(mid-Atlantic): hard clam - USGS National Wetlands Research Center

This is one of the first reports to be published in the new "BiologicalReport" series. This technical report series, publ ished by the Researchand Development branch of the U.S. Fish and Wildlife Service, replacesthe "FWS/OBS1' series publ ished from 1976 to September 1984. The BiologicalReport series is designed for the rapid publication of reports withan application orientation, and it continues the focus of the FWS/OBSseries on resource management issues and fish and wi 1 dl i fe needs.

Biological Report 82(11. 41)TR EL-82-4February 1985Species Profiles: Life Histories and Environmental Requirementsof Coastal Fishes and Invertebrates (Mid-At1 antic)HARD CLAMJon G. StanleyMaine Cooperative Fishery Research Unit313 Murray Hal 1Uni vers i ty of MaineOrono, ME 04469Project OfficerJohn ParsonsNational Coastal Ecosystems Team1J.S. Fish and Wildlife Service1010 Gause BoulevardSlidell, LA 70458Performed forCoastal Ecology GroupWaterways Experiment Stat ionI1.S. Army Corps of EngineersVicksburg, MS 39180andNational Coastal Ecosystems TeamDivision of Biological ServicesResearch and DevelopmentFish and Wildlife ServiceU.S. Department of the InteriorWashington, DC 20240

This series should be referenced as followsU. S. Fish and Wildl ife Service. 1983-19-. Species profiles: 1 ife historiesand environmental requirements of coastal fishes and invertebrates. U. S. FishWildl. Serv. Biol. Rep. 82(11) U.S. Army Corps of Engineers, TR EL-82-4.This profile should be cited as follows:Stanley, J.G. 1985. Species profiles: 1 ife histories and environmentalrequirements of coastal fishes and invertebrates (mid-Atlantic) -- hard clam.U.S. Fish Wildl. Serv. Biol. Rep. 82(11.41). U.S. Army Corps of Engineers, TREL-82-4. 24 pp.

PREFACEThis species profile is one of a series on coastal aquatic organisrls,principal ly fish, of sport, commercial , or ecological importance. The profil esare designed to provide coastal managers, engineers, and biologists with a briefcomprehensive sketch of the bi 01 og ical characteristics and environmental requirementsof the species and to describe how populations of the species may beexpected to react to environmental changes caused by coastal development. Eachprofile has sections on taxonomy, 1 i fe history, ecol ogical role, environmentalrequirements, and economi: importance, if appl icabl e. A three-ring binder isused for this series so that new profiles can be added as they are prepared. Thisproject is jointly planned and financed by the U.S. Army Corps of Engineers andthe U.S. Fish and Wildlife Service.Suggestions or questions regarding this report should be directed to oneof the foll owing addresses.Information Transfer Special istNational Coastal Ecosys tems TeamU.S. Fish ahd Wildlife ServiceNASA-Sl idel 1 Computer Compl ex1010 Gause Boul evardSl idel 1 , LA 70458U.S. Army Engineer Waterways Experiment StationAttention: WESER-CPost Office Box 631Vicksburg, MS 39180

CONVERSION TABLEMetric to U.S.CustomaryMu1 tiply & To Obtainmil 1 imeters (mn)centimeters (an)meters (m)ki 1 ometers ( km)inchesinchesfeetmil es2square meters (m ) 10.76 square feetsquare ki 1 meters ( km2) 0.3861 square mileshectares (ha) 2.471 acres1 i ters ( 1 ) 0.2642cubic meters (m3) 35.31cubic meters 0.0008110gal 1 onscubic feetacre-feetmil 1 igrams (mg) 0.00003527 ouncesgrams (g) 0.03527 ounceskil ograms (kg) 2.205 poundsmetric tons (t) 2205.0 poundsmetric tons 1.102 short tonski 1 ocal ori es ( kcal ) 3.968 British thermal unitsCel s ius degrees 1.8("C) + 32 FahrenheitdegreesU.S.Customary to Metricinches 25.40inches 2.54feet (f t) 0.3048fathoms 1.829miles (mi) 1.609nautical miles (mi) 1.852square feet (ft2)acres2square miles (mi )gallons (gal) 3.785cubic feet (ft3) 0.02831acre- feet 1233.0ounces (oz) 28.35pounds (Ib) 0.4536short tons (ton) 0.9072British thermal units (Btu) 0.2520mil 1 imeterscentimetersmetersmeterskil ometerski 1 ometerssquare metershectaressquare ki 1 ometers1 i terscubic meterscubic metersgramski1 ogramsmetric tonski 1 ocal oriesFahrenheit degrees 0.5556("F - 32) Celsius degrees

CONTENTSPagePREFACE ............................. iiiCONVERSION FACTORS ....................... ivACKNOWLEDGMENTS ......................... viNOMENCLATURE /TAXONOMY /RANGE .................. 1MORPHOLOGY/IDENTIFICATION AIDS ................. 1REASON FOR INCLUSION I:N SERIES ................. 2LIFE HISTORY .......................... 2Spawning ........................... 2Fecundity and Eggs ...................... 3Larvae ............................ 3Juvenileseedclam ...................... 4Adult ............................. 4COMMERCIAL/SPORTFISHERIES ................... 5She1 lf isheries ........................ 5Popul at i on Dynamics ...................... 9GROWTH CHARACTERISTICS ..................... 10ECOLOGICAL ROLE ......................... 11Food and Feeding Habits .................... 11Predation ........................... 11ENVIRONMENTAL REQUIREMENTS ................... 12Temperature .......................... 12Salinity ........................... 13Dissolved Oxygen ....................... 13Substrate ........................... 14Currents ........................... 14Turbidity ........................... 15Habitat Alteratio~ ...................... 15LITERATURE CITED ........................

4CKNOWLE DGMENTSThanks are due to Robert L. Dow, Maine Department of MarineResources, Augusta, for helpful information; and Herbert Hidu, Universityof Maine, Michael Castagna, Virginia Institute of Marine Sciences, andJohn R. Moring, Maine Cooperative Fish and Wildlife Unit, University ofMaine, for reviewing the manuscript. McHugh et al. (1982) was a majorsource of information.Figure 1 is used with the permission of Grass Medical Instruments andthe artist, Trudy Nicholson. Figure 4 is reproduced with permission fromthe General Secretary of the Counsei 1 International Pour L' Expl oration dela Mer.

Figure 1. The hard clam.HARD CLAMNOMENCLATURE /TAXONOMY /RANGEScientific name . . . . . Mercenariamercenaria L. Widely known as Venusmercenaria before We1 1s (1957) reassignedthe species to the genus Linneausoriginally appliedPreferred common names . . Quahog inthe Northern United States, hardclam in the Southern United States(Figure 1)Other common names . . . . Quahaug,hard-she1 led clam, round clam, cherrystoneclam, 1 ittle-necked clamClass . Bivalvia (Pelecypoda)Order . Eu 1 amel 1 i branchi aSuborder HeterodontaFamily . . . . . . . . . . . Veneri daeGeographical range: The hard clamlives in intertidal areas andsubtidal waters to depths asgreat as 15 m along the Atlanticand Gulf coasts from the Gulf ofSt. Lawrence to Texas (Abbott1974). It is most abundant fromMassachusetts to Virginia and hasbeen introduced to Europe andCalifornia. A similar species(M. campechiensis) that li'ves incoastal waters from North Carolinasouthward to Florida andTexas is also called the hardclam. Abbott (1974) stated that- M. cam~echiensis mav ., be a subspeciesof M. mercenaria becausethey hybridi?e.MORPHOLOGY/IOENTIFICATION AIDSThe hard clam has a thick shell,a violet interior border, and shortsiphons (Verrill 1873; Stanley 1970;Morris 1973). The mean length of thethick solid shell is usually 60 to 70mm, but sometimes reaches 120 to 130mn. The ratios of length (L), height(H), and width (W) are: L/H = 1.25;H/W = 1.52; L/W = 1.90. The thicknessindex (ratio of she1 1 volume tointernal volume) is 0.60.

The external surface has numerousconcentric 1 ines that are conspicuousand closely spaced near the outermargins, but more widely spaced aroundthe umbo, especially in youngershells. The center of each valve issmoother than the distal portion. Theumbo is far anterior and projectstoward the front of the she1 1. Theshell is el 1 iptical, somewhat pointedposteriorly, and has a grayish-whiteexterior and a white interior with adark violet border near the margins.The colored part of the shell wasfashioned into wampum by the AmericanIndians for use as money, hence thescientific name (Morris 1973). Theinterior ventral margins are denticulate.The internal anatomy also has distinctive characteristics (Verri 111873). Short siphons are united fromtheir bases to near the ends; theincurrent siphon has a short fringe oftentacles. The siphon tubes areyellowish or brownish orange towardthe end, and may be streaked with darkbrown, black, or opaque white. Thefoot is large, muscular, and plowshaped. The mantle lobes are separatealong the front and ventral edges ofthe shell and have thin edges foldedinto delicate frills, some of whichare elongated near the siphons. Footand mantle edges are white.The vel iger larvae can be distinguishedfrom other bivalves by theshape of the shell and hinge structure(Loosanoff et al. 1966; Chanley andAndrews 1971; Lutz et al. 1982). Themargin of the shell is circular,tapering toward the hinge; thehinge is short and narrow.REASON FOR INCLUSION IN SERIESHard clams are the most extensivelydistributed commercial clam inthe United States and have the greatesttotal market value (Ritchie 1977).Their abundance in clean substratesaccessible to the public makes thehard clam a popular recreational species.Their habitat is vulnerable tocoastal construct ion projects and pollutionfrom urban and industrialdevel opment . Because adults do notmigrate, repopul at ion of over-fishedhard clam beds depends on the transportof larvae from other areas andseveral years for growth, maturation,and reproduction. Any disturbance,however temporary, may cause along-term impact.LIFE HISTORYSpawningThe spawning season extends fromMarch through November, dependi ng onlatitude and temperature. In temperatecl imates, spawning is heaviest inJuly (Carriker 1961). The peak is inMay in the York River, Virginia, andis progressively later in Raritan Bay,New Jersey, and Narragansett Bay,Rhode Is1 and (Jeffries 1964). Spawningbegins in Greenwich Bay, RhodeIsland, about the first of June and iscompleted by mid-July (Landers 1955).In Delaware Bay, spawning lasts fromJune to October but is most intense inAugust (Keck et al. 1975). In Chincoteagueand Sinepuxent Bays, Mary1 and,spawning extends from early Junethrough August (Sieling 1956). Individualfemale hard clams require 2.0to 2.5 months to complete spawning,b ~ the ~ t release of eggs is greatestduring the initial spawning of theseason (Ansell 1967). Spawning is moreintense during neap than during springtides, presumably becallse water temperaturesare higher during neap tides(Carri ker 1961 ).Water temperature is the decisivefactor governing final gamete maturation.In a 2-year study in LowerLittle Egg Harbor, New Jersey, themedian daily spawning temperature was25.7"C and the range was 22' to 30°C(Carriker 1961). In Delaware Bay,

clams spawn at 25" to 27°C (Keck etal. 1975). About 73% of the clamsspawn during the first 2 to 3 days ofrising water temperatures (Carriker1961). The required or preferredwater temperature range for spawningis 21" to 25°C (Kennish and Olsson1975). In England, the clams spawn ata water temperature of 18" to 20°C(Mitchell 1974). When thresholdtemperatures are reached, ma1 es re-1 ease semen that contains pheromones.The pheromones are carried by watercurrents to the females, which arethen stimulated to release eggs(Nelson and Haskin 1949).Sexual maturity usual ly is reachedduring the second year of life.Because size, not age, determines sexualmaturity, slower growing individualsmature at an older age.Reproductive potential peaks at 60 mm,hut then declines as the clams growlarger (Belding 1931).Fecundity and EggsThe average number of eggs releasedby a 60-mm female in nature isabout 2 million (Belding 1931). Inlaboratory tests, the average-sizedfemale released 8 mil 1 ion eggs perseason (Davis and Chanley 1956; Ansell1967). The fecundity of one large femalewas 16.5 million eggs, whereassmall clams (ahout 33 mm) have farfewer eggs (Brice1 j and Malouf 1980).About 2,000 spermatozoa are shed foreach ovum.The spherical eggs are 78 Prn indiameter and yolk granules are closelypacked (Belding 1931). A large gelatinouscapsule distinguishes the hardclam egg from the eggs of other mollusks.Eggs are released through theexcurrent siphon, and the capsuleswells after contact with seawateruntil it is 3.2 times the diameter ofthe egg. Because the gel at i nouscapsule imparts buoyancy, the eggs arepelagic and carried hy tidal andcoastal currents. Spermatozoaswimming in water come into contactwith and penetrate the capsule,fertilizing the egg.After 10 h the embryo developingwithin the capsule becomes coveredwith cilia. The lashing of the ciliatears the membrane and gelatinous capsuleand the ciliated gastrula escapesinto the water. Eggs may be carriedas far as 25 km from the spawningsite.LarvaeTrochophore larvae are formedabout 12 to 14 h after hatching(Belding 1931). The shape resembles achild's top, and the cilia on theblunt anterior end cause spiralswimming and rotation around the longaxis in either direction. 4 functionalmouth develops and the 1 arva beginsfeeding on suspended particulates,especially dinoflagellates. Thelarvae concentrate about 1 m below thesurface during daylight but at nightare more evenly mixed in the watercolumn (Carri ker 1952).1,i;cil.t 24 h after hatching, a shellgland forms opposite the mouth, athin transparent shell is secreted,and the larva becomes a veliger (Belding1931). The veliger drifts inocean and estuarine currents, but itis able to move 7 to 8 cm/min vertically by extending the ci 1 iatedvel um (Mi 1 ei kovsky 1973). Verticalmigration is stimulated by turbulence,which carries veligers intohorizontal water currents fortransport (Carri ker 1961). The numberof veligers is greatest in the watercolumn 3 h after low tide (Moul tonand Coffin 1954). By drifting withthe incoming tide, the veligers aretransported into the estuary and tosea. Veligers of hard clams areabundant in the zooplankton inestuaries during the summer, wheredensities may exceed 500/1 (Carri ker1952; Moulton and Coffin 1954;Jeffries 1964).

The veliger stage lasts 7 to 30days, depending on temperature. Metamorphosisof the veliger of the hardclam is a gradual process that takesplace 16 to 30 days after hatching atlB°C, 11 to 22 days at 24OC, and 7 to16 days at 30°C (Loosanoff et al.1951).Juvenile Seed ClamWhen the vel iger becomes 0.2 to0.3 mm long, the shell thickens, afoot replaces the velum, and a byssalgland develops, indicating metamorphosisto the seed clam. Metamorphosisis inhibited at salinities below 17.5to 20 parts per thousand (ppt)(Castagna and Chanley 1973), ensuringthat seed clams avoid setting in anenvironment with salinities unsuitablefor adults. Seed clams usually aremost abundant in years when freshwaterinflow into the estuary is belownormal and salinity is above normal(Hi bbert 1976).The byssal gland of the seed clamsecretes a tough thread, the byssus,which anchors the clam to the substrate.Seed clams set more denselyin sand than mud (MacKenzie 1979);bits of shell or detritus may alsoserve as anchors. In the laboratory,sand is preferred to mud for setting,but the size of sand grains is notimportant (Keck et al. 1974). InLittle Egg Harbor, New Jersey, theseed clams prefer to set on a firmsurface with a thin layer of detritus(Carriker 1952) or on shells coatedwith mud (Carriker 1961).The set may exceed 125 clams/mL ingood habitat (Carriker 1961): extra-0rdinar.y 2sets may be as high as270,00O/m (Dow and Wallace 1955).The density of the set is not necessarily related to adult concentrationsbecause of movements and mortality ofthe seed clams. Seed clams seek apreferred habitat -- a sandy or siltybottom with small rocks and shells.They hide under shells or rocks toavoid predators (Lee 1977). The seedclams may reach their ultimate habitatin their second year of life (Burbancket al. 1956). A 25-mm clam may betumbled along by currents of 25 cm/secand deposited behind obstruct i ons (M.Castagna, Va. Inst. Mar. Sci.; pers.comm.) .To move, the clam byssus is castoff and the foot is used for locomotion(Belding 1931). When the youngclam reaches a desirable habitat, itspins a new byssus and reattaches to asmall object. Byssal fibers are usedfor anchorage until the young clam is10 n long; it then metamorphoses andassumes the burrowing habits of theadult.The distribution of seed clams isalso altered by predation. Clams thatset among oyster shells or stones areprotected (Maurer and Watling 1973);without cover, seed clams are subjectto heavy predation. Normally they donot live in areas exposed to waveaction or strong currents (Anderson etal. 1978), but in the absence ofpredators, Carriker (1959) reportedthat s~~rvival on unstable bottoms waspossible.AdultTle adult hard clam lives in thesubstrate and burrows with a muscularfoot. It remains in the location atwhich it first burrows for theremainder of its life. In the first38 days after first burrowing, adultsmoved laterally an average of only 5cm and a maximum of 15 cm from thepoint of origin (Chestnut 1951).Clams 20 to 30 mm long are known totravel as far as 30 cm in 2 months(Kerswi 11 1941).Adults bury deeper in sand (meandepth 2 cm) than in mud (mean depth 1cm), and small adults burrow proportional ly deeper than 1 arger ones(Stanley 1970). If dug up, the hardclam reburrows, and if covered withoverburden it can escape upward (Belding1931 ). A clam can escape through

10 to 50 cm of overburden, if thesediment dumped is similar to thelocal substrate (Kranz 1974). Foreignsediment reduces escapement.The adult is most common in theintertidal and subtidal areas ofestuaries and bays. Hard clams aremost abundant in the lower estuary andare seldom found in the upper estuarywhere salinities are lower (Turner1953). They are absent in places withsalinity less than 15 ppt in upperDelaware Bay (Maurer et al. 1974) andin upper Chesapeake Bay (Siel ing 1956;Lippson 1973). In Newport River,North Carolina, they are absent in theupper estuary at average salinitiesless than 19 ppt (Wells 1961). InGreenwich Cove, Maine, clams wereabout three times more dense at theseaward end of the cove than in theupper cove (Tiller 1950).Hard clams tend to be found inprotected locations within bays andestuaries (Loosanoff 1946). In Rand'sHarbor, Massachusetts, about 50% ofthe population lived on the gravelslope, 25% in the muddy channel, and25% in the subtidal zone (Burbanck etal. 1956). In South Carolina, thehard clam usually avoids openestuaries, but 1 ives in small channelsand protected areas (Anderson et al.1978). In Georgia, hard clams livelargely in intertidal areas protectedfrom wave action (Godwin 1968).Loosanoff (1946) a1 so mentioned thei rintolerance to rough waves. In theTest and Itchen Rivers, England, theyare absent above the mean tide line(Hi bbert 1976).Some populations are oceanic,e.g., those in the shoals of NantucketSound (Turner 1953). An offshorepopulation of hard clams is locatedbetween Cape Lookout and BeaufortInlet, North Carolina (Porter andChestnut 1962). Reviews by Belding(1931) and Loosanoff (1946) state thathard clams live at depths up to 15 m,whereas Rurbanck et al. (1956) reporteda maximum depth of 8 m. Hardclams were lacking in 9,000 bottomsampl es coll ected at depths greaterthan 24 m in the mid-Atlantic Bight(Theroux and Wigley 1983). The 1982landings of about 13 million poundswere taken within 3 mi of the 1J.S.coast (Thompson 1983).COMMERCIAL /SPORT FISHERIESShellfisheriesThe hard clam is more widelydistributed than any other commercialclam species in U.S. waters and is themost valuable commercial and sportspecies (Ritchie 1977). The fisheryis located chiefly along the mid-Atlantic Bight (Figure 2). North ofCape Cod and in the Gulf of Mexico itis important only in relativelyisolated waters (McHugh 1979).Hard clams are taken commerciallywith hoes, bull rakes, hand tongs, andpower dredges (Engle 1970). Of thecommercial landings f rom NarragansettRay, 90% are taken by handraking(Holmsen 1966), whereas in ChesapeakeBay, 95% of hard clams are taken withpatent tongs (Haven and Loesch 1973).Although a power dredge is effective,it is not permitted in many areas,even though it disturbs the substrateno more than bullraking, and allevidence of harvesting disappearswithin 500 days (Glude and Landers1953). A power dredge with an escalatorincreases the catch of the morevaluable small clams, but causes disturbanceof the substrate (Godcharles1971). Because dredging destroys seagrassesand benthic algae and recolonizationis slow, dredging has a relativelylong-term envi ronmental impact.About 6 million kg (meat weight)of hard clam are landed annually alongthe Atlantic seaboard (McHugh 1979;Thompson 1983). The fishery is

NEW YORKATLANTIC OCEANMILESKILOMETERS'@&primaryharvest areasFigure 2.Primary areas of hard clam harvest in the mid- Atlantic coastal region.6

characterized by large fluctuations.The landings in New York and NewJersey were high in the late 18001s,low in the early 19001s, and highagain in the late 1940's and early1950's (McHugh 1977). More recently,production in New York graduallyincreased from 1.8 million kg in 1960to 4.1 million kg in 1976, anddeclined thereafter to 1.5 mil lion kgin 1982 (Table 1). New Jersey had aperiod of moderate production of about0.8 million kg from 1961 to 1965, aperiod of high production of about 1.1million kg from 1966 to 1971, and asubsequent gradual decline to a low of0.4 million kg from 1977 to 1982.Hard clam landings for Rhode Islandare almost mirror images of those forNew Jersey; high production in theearly 19601s, low production from 1966to 1977, and high production from 1979to 1982.The hard clam fishery in the mid-Atlantic region is most intense in afew bays with large populations. Inthe mid-1970's about 40% of the U.S.landings were from Great South Bay onLong Island (MacKenzie 1977). Otherareas of high production are GreenwichBay in Rhode Island (Stringer 1952);Little Egg Harbor, New Jersey (Figleyand Townsend 1980); Raritan Ray,between New York and New Jersey(Jacobson and Gharrett 1967); andChi ncoteague and Si nepuxent Bays,Mary'l and (Siel ing 1956). The landingsof hard clams in the mid-Atlanticregion are about 83% of the U.S. total(Kinoshita and Vondruska 1980).The value of U.S. landings hasprogressively increased. The U.S.1 andi ngs (meat wei ght) decl inedbetween 1965 and 1975, but the valueper unit increased (Zakaria 1979).The landings were 13.3 million lbvalued at $29.7 million in 1978(Pileggi and Thompson 1979), 18million 1b worth $51 million in 1981,and 12.9 mil 1 ion lb worth $43 mil 1 ionin 1982 (Thompson 1983).The price of hard clams varieswith size and the season (Ritchie1977). Littlenecks (about 46 long)command a higher price ($60/bu) thancherrystones (77 mm, $22/bu), or chowderclams (97 n, $13/bu). Prices in1983 averaged about $26/bu. Hardclams are also processed and marketedas clam juice. The market for freshhard clams is possible because theanimals, if kept cool, live for 1 to 3weeks out of water.The recreational catch of hardclams is not included in the landingdata. In New Jersey, one-third of thecatch is taken by 21,600 shellfishermenwith recreational 1 icenses, andthe rest by 1,000 comnercial licenseholders (Fi gl ey and Townsend 1980).In the town of Islip, New York,524,000 bu were taken commercial ly and21,000 bu in the recreational fishery(Buckner 1979). El sewhere, comparisonwith commercial fisheries is difficultbecause of differences in the way thecatch is reported. In Rehoboth andIndian River Bays, Delaware, the commercialcatch was 0.6 millionkilograms in 1957 compared to arecreational catch of 1 million clams(Shuster 1959). In Massachusetts,the commercial fishery was worth$788,000 in 1975 and the recreationalfishery was worth between $31,000 and$195,000 (Conrad 1979). In GreatSouth Bay, New York, the recreationalfishery was 4,806 bu in 1977 (Fox1978), compared with a commercialfishery of about 8 million lb in 1976(MacMi 11 an 1978). Because a bushelof hard clams yields about 10 lb ofmeat (Shuster 1959), the recreationalfishery in Great South Bay accountedfor only 50,000 lb -- an insignificantamount.In heavily fished areas, manyclams are cropped about as soon asthey reach a marketable size (Ritchie1977), i .e., when 3 to 4 years old and40 to 50 mm long. This method ofcropping makes good use of theresource because it leaves the morevaluable smaller clams and sufficient

Table 1. Hard clam landings (meat weight in thousands ofki lograms) in the mid-Atlantic region (Kinoshi ta andVondruska 1980). Data for 1980-82 are taken from unpublishedrecords.Year1960196119621963196419651966196719681969197019711972197319741975197619771978197919801981NA = Data not available.From State of Virginia records.StateNJ VA NC MI) CT DE1,158 753 N A ~ NA NA NA765 544 NA NA NA NA608 766 NA NA NA NA718 951 NA NA NA NA859 1,113 116 151 NA NA850 1,128 142 108 68 1651,212 844 106 78 111 1201,305 844 91 134 109 1361,158 848 92 360 109 1081,027 863 115 238 NA NA1,168 604 128 257 NA NA1,124 833 115 151 NA 52996 607 124 85 176 NA859 614 172 31 109 NA790 505 130 32 56 46735 494 129 34 54 15677 406 139 16 65 24484 463 335 11 65 18365 226 405 11 81 13407 281 70 9 82 19383 341 699 19136 11419 504~ 661 29

ood stock to repopulate the clambeds.Population DynamicsLarval hard clams may be one ofthe most abundant plankters in estuaries.Larval densities of 25/1(Carriker 1952) and 572/1 (Carriker1961) have been measured. Based onthese densities, the author calculatedthat there would be 50,000 to 1.1mi 11 ion larvae per square meter in anestuary 2 m deep. There were 501 arvae/l in Wickford Harbor, RhodeIsland, which were reduced to 3larvae/l hy the time of setting(Landers 1953). The number of seedclams that set in Little Egg Harbor,New Jersey, was estimated to be125/m2 (Carriker 1961). Populationsof seed clams in&asco Bay, Maine, mayreach 270,00O/m (Dow and Wallace1955).Adult population density varieswidely. Populations in Greenwich Bay,Khod? Island, ranged from 2 to12/m (Stickney and Stringer 1957).At some places in Greenwich Bayhard clams densities >averaged215/m (Stringer 1955). Populationselsewhere in Na5ragansett Bay rangedfrom 5 to 189/m (U.S. Department ofthe Interior 1956); the highestaverage densities were ip theProvidence River Egtuary (17/m ) andBristol Harbor (9/m ). The populationdensity in Nantucket Sou2d, Massachusetts,was about 0.06/m (Ropes andMartin 1960). The population densityin waters of th? Town of Is1 ip, NewYork, were 16/m rhere fishing waspermitted and 30/m in closed waters(Buckner 1979). Popul atyn densitiesin Raritan Bay wef;e ll/m on the NewYork side and 5/m- on the New Jerseyside (Campbell 1965). Bioyss (meatweight) ranged frfm 1.6 g/m in poorhabitat to 36 g/m in good habitat ofMoriches Bay, New York (O'Conner1972). Annual recruitment in theJames 2River Estuary, Virginia, was0.84/m (Haven 1970). Along theGeorgia coast abundance ranged from20.1/m2 to 21/m (Godwin 1958). Hardclams introduced in Great Britaincoastal2 waters reached densities of 6to 8/p (Ansell 21963). Densities of110/m and 540/m in Casco Bay, Maine,were mentioned by Dow (1952).Natural mortality is high in thelarval and seed clam stages, butalmost nil once the shell becomesthick enough to resist predators(Figure 3). Based on densities ofdifferent life stages in the field, Icalculated monthly mortality coefficients(Z) of 1.7 for the eggs(monthly mortality = 81%) and 1.5 forthe larvae (monthly mortality = 78%).In Rhode Island, observed mortality oflarvae over the summer was 95% to 97%in Wickford Harbor and 94% to 99.7% inGreenwich Bay (Landers 1955). I calculatedan annual mortal ity coefficientfrom seed clam to adult of 3.0(annual mortality = 95%). InChesapeake Bay, usually less than 10%.of small clams survive for 1 yearand in some locations none survive(Haven and Loesch 1973). On the basisof nine estimates of adult mortal i tyin England, Hi bbert (1976) calculatedan average annual mortal i tycoefficient of 0.8 (annual mortal i ty= 55%). The mortality coefficient ofadult clams held in trays andprotected from predators in SouthCarolina was only 0.13, or about 12%Month: 5 10 ISFigure 3. Abundance of hard clams atdifferent life stages, from eggs toadult, based on a composite of thedata cited in the text.

annual ly (Eldridge and Eversole1982). These mortal i ties representnatural mortal i ty, which wasapproximately equal to instantaneoustotal mortality Z in the ahsence ofharvest. Overwinter mortality of hardclams in Maine was 40% (Dow andWallace 1955).Because hard clams tend to becompletely harvested in any particularbed, it was not possible to arrive ata sound estimate of fishing mortalityF. Mortality of hard clams smallerthan the legal size was estimated tohe 30% each time a flat was disturbedby digging (Dow 1953).GROWTH CHARACTER ISTICSThe hard clam grows rapidly infavorable envi ronments. The vel i gerlarvae grow from 10 um to 200 um in 7days in Little Egg Harbor, New Jersey(Carriker 1952). At 18°C the larvaeincreased from 105 Pm to 183 um in 20days, whereas at 30°C they grew tothis size in 12 days (Loosanoff et al.1951). The daily percent growth rateof vel igers, as a function of temperatureand salinity is as follows:Growth = -288 + 12.40T + 14.09swhere T is the temperature in "C and Sis the salinity in ppt (Lough 1975).At 20°C and 30 ppt, for example, thedaily growth would be 68%. Growthstops at temperatures below 9°C andabove 31°C (Ansell 1968).At the end of their first summer,seed clams are about 5 to 7 mm long inNew York, and 16 mm long in Florida(Ansell 1968). Annual growth dependson the length of the growing season,which is largely a function of latitude(Figure 4). The average annualgrowth i ncrements based on she1 1length, estimated from Figure 4 forages 2 to 5 years, were about 10 mm inCanada, 13 mm in Vaine, 14 mm in NewJersey, and 23 m in North CarolinaYear of lifeFigure 4. Shell lengths of hard clamsof different ages from Florida, NorthCarolina, New Jersey, Maine, andPrince Edward Island, Canada (Ansell1968).and Florida. Growth increment isabout the same during the peak growthperiod of midsummer regardless oflatitude (Ansell 1968).The growth rate of adults slowswith increase in length. Clams 35 to39 m long grow about three timesfaster than clams 65 to 69 rnm long(Pratt and Campbell 1956).O f interest to resource managersis the time required for clams toreach the minimum legal size (based onshell length), which in most States isreached in about 3 years (Ansell1968). In Rhode Island and Connecticut,clams reach the 44-mn legalsize in about 2.5 years. In New York,the 50-mm minimr~m size is reached in3.0 years, whereas in New Jersey theminimum size is reached in 3.3 years.In Chesapeake Bay off GloucesterPoint, hard clams require 4 to 5 yearsto grow to commercial sizes of 38 to50 m (Haven 1970). At the extremesof the I1.S. range, the legal size isattained in 3 years in Florida at a

size of 57 mm and in 5 years in Maineat a size of 51 mm.ECOLOGICAL ROLEFood and Feeding Habits--Mu1 t hard clams feed by filteringout plankton and micro-organisms thatare carried along the bottom bycurrents (Chestnut 1951). Hard clamsdepend on plankton for food before andduring spawning to furnish sufficientenergy to ripen the gonads (Ansell1967). If the food supply is inadequate,spawning is diminished or nil.In the laboratory, food densities of300 mg/l of carbon are optimal for depositionof biomass (Tenore andDunstan 1973).Food and other materials are takenin b,y the clam through the incurrentsiphon. Tentacles on the siphon detectexcessi ve concent rations ofoversized particles in the water andcause the siphon to close. The mantle,visceral mass, and gills are ciliatedand secrete mucus. Particlesbrought in through the incurrentsiphon attach to the mucus. Depositson the gills are collected by thecilia and carried towards the mouth(Kellogg 1903). The palps at themouth entrance determi ne, by vol ume,whether the particle mass is ingestedor rejected. Only small masses areselected for digestion. C omp 1 expatterns of cilia movement remove thewaste, call ed pseudofeces, from pal psand gills. Eventually a1 1 wastematerials are collected on the mantleand carried to the base of the siphon,avoiding the stream of incoming seawater.When sufficient waste has beencollected, the adductor muscle suddenlycontracts, forcibly ejecting astream of water containing the wastemass from the incurrent siphon (Kellogg1903).P redat i onPredation is the primary naturalcontrol of hard clam populations (Virstein1977). The clams are preyed onby fish, birds, starfish, crabs, andother mollusks. For defense theyburrow or live among shells or rocks.Without shell or rock cover, thejuvenile hard clam may be exterminatedby predators. In one experiment, survivalin penned sites was 94% comparedwith 9% in an unpenned area (Kraeuterand Castagna 1980).Crabs are the most serious predatorsof hard clams; in one study 88%of the predators were crabs (Whetstoneand Eversole 1978). The crabs crushsmaller clams with their claws andchip the edges of the shells of largerclams. A rock crab (Cancer irroratus)consumes up to 30 small clams/h, and amud crab (Neo ano e sayi) consumes upto 14 clam*Kenzie 1977). Insome are s, mud crabs may be as denseas 50/mt Mortality of young clamsparallels the frequency at which shellbits occur in the stomachs of the mud+crab Pano eus herbsti i (Whetstone andEversole 19 8). Crabs are effectivepredators because they can pry theclam out of the sediment. The rockcrab, blue crab (Call inectes sa idus)and green crab (Carcinides r n a h i ; ~up the clams, whereas mud crabs burythemselves in the sediment to crushthe clam in place (MacKenzie 1977).Hard clams longer than 7 mn are notvulnerable to mud crabs, and thoselonger than 15 mm are not vulnerableto rock crabs (MacKenzie 1977).Mollusks are+the next most importantpredator. Oyster dri 11s (Urosalpinxcinerea and Eu leura caudamdthe moon snails Polinices duplicataand Lunatia heros) drill holes in theshell and remove the clam's body tissues(Buckley 1974). Hard clams largerthan the predator are thick enoughto withstand being drilled by moonsnai 1s (Kitchell et a1 . 1981). Whelks(Busycon canaliculatum and B. caria)chip off the outer edge of the shell

to make a hole through which theyinsert their proboscises and ingestthe clam's soft parts by alternatelyrasping and swallowing (Carriker1951). Hard clams are vulnerable tooyster drills until 20 mm long and tomoon snails until 50 rrnn long(MacKenzie 1977). In addition, theadult hard clam may destroy its ownlarvae by ingestion.The sea star (Asterias forbesi)pulls the valves of adults apart withits tube feet and inverts its stomachinto the body cavity (MacKenzie 1979;Doering 1982a). If a sea star ispresent, hard clams bury deeper(Pratt and Campbell 1956; Doering1982b) and reduce activity (Doering1982~). Fish, such as flounder, andwaterfowl also feed on larvae andyoung cl ams (Be1 d i ng 1931).ENVIRONMENTAL REQUIREMENTSTemperatureUater temperature is the most importantfactor in growth and reproduction.The harvest of the hard clam inMaine was highly correlated (r = 0.80)to the August sea temperature 2 yearspreviously (Sutcl iffe et al. 1977).Dow (1977) recorded a highly significantcorrelation between mean annualsea temperature and populations ofadult hard clams.Hard clams spawn at temperaturesof 22" to 30°C in Little Egg Harbor,New Jersey (Carriker 1961) and from21' to 25°C in Barnegat Ray, NewJersey (Kennish and Olsson 1975).They spawn in Delaware Bay at 25' to27OC (Keck et al. 1975). Spawning istriggered by rising temperatures.The optinun temperature range forlarval growth is 22.5" to 25°C inbrackish water and 17.5" to 30°C at ahigher salinity (Davis and Calabrese1964). According to Carriker (1961)larvae to1 erate water temperatures of13" to 30°C. Eggs require temp-eratures above 7.2"C, but larvalsurvival is highest between 19" and30°C (Lough 1975). Growth is greatestfrom 22' to 36OC. Embryos and vel igerlarvae develop abnormally and die at15°C and 33"C, but straight hingedlarvae tolerate these temperature extremes(Loosanoff et al. 1951). Theminimum temperature for growth whenclams are fed naked dinoflagellates is12.5OC, but higher temperatures areneeded to digest a1 gae (Davis andCalabrese 1964).Adult hard clams tolerate temperaturesfrom below freezing to about35°C. Adults survive at -6OC, but diewhen 64% of the water in the tissueshas changed to ice (Williams 1970).Hard clams located in bars elevatedabove the gradient of the mud flatsusually suffer 100% winter mortality,almost surely caused by freezing (Dowand Wall ace 1951). Summer temperaturesas high as 34°C are tolerated(Van Winkle et al. 1976; MacKenzie1979).Growth is reduced at water temperaturesbelow 10°C (Pratt andCampbell 1956) and growth stops at 8°C(Relding 1931). Hard clams hibernateat temperatures below 6OC (Loosanoff1939). Pumpi ng water, requi red forfeeding, ceases below 6°C and above32OC (Hamwi 1968). The extension ofthe siphon also indicates pumping; thetemperature range for siphon extensionis 1" to 34°C (Van Winkle et al.1976).Estimats of the optimum temperaturefor hard clam growth vary fromabout 20°C (Ansell 1967) to 23OC(Pratt and Campbell 1956). Otherbiological activities indicate thermaloptima. Hamwi (1068) found maximumpumping at 24" to 26°C. Siphon extensionwas greatest in the range of 11°Cto 22OC (Van Winkle et al. 1976).Storr et al. (1982) reported twooptima for shell calcium deposition:13' to 16OC, and 24OC. Optimumtemperatures for burrowing are 21' to31°C (Savage 1976).

Hard clams are adversely affectedby rapid temperature changes. A rapidtemperature increase of + 5OC in thedischarge from a nuclear power plantstopped she1 1 growth (Kennish 1976).The summer growth of hard clams wasreduced 60% to 90% when the clams weretransplanted to the warrner waters ofthe discharge site.Sal i nityThe salinities at which hard clamsare found usually range from ab0llt 10ppt to 35 ppt, allowing for possiblegeographic di fferences. Be1 di ng(1931) reported 23 to 32 ppt as thegeneral range of tolerance. In WellfleetHarbor, Massachusetts, salinityin cl am beds ranged from 20 to 34 ppt(Curley et al. 1972). The range ofsalinities in a New York clam' habitatwas 15 to 35 ppt (MacKenzie 1979). InNew Jersey, clams are found only inbays where salinity is above 15 ppt(Figley and Townsend 1980). Hardclams do not live in salinities below19 ppt in the Newport River Estuary,North Carolina (We1 1s 1961), or atsalinities below 18 ppt in SouthCarol ina (Anderson et a1 . 1978). Thesalinities of natural clam beds rangefrom 10 to 28 ppt in the mid-Atlanticregion (Loosanoff 1946).Salinity is most critical duringthe egg and larval stages. The embryosin Long Island Sound developonly in the range of 20 to 32 ppt; at35 ppt only 10% develop (Davis 1958).Veliger survival is low during highrainfall (Carriker 1961 ). Vel igergrowth is best at 20 to 27 ppt. Larvaeapparently requi re higher sal i ni -ties than adults, and metamorphosis toseed clams is rare below 18 ppt (Castagnaand Chanley 1973). Embryos developnormally between 20 and 35 ppt;the optimum is about 28 ppt. The minimumsalinity at which larvae survivewas 15 ppt. In Southampton Water,England, young cl ams were abundantonly in years of low freshwater inflowfrom the River Test (Mi tchel 1 1974).Juvenile and adult clams closetheir shells when exposed to dilutedseawater to increase their to1 eranceto low salinities. Juveniles can livein freshwater for 22 days in the laboratory(Chanley 1958). At 10 pptthey begin dying at 28 days and at 10and 15 ppt there is little feeding orburrowing. Adult hard clams exposedto salinities as low as 0.3 ppt in theSantee River system in South Carolinasurvived for 14 days (Burrell 1977).Laboratory tests showed that pumpingceased below 15 ppt and ahove 40 ppt,and that the rate of pumping was highestbetween 23 and 27 ppt (Hamwi1968). In the laboratory, siphons arerarely extended at salinities below 17ppt or above 38 ppt (Van Winkle et al.1976). The optimum salinity range forsiphon extension is 24 to 32 ppt.The optimum salinity for larvalsurvival is about 27 ppt (Davis andCalabrese 1964). At about 22 ppt, thetemperature to1 erance was reduced. Astrong interaction between temperatureand salinity was reported by Lough(1975). The maximum survival of eggswas ahove 28 ppt and above 7.2OC. For1 arvae, survi val was highest between21 and 29 ppt at 19' to 29.5OC. Thelarvae grew best between 22 and 30 pptat 22' to 36OC.Dissolved OxygenChanges in dissolved oxygen do notaffect hard clams as much as changesin temperature and salinity. All lifestages survive nearly anoxic conditionsfor relatively long periods, butthe,y stop growing. Embryos requireonly 0.5 mg/l dissolved oxygen and dieonly at oxygen levels below 0.2 mg/l(Morrison 1971). Embryos fail todevelop to the trochophore stage whendissolved oxygen is 0.34 mg/l or less.Larval growth is nearly zero at such1 ow oxygen concentrations but picks upat 2.4 mg/l and is best at 4.2 mg/l.Adults tolerated low oxygen in thelaboratory, but their metabolism

ecame depressed. The hard clam cantolerate less than 1 mg/l for 3 weeksand still be capable of reburrowing(Savage 1976). Growth is suppressedwhen oxygen concent rations are 1 ow.Below 5 mg/l, oxygen consumptionprogressively decl i nes and an oxygendebt is incurred (Hami 1969). Theoxygen debt is rapidly repaid in a fewhours after return to aerobic conditions.Ultimately, hard clams succumbto hypoxic environments. Hardclams nearly disappeared because ofaccelerated eutrophication and reducedoxygen in coastal waters near a duckrearing area on Long Island, New York(08Conner 1972).Subst rate --Numerous studies have shown thathard clams are more likely to live ona sandy bottom than on a mud bottom(Allen 1954: Maurer and Watling 1973;Mi tchell 1974). Recause water currentssort bottom substrates, there isa high correlation between currentsand bottom type; consequently, watercirculation may be the decisiveelement in the distribution of hardclams (Greene et a1 . 1978).Clam larvae set more frequentlyand more densely on sand than on mud(MacKenzie 1979). There also appearsto be some correlation between grainsize and the density of setting (Kecket al. 1974). In a laboratory test,781 larvae set on mud particles 0.05mm in diameter whereas 2,083 set onsand particles 0.50 mrn in diameter.There was little difference in thedensities of setting on sand graindiameters of 0.25, 0.50, 0.71, and1.00 mm. Larvae much prefer sand(0.25 mm) over mud (0.50 mm), yet thehighest concentration of seed clamswas on shells coated with mud(Carri ker 1961). Seed clams canemerge from a depth of sediment atleast five times their shell height.Abundance also is related to othersubstrate. Twice as many hard clamslive in gravelly substrate than in mud(Burbanck et al. 1956). The biomassof living clam tissue is related tothe type of substrate in Moriches Bay,New York, as follows: sand, 25.5g/m; sand without vegetation 34g/m2; sand with vegetation, 11.3g/m2; and sand with clayey silt, 1.6g/m (08Conner 1972). The presence ofshells was more important thanparticle size in determining clamabundance in Greenwich Bay, RhodeIsland. The ahundance was as follows:16/m2 in mud, sand and shell ; 10/m2 insand and shell; 6/m2 in mud and shell,or mud and sand, or sand; and 3/m2 inmud (Stringer 1955). The density ofhard clams was correlated to the abundanceof particles with diametersgreater than 2 n (Saila et al. 1967).Not a1 1 reports agree. For example,in the Woods Hole region, Allee(1923) reported a re1 ati ve density(per m ) of 19 in mud, 14 in sand, 4in rockweed, 2 in gravel, and 1 ineelgrass. Hard clams in Bogue Sound,North Carolina, tended to be in finersediments (Brett 1963).The growth of hard clams sometimesreflects the substrate type. Clamsgrew 50% faster in sand than in mud inGreat South Bay, New York (Greene1975). Clams placed in sand inNarragansett Bay, Rhode Island, grew24% faster than those placed in mud(Pratt 1953). There was a highcorrelation (r = 0.88) between shelllength and substrate particle size inLittle Bay, New Jersey (Johnson 1977).CurrentsWater movement is important to alllife stages of the hard clam. Currentstransport eggs and 1 arvae and bri ngfood to the adults. Hard clams ofUickford Harbor, Rhode Island, live incurrent velocities 1 ess than 0.5 m/sec(Landers 1953).Larvae prefer currents from 12 to130 cm/sec (Carriker 1952). Densitiesof larvae were low near the inlet of

an estuary where tidal exchange wasgreatest and currents fastest (Carriker1961). The planktonic abundancedistribution of larvae is not affectedby individual tidal stages, but observationssuggest that the abundance washighest 3 h after low tide (Moultonand Coffin 1954).The growth of adults also is corre1ated with tidal currents (Kerswill1949; Haskin 1952; Wells 1957). Hardclams grow better at a velocity of 7.5cm/sec than in a sluggish slough (Kerswi11 1949). Strong currents, however,may scour the bottom and reducehabitat quality (We1 1s 1957).TurbidityRecause hard clams filter water toobtain food material, they also trapother suspended material. Dischargingthis materi a1 reduces energy avai 1 ablefor growth (Pratt and Campbell 1956).Excess turbidity can clog the filteringapparatus and cause death. Eggsand larvae are also sensitive to turbidity.Embryos develop normal ly ifsuspended si lt or sediment arepresent, unless concent rat ions areunusually high (Davis 1960). Siltabove 3 g/l impedes development, butsome emhryos devel op normal 1 y i nwaters with 4 g/l of clay, chalk, orFul ler's earth. Embryo development isnormal at 2 g/l of particles between 5and 50 um diameter. Sand had littleeffect on eggs except for the smallestparticles at the highest concentrations(Davis and Hidu 1969).Larvae are more sensitive thanembryos to turbidity. In a laboratorystudy, 90% of the larvae died atconcentrations of chalk above 0.25 g/land of Fuller's earth above 0.5 g/l(Davis 1960). Larvae tolerate silt upto 4 g/l, and even grow faster in lowconcentrations of silt than in siltfreewater. Larval growth is depressedby concentrations of clay 0.5g/l and higher (Davis and Hidu 1969).Although turbidity may have profoundeffects on adult clams, the 1 imitsof the reaction of the clams toturbidity is not well defined. Menzel(1963) reported that high turbidity insummer may inhibit the growth ofadults in Florida. Another view isthat clearing of particles from thefi 1 teri ng apparatus reduces growth inmuddy habi tats (Pratt and Campbell1956). Adults expel 1 ed pseudofecesproduced when clams clear thefiltering apparatus 107 timeslh inmud, 19/h in fine sand, and 7/h incoarse sand. Rhoads et a1 . (1975)believed that a turbid layer near thebottom in Buzzards Bay, Massachusetts,enhanced the growth of hard clamsbecause it contained detrital food.Habitat Alteration -Dredging of coastal waters reducesthe abundance of hard clams in thearea of impact. For example, hardclams in the path of a dredged channelthrough a lagoon on Long Island, NewYork, were destroyed, and those oneither side of the path were adverse1.yaffected by sedimentation (Kaplan etal. 1974). Hard clams further than400 m from the dredge site wereunaffected. Commercial clammers inthis area reported no noticeablereduction in harvest the followingyear, whereas scientists found asignificant reduction in standingcrop. In Roca Ciega Bay, Florida, thehard clam pop~~lation failed to returnto its previous abundance 13 yearsafter dredging (Taylor and Saloman1968).

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10171 -101REPORT DOCUMENTATION ! 1. RE*"T No.PAGE (Biological Report 82(11.41)*4. nil. and h~ltl.Species Profiles: Life Histories and Environmental Requirementsof Coastal Fishes and Invertebrates (Mid-Atlantic) -- Hard ClamJ. ROCID~~WI ~ccascon NOL RODOrl OmtoFebruary 1985I7. Author(.)Jon G. Stanley+. hdormlry Organization Name and AddrossMaine Cooperative Fishery Research Unit313 Murray HallUniversity of MaineOrono, ME 0446912. Igonwdn# Organlzmtlon Name and Addnss(GINational Coastal Ecosystems Team U.S. Army Corps of Engineers 1% TYD. d Regon L hdod CowndFish and Wildlife ServiceWaterways Experiment StationU.S. Dep. of the Interior P.O. Box 631Washington, DC 20240 Vicksburg, MS 39180IS. Supplamontafy Not-I *U.S.1L -rut (Umlt: ZM words)Army Corps of Engineers Report No. TR EL-82-4.Species profiles are literature summaries on the taxonomy, morphology, range, lifehistory, and environmental requirements of coastal aquatic species. They are designed toassist in environmental impact assessment. The hard clam (Mercenari a mercenaria) is themost extensively distributed commercial clam in the United States. They spawn offshore insummer when water temperatures are between 18" and 30°C. The eggs and larvae are carriedby currents into estuaries, where seed clams set on sand or pebbles. Seed clams that lackcover of shells or stone largely perish because of predation. Adults filter-feed onphytoplankton and particulate material. Adults survive temperatures of -6" to 30°C andsalinities of 10 to 35 ppt, and can withstand freshwater for several days by closing theirshell. When the shell is closed, they must tolerate anoxic conditions, and they surviveless than 1 mg/l oxygen in the water for several days. Even the larvae tolerate 0.5 mg/lof oxygen.I 17. Drrmmnt Analysisa O.r*ptomEstuariesClamsGrowthFebei~~l%nlOD.n-Endad TarmsHard clamMercenari a mercenariaI Habitat1 Salinity requirementsc. COSATI fieldlGmupTemperature requirementsspawningFisheriesrZ. Pric.C(See ANSl-239.18IOPTIONAL CORY 272 (4-7'1(Forrnorly NTIE35)Dopa-mont d Commaf'ca

REGION 1Regional DirectorU.S. Fish and Wildlife ServiceLloyd Five Hundred Building, Suite 1692500 N.E. Multnornah StreetPortland, Oregon 97232REGION 2 REGION 3Regional DirectorRegional DirectorU.S. Fish and Wildlife ServiceU.S. Fish and Wildlife ServiceP.O. Box 1306Federal Building, Fort SnellingAlbuquerque, New Mexico 87 103 Twin Cities, Minnesota 55 1 1 1REGION 4Regional DirectorU.S. Fish and Wildlife ServiceRichard B. Russell Building75 Spring Street, S.W.Atlanta, Georgia 30303REGION 5 REGION 6Regional DirectorRegional DirectorU.S. Fish and Wildlife ServiceU.S. Fish and Wildlife ServiceOne Gateway Center P.O. Box 25486Newton Corner, Massachusetts 02158 Denver Federal CenterDenver, Colorado 80225REGION 7Regional DirectorU.S. Fish and Wildlife Service101 1 E. Tudor RoadAnchorage, Alaska 99503

DEPARTMENT OF THE INTERIORU.S. FISH AID WILDLIFE SERVICEAs the Nation's principal conservation agency, the Department of the Interior has responsibilityfor most of our.nationally owned public lands and natural resources. This includesfostering the wisest use of our land and water resources, protecting our fish and wildlife,preserving theenvironmental and cultural values of our national parks and historical places,and providing for the enjoyment of life through outdoor recreation. The Department assessesour energy and mineral resources and works to assure that their development is inthe best interests of all our people. The Department also has a major responsibility forAmerican Indian reservation communities and for people who live in island territories underU.S. administration.