecurrent. Water is directed by thescoop into the silo chamber and flowsout through sideports at the waterlevel. In pilot testing, this systemappeared to foster more rapid growththan a land-based nursery. It offers

the obvious advantage of low operat-ing costs (maintenance only), but isvulnerable to storm damage.

Bottom culture

Various bottom culture nursery sys-tems are in use in the southern U.S.These include trays supported off thebottom on racks in subtidal areas,trays planted on or just above thebottom in intertidal and shallow sub-tidal areas, and “soft” bags or pens.All bottom culture systems requirefrequent maintenance to remove silt,fouling organisms and predators. The subtidal rack and tray systemmust be deployed in deep water so itwill not be a navigation hazard.Therefore, a large boat is needed todeploy, maintain and harvest the trays.Trays are made of wood frames withscreening top and bottom. Severaltrays are supported on one rack in atiered arrangement, with spacebetween the trays for water circula-tion. A rack and tray system is labor-intensive to maintain but, if properlysited, promotes good growth and sur-vival even for very small seed (1 mmand up). Small seed (1 to 2 mm) canbe planted at very high densities(50,000 to 150,000/m2, about 5,000 to15,000/ft2) initially, but must bethinned as they grow. The fine meshthat must be used to retain very smallseed has to be cleaned weekly toremove fouling organisms and silt.

Table 5. Flow requirements and stocking densities for seed clams inan upwelling nursery.

Size Velocity Flow ratio Density Densitymm cm/sec L/min:L clams ml/cm2 clams/cm2

1 0.6 240 0.15 4502 0.6 90 0.4 1202 0.5 100 0.3 902 0.4 110 0.2 603 0.4 70 0.3 303 0.2 80 0.15 154 0.2 40 0.3 125 0.2 30 0.4 86 0.2 20 0.6 77 0.2 15 0.8 78 0.2 12 1 79 0.2 10 1.2 6

10 0.2 9 1.4 4

Smaller trays can be deployed inintertidal or shallow subtidalareas if substrate is firm (sand orshell). The trays are buried so thattheir tops are level with the surround-ing substrate; then they are filledwith sand and covered with screening(6-mm or 0.25-inch openings). If thetrays are buried too deep they maybe covered over by shifting sand orsilt. If they are not buried deepenough, they may create scouringcurrents that will wash out the tray.Banks that have sandy substrate areoften subject to strong currents thatmay wash substrate out of the trays.Screens must be checked often andany silt removed from the top screen.Site selection is important becauseheavy silt can kill clams. These traysare probably not suitable for verysmall seed (less than 2 mm). Trays or polyethylene mesh bags maybe placed on racks in intertidal or shal-low subtidal areas. Racks may be con-structed of wood, rebar or PVC. Trayswith solid bottoms are filled with sandor similar substrate. Bags will notretain substrate and will work only inwell-protected sites with no waves orstrong currents; bags and are not well-suited to seed less than 4 mm.“Soft” bags are used extensively inFlorida for nursery culture and havebeen tried farther north with mixedsuccess. In Florida, cages with severaltiers of bags have also been tried.Predation of small seed clams (lessthan 7 mm SL) by crabs and rays hasbeen a problem in “soft” bag nurseries.The bags are made of woven polyesterin an appropriate mesh size to containthe seed. Bags are anchored to the bot-tom with a weighted rope, rebar orstakes. A float inside the bag holds theupper side above the seed. In verysilty areas, the float is often not ade-quate to prevent silt from collecting onthe top of the bag. A modification of the “soft” bag is the“soft pen.” These pens are construct-ed of the same soft, woven materialbut have a rigid frame that supportsthe upper edge 25 to 36 cm (12 to 18inches) above the substrate. Theframe is made of PVC pipe andplaced on the inside or outside of themesh. A recent development is theuse of a vinyl covered wire (14-gauge,0.5-inch openings) skirt to support thesoft pen. The wire fencing helps keep

out predators. Pens have removabletops made of the same soft mesh orof lighter weight plastic netting. Thelid must have a mesh size smallenough to exclude most crabs and itmust fit securely over the edge of thepen so that there is no gap wherecrabs can enter. These systems arenot suitable for small seed (less than 4mm) because the mesh size requiredto retain such little seed would foulrapidly and restrict water flow.

Pond culture

Ponds and impoundments can beused as nurseries to some advan-tage. They protect clams somewhatfrom environmental disturbancesand predators and are usually moreaccessible than field sites, whilebeing less expensive to operate thanland-based nurseries. However, it isvery expensive to construct a pondso this method is probably an

PVCpipe

Silo

Airline Air

flow

Crosssupport Airlift

tube

Supportrods

Hose fromair blower Water

level

PVCpipe

Silo PVCpipe

Airflow

Supportrod

Airlift

Seed

Mesh Water flow

Side view (partial)

Top view

Cross-section

Working deck(partially cut away)

Seed bins

To mooring

Flotation(partially cut away)

Outflow trough(front section cut away)

Bin screen

Tidal scoop

Tidalflow

Figure 4. Floating upwelling nursery system.

Figure 5. Tidal-powered upwelling nursery system.

option only where a pond alreadyexists.With pond culture the natural foodsupply can be enhanced by managingfor algal blooms. However, pond man-agement is complex and poorly under-stood, particularly as it relates to therequirements of bivalve culture. Inaddition to the food supply, the pondmanager must consider many other fac-tors that will affect clam growth andsurvival, such as dissolved oxygen, pH,temperature and waste products. Various sizes of seed have been plant-ed in a variety of containment sys-tems, including airlift-drivenupwellers, trays and bags supportedon racks, trays and bags placed on thebottom, and pond-side upwellers towhich water is pumped. Seed havealso been planted on window screen-ing placed directly on the pond bot-tom. In pond culture, containers main-ly facilitate the handling and harvest-ing of seed and are less important forpredator control, as predators arephysically excluded from the pond.

ReferencesHadley, Nancy H., J.J. Manzi, A.G. Eversole,R.T. Dillon, C.E. Battey and N.M. Peacock.1997. A manual for the culture of the hard clamMercenaria spp. in South Carolina. SouthCarolina Sea Grant Consortium.

Manzi, J.J. and M. Castagna, eds. 1989. ClamMariculture in North America. Elsevier, NY. 461 pp.

Hatchery culture

Brown, C. 1983. Bacterial disease in bivalve lar-val cultures and their control. pp. 230-242 inC.J. Berg, ed. Culture of Marine Invertebrates.Selected Readings. Hutchinson Ross Pub. Co.Stroudsburg, PA.

Castagna, M. and J. Manzi. 1989. Clam culturein North America: Hatchery production of nurs-ery stock clams. pp 111-126 in Manzi, J.J. andM. Castagna, eds. Clam Mariculture in NorthAmerica. Elsevier, NY.

Chanley, P. 1972. Laboratory cultivation ofassorted bivalve larvae. pp. 297-318 in W.L.Smith and M.H. Chanley, eds., Culture ofMarine Invertebrate Animals. Plenum Press.New York.

Hartman, M. 1989. Manual for the design andoperation of a low budget hatchery for the hardclam Mercenaria mercenaria in Florida.Aquaculture Report Series, Florida Dept. ofAgriculture and Consumer Services, Division ofMarketing. (To get a copy, write AquacultureProgram, Room 425, Mayo Building,Tallahassee FL 32399-0800 or call 904-488-4033.Free or nominal cost.)

Nursery culture

Baldwin, R.B., W. Mook, N.H. Hadley, R.J.Rhodes and M.R. DeVoe. 1995. Constructionand Operations Manual for a Tidal-PoweredUpwelling Nursery System. South Carolina SeaGrant Consortium.

Castagna, M. 1984. Methods of growingMercenaria mercenaria from postlarval to pre-ferred-size seed for field planting. Aquaculture39:355-359.

Hadley, N. and J. Manzi. 1984. Growth of seedclams (Mercenaria mercenaria) at various densi-ties in a commercial scale nursery system.Aquaculture 36:369-379.

Malinowski, S. 1988. Variable growth rates ofseed clams in an upflow nursery system andthe economics of culling slow growing animals.Journal of Shellfish Research 7(3):359-366.

Malinowski, S. and S. Siddall. 1989. Passivewater reuse in a commercial-scale hard clamupflow nursery system. Journal of ShellfishResearch 8(1):241-248.

Manzi, J.J. and M. Castagna. 1989. Nurseryculture of clams in North America. pp 127-148in Manzi, J.J. and Castagna, M. ed., ClamMariculture in North America. Elsevier, NY.

Manzi, J., N. Hadley, C. Battey, R. Haggerty, R.Hamilton and M. Carter. 1984. Culture of thenorthern hard clam in a commercial-scale,upflow, nursery system. Journal of ShellfishResearch 4(2):119-124.

Manzi, J. and N. Hadley. 1988. Recent advancesin nursery culture of bivalve mollusks in NorthAmerica. NOAA, Tech. Rep. NMFS 70, NMFS.

Mook, W. 1988. Guide to construction of a tidalupweller. Mook Sea Farm, Inc. Damariscotta,ME.

Mook, W. and A.C. Johnson. 1988. Utilizationof low-cost, tidal-powered floating nurseries torear bivalve seed. Mook Sea Farm, Inc.Damariscotta, ME.

Pfeiffer, T.J. and K.A. Rusch. 2001. Integratedsystem for microalgae nursery seed clam cul-ture. Global Aquaculture Advocate 4(5)p.33-35.

Vaughan, D. and L. Creswell. 1990. Field grow-out techniques and technology transfer for thehard clam Mercenaria mercenaria. FloridaDepartment of Agriculture and ConsumerServices, Aquaculture Report Series. Tallahassee,FL.

Algal culture and diets

Benemann, J.R. 1992. Microalgae aquaculturefeeds. Journal of Applied Phycology 4(3): 233-245.

Guillard, R.L. 1983. Culture of phytoplanktonfor feeding marine invertebrates. pp. 108-132 inC.J. Berg, Jr. ed. Culture of MarineInvertebrates. Hutchinson Ross Publishing,Stroudsburg, PA.

Miyachi, S., O. Nakayame, Y. Yokohama, M.Ohmari, K. Komagata, H. Sugawara and Y.Ugawa, eds. 1989. World Catalogue of Algae.Japan Scientific Societies Press, Tokyo.

Ogle, J.T. 1982. Operation of an oyster hatcheryutilizing brown water culture technique. Journalof Shellfish Research 2:153-156.

Walsh, D.T., C.A. Withstandley, R.A. Krauseand E.J. Petrovitz. 1987. Mass culture of select-ed marine microalgae for the nursery produc-tion of bivalve seed. Journal of Shellfish Research6:71-77.

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The work reported in this publication was supported in part by the Southern Regional Aquaculture Centerthrough Grant No. 2004-38500-14387 from the United States Department of Agriculture, Cooperative StateResearch, Education, and Extension Service.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software SteeringCommittee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have notbeen revised are considered to reflect the current state of knowledge.