QL155.S63no. 8211.65

Biological Report 82 (11.85)July 1986

TR EL-82-4

Lihravr;21;nl;T,l Wetlands Research CenterI!. s. yrsh md wildlife Service7 i; j (&pxionle Boulevard

Species Profiles: Life Histories andLafayette, La. 70506

Environmental Requirements of Coastal Fishesand Invertebrates (Mid-Atlantic)

AMERICAN OYSTER

Fish and Wildlife ServiceCoastal Ecology Group

Watetwavs Experiment StationU.S. Department of the Interior U.S. Army Corps of Engineers

NWRC

This page has been left blank intentionally.

Biological Report 82 (11.65)TR EL-82-4July 1986

Species Profiles: Life Histories and Environmental Requirementsof Coastal Fishes and Invertebrates (Mid-Atlantic)

AMERICAN OYSTER

Jon G. StanleyMaine Cooperative Fishery Research Unit

313 Murray HallUniversity of MaineOrono, ME 04469

and

Mark A. SellersProgram in Oceanography

University of Maine at OronoIra C. Darling CenterWalpole, ME 04573

Project ManagerLarry Shanks

Project OfficerJohn Parsons

National Wetlands Research CenterU.S. Fish and Wildlife Service

1010 Gause BoulevardSlidell, LA 70458

Performed forU.S. Army Corps of Engineers

Coastal Ecology GroupWaterways Experiment Station

Vicksburg, MS 39180

and

National Wetlands Research CenterResearch and Developmentfish and Wildlife Service

U.S. Department of the InteriorWashington, DC 20240

This series should be referenced as follows:

U.S. Fish and Wildlife Service. 1983-19 . Species profiles: life historiesand environmental requirements of coasts 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., and M.A. Sellers. 1986. Species profiles: life histories andenvironmental requirements of coastal fishes and invertebrates (Mid-Atlantic)--American oyster. U.S. Fish Wildl. Serv. Biol. Rep. 82(11.65). U.S.Army Corps of Engineers, TR EL-82-4. 25 pp.

PREFACE

This species profile is one of a series on coastal aquatic organisms,principally fish, of sport, commercial, or ecological importance. The profilesare designed to provide coastal managers, engineers, and biologists with a briefcomprehensive sketch of the biological characteristics and environmentalrequirements of 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, life history, ecological role, environmentalrequirements, and economic importance, if applicable. A three-ring binder isused for this series so that new profiles can be added as they are prepared.This project is jointly planned and financed by the U.S. Army Corps of Engineersand the U.S. Fish and Wildlife Service.

Suggestions or questions regarding this report should be directed to one ofthe following addresses.

Information Transfer SpecialistNational Wetlands Research CenterU.S. Fish and Wildlife ServiceNASA-Slide11 Computer Complex1010 Gause BoulevardSlidell, LA 70458

or

U.S. Army Engineer Waterways Experiment StationAttention: WESER-CPost Office Box 631Vicksburg, MS 39180

iii

CONVERSION TABLE

Multiply

millimeters (mm) 0.03937 inchescentimeters (cm) 0.3937 inchesmeters (m) 3.281 feetkilometers (km) 0.6214 miles

square meters cm*)square kilometers (km*)hectares (ha)

10.76 square feet0.3861 square lniles2.471 acres

liters (1) 0.2642cubic meters (m')

gallons35.31 cubic feet

cubic meters 0.000811c) acre-feet

milligrams (mg) 0.00003527 ouncesgrams (g) 0.03527 ounceskilograms (kg) 2.205 poundsmetric tons (t) 2205.0 poundsmetric tons 1.102 short tonskilocalories (kcal) 3.968 British thermal units

Celsius degrees 1.8("C) + 32 Fahrenheit degrees

inches 25.40 millimetersinches 2.54 centimetersfeet (ft) 0.3048 inetersfathoms 1.829 metersmiles (mi) 1.609 kilometersnautical miles (nmi) 1.852 kilometers

square feet (ft2)acressquare miles (mi')

0.0929 square meters0.4047 hectares2.590 square kilometers

gallons (gal) 3.785 literscubic feet (ft3) 0.02831 cubic metersacre-feet 1233.0 cubic meters

ounces (02)pounds (lb)

28.350.45360.9072

gramskilogramsmetric tonskilocalories

Metric to U.S. Customary

!Y

U.S. Customary to Metric

short tons (ton)British thermal units (Btu) 0.2520

Fahrenheit degrees 0.5556("F - 32)

iv

To Obtain

Celsius degrees

CONTENTS

Page

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiCONVERSION TABLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

NONMENCLATURE/TAXONOMY/RANGE .MORPHOLOGY/IDENTIFICATION AIDSREASON FOR INCLUSION ON SERIESLIFE HISTORY . . . . . . . . .Larvae . . . . . . . . . . .Juveniles . . . . . . . . . .Adults

GROWTH CHAiAiT;RkiI&' : : : :COMMERCIAL SHELLFISHERIES . . .Population Dynamics . . . . .

ECOLOGICAL ROLE . . . . . . . .ENVIRONMENTAL REQUIREMENTS . .Temperature . . . . . . . . .Salinity . . . . . . . . . .Substrate and Current . . .Oxygen . . . . . . . . . . .Turbidity and Sedimentation .Acidity . . . . . . . . . . .

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LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

V

ACKNOWLEDGMENTS

We thank Herbert Hidu, Professor of Zoology, University of Maine; Edwin W.Cake, Jr., Head, Oyster Biology Section, Gulf Coast Research Laboratory; MarkChatry, Marine Biological Laboratory, Louisiana Department of Wildlife andFisheries; and Dexter S. Haven, Virginia Institute of Marine Science, forreviewing the manuscript and offering many helpful suggestions.

V i

I I I I I0

I

Centimeters5

Figure 1. American oyster (Galtsoff 1964).

AMERICAN OYSTER

NOMENCLATURE/TAXONOMY/RANGE

Scientific name . . . . . Crassostreavirginica (Gmelin)

Preferred common name . . . Americanoyster (Figure 1)

Other common name . . . Eastern oysterClass . . . . . Bivalvia (Pelecypoda)Order . . . . . .Mytiloida (Pteroidea)Family. . . . . . . . . . . Ostreidae

Geographic range: The American oysterlives in estuaries and behindbarrier islands along the eastcoast of North America, from theGulf of St. Lawrence, Canada, toKey Biscayne, Florida. Its rangeextends to the Yucatan Peninsulaof Mexico and the West Indies toVenezuela. This species wassuccessfully introduced in Japan,Australia, Great Britain, Hawaii,and the west coast of NorthAmerica (Ahmed 1975). In themid-Atlantic region, the Americanoyster is most abundant in LongIsland Sound, Delaware Bay, andChesapeake Bay.

MORPHOLOGY/IDENTIFICATION AIDS

The left valve is almost alwaysthicker and heavier than the right,and more deeply cupped (Yonge 1960;Galtsoff 1964). The oyster iscemented to the substrate on its leftvalve. Hinge teeth are absent, but abuttress on the right valve fits intoa depression on the left. There is nogap between the valves when fullyclosed.

Shell shape is variable. On hardbottoms, beaks (umbones) usually arecurved and point toward the posterior,whereas in silty environments or onreefs, umbones are usually straight.Single oysters from hard substratesare rounded and ornamented with radialridges and foliated processes (Figurel), whereas those from soft substratesor reefs are more slender and aresparsely ornamented. Shell thicknessalso depends on environment. Oysterson hard substrates have thicker andless fragile shells than those on softsubstrate. The index of shape (height+ width/length) varies from 0.5 to 1.3

1

in southern populations and from 0.6to 1.2 in northern populations.

The shell grows along adorsal-ventral axis, but the angle ofthe axis is not permanent and maychange several times during thelifespan of an individual, resultingin a zigzag pattern. The growth axismay change as much as 90 degrees. Theshell is usually 10 to 15 cm long whenthe oyster is 3 to 5 years old.Although tissue mass reaches an upperlimit, the shell continues to grow,primarily in thickness, over thelifespan of the oyster (Stenzel 1971).

The American oyster is monomya-rian (anterior adductor muscle hasbeen lost). The interior of the shellhas a purple-pigmented adductor musclescar situated slightly posterior andventral. A second muscle scar, of theQuenstedt's muscle, is situatedventral to and a short distance fromthe hinge. The purple pigmentation onthe adductor muscle scar distinguishesthe American oyster from similarspecies. In the mangrove oyster (C.rhizophorae) and Pacific oyster (5.

ejliedthed miu;c;e rsic~a;;s'i$t~~

unpigmented. T& shell of themangrove oyster is less plicated thanthat of the American oyster. Thereare no other species of Crassostreasympatric with the American oyster inthe Mid-Atlantic region. Species ofCrassostrea are distinguishable fromspecies of Ostrea species by the pro-myal chamber,which is well developedin Crassostrea species, but not inOstrea. By trapping salt water, thischamber may allow the American oysterto tolerate wider fluctuations insalinity in estuaries.

Crassostrea species are oviparous(gametes arstFelaeased into theawater),whereas species incubatefertilized-in the mantle cavity.Advanced larvae of American oystersare distinguished from the larvae ofother bivalves by length-widthmeasurements and an asymmetric umbo.

The dentition on the hinge of thelarvae of the American oyster isdistinctly different from that inother bivalves (Lutz et al. 1982).

REASON FOR INCLUSION IN SERIES

The American oyster supports animportant commercial fishery from theGulf of St. Lawrence to the Gulf ofMexico, and is an important maricul-ture species. More oysters areprocessed than any other singlefishery product in the United States,and over 10,000 people work in theoyster industry. Oysters are valuedas a luxury food item. The Americanoyster is the keystone species of areef biocoenosis that includes severalhundred species (Wells 1961). Becausethe oyster inhabits estuaries, it isparticularly vulnerable to urban andindustrial disturbances.

LIFE HISTORY

Gametogenesis and spawning are 4stimulated by changes in water tem-perature (Kaufman 1978: Andrews1979a, 1979b; Kennedy and Krantz1982b). Spawning temperatures differamong populations. Based on spawningtemperature, Stauber (1950) recognizedthree physiological races, one fromthe Gulf of Mexico that spawns whenwater temperatures are near 25 "C, andtwo from the east coast that spawn at16 "C and 20 'C. Evidence forphysiological races also was reportedby Loosanoff (1969), who found thatgametes of 60% of the oysters fromLong Island Sound populations ripenedat a water temperature of 15 "C after45 days, whereas only 20% of theoysters from a New Jersey populationhad ripe gametes after 72 days at thesame temperature. At 18 "C none ofthe oysters south of New Jerseymatured in a holding pond inConnecticut, even after 3 years. InBarnegat Bay, New Jersey, oystersfirst spawn when water temperaturereaches 20 OC, but subsequent spawning

2

requires water temperatures of atleast 22 “C (Nelson 1928). In Dela-ware, native oysters matured in 150days at 15 'C (Price and Maurer 1971),which is a much longer time than thatreported by Loosanoff (1969). Thedays required for gonad maturation (D)in Long Island Sound oysters areinversely proportional to temperature(T in 'C):

D = 4.8 + 4205 e-00355T

Delaware oysters require six times aslong to ripen at water temperaturesfrom 12 to 20 "C as do Long IslandSound oysters (Price and Maurer 1971).In Chesapeake Bay, spawning occurswhen water temperatures are 21 to 24"C, with limited spawning at 15 to 20"C (Kennedy and Krantz 1982).

The variation in spawningtemperature may be caused by otherfactors. Kennedy and Krantz (1982)postulated that phytoplankton bloomsand nutrition may be responsible forstimulating spawning in Chesapeake Bayoysters.

The time and intensity ofspawning do not depend directly ontidal cycles (Loosanoff and Nomejko1951). During low tide, however, thesunlight may warm the water andstimulate spawning (Drinnan andStallworthy 1979).

Spawning is initiated by one ormore males that release their spermand a pheromone into the water. Thefemales spawn when sperm enter thewater transport system (Andrews1979a), or when pheromone stimulatesfemales to release their eggs in amass spawning (Bahr and Lanier 1981).Each female produces 23 million to 86million eggs per spawning; the numberis proportional to the size of theindividual (Davis and Chanley 1955).Individual fecundities of 15 millionto 115 million were cited by Yonge(1960). Females may spawn severaltimes in one season; as the intervalbetween spawnings increases, the

number of eggs produced per seasondecreases (Davis and Chanley 1955).The quantity of sperm produced dependson the quantity of stored glycogen ofthe male oyster at the beginning ofthe spawning season (Loosanoff andDavis 1952).

The spawning season is longer inwarmer climates: from April to Octoberin the Gulf of Mexico (Hayes and Men-zel 1981); but only in July nearPrince Edward Island, Canada (Drinnanand Stallworthy 1979). In Long IslandSound, spawning begins around July 1and lasts into August (Loosanoff andNomejko 1951). In Chesapeake Bay,Maryland, spawning is from May toSeptember; oysters in shallower waterspawn first (Beaven 1955). Oystersat an unusual depth of 40 m in coolerwaters of Patuxent River estuary donot release all their gametes duringthe spawning season (Merrill and Boss1966). Kennedy and Krantz (1982)documented the spawning season for 18beds in Chesapeake Bay. Spawningbegins in May in some beds, and Junein others. Spawning continues intoAugust in most beds and into Septemberin a few (to December in one bed).The spawning season in eastern shorebars extended over a longer periodduring the early 1960s than in 1977 to1978. In the James_ River estuary,Virginia, spawning commences in lateMay and continues until October(Loosanoff 1932). The eggs hatchabout 6 hours after fertilization atwater temperatures near 24 "C(Loosanoff 1965a).

Larvae

Oyster larvae are meroplanktonicand remain in the water column for 2to 3 weeks after hatching (Bahr andLanier 1981). In Milford Harbor,Connecticut, the larval period is 13to 16 days at 22 "C (Prytherch 1929).During the planktonic phase, thelarvae pass through several stages of;;;;;opment (Carriker and Palmer

. After the blastula (3.2 hr),

3

gastrula (4.5 hr), and trochophore (10hr) stages (Parrish 1969), the larvasecretes a straight-hinge shell anddevelops a ring of locomotory ciliacalled the velum. This prodissoconchI larva (also termed straight-hingelarva or veliger) is about 75 Pm indiameter. It develops into the pro-dissoconch II larva (also termed eyedlarva or pediveliger), which is char-acterized by pronounced umbones. Thislarva is a vigorous swimmer, and has apair of pigmented eyes and an elon-gated foot with a large byssal gland(Andrews 1979a). The prodissoconchlarva is about 0.3 mm in diameter(Galtsoff 1964).

Young larvae (prodissoconch I) inLittle Egg Harbor, New Jersey, stay inthe water column about 1.0 m below thesurface (Carriker 1951). Older larvae(prodissoconch II) are near the bottomin the halocline of estuaries duringflood tide and rise nearer the surfaceduring the ebb tide. The late stagelarvae congregate near the bottom ofDelaware Bay during slack tide and aredistributed throughout the watercolumn during flood tide (Kunkle1957). Andrews (1979a) doubted thevalidity of these findings. In thelaboratory, older larvae are stim-ulated to swim by increased salinitiesand inhibited by decreased salinities(Haskin 1964). Larvae are on the bot-tom of Milford Harbor, Connecticut,when the current is greater than 0.6m/set and drop to the bottom inholding tanks at velocities of 0.3 to0.5 m/set (Prytherch 1929). Swimmingvelocity increases by threefold at asalinity near 100% seawater (Hidu andHaskin 1978). Upward swimming is atabout 1 cm/set (Wood and Hargis 1971;Andrews 1979a). These behavioraltraits may result in selective tidaltransport so that larvae avoid beingflushed from the estuary. Generally,larvae are transported toward the headof an estuary against a net downstreamflow (Seliger et al. 1982) by usingthese behavioral responses.

Juveniles

Two to three weeks afterspawning, oyster larvae seek a solidsurface for attachment (called a set,or the process of setting) and com-mence crawling in circles (Andrews1979a). In Long Island Sound, thefirst set is 18 days after the firstspawning (Loosanoff and Nomejko 1951).After attachment with a droplet ofliquid cement exuded from a pore inthe foot, they lose the velum and footand are now calledattached oysters).

sSqpet,,,(newlyare

preferred as attachment, but stonesand other firm surfaces may be used.Spat that set during the first 3 daysafter metamorphosis may grow fasterthan those setting later (Losee 1979).Metamorphosis may be delayed if suit-able substrate is not located (Newkirket a?. 1977). Burke (1983) definedsettlement as the behavior of droppingto the bottom, in contrast to meta-morphosis, which is the irreversibledevelopmental process.

Several factors influence thesetting behavior of larvae. Hidu andHaskin (1971) suggested that risingtemperatures over tidal flats duringflood tides stimulate setting. In thelaboratory, rising temperaturetriggered setting (Lutz et al. 1970).Swimming larvae have positive photo-taxis, which becomes negative with anincrease in temperature (Bahr andLanier 1981). Light inhibits settingin holding tanks (Shaw et al. 1970)and oysters prefer to set on theundersides of shells (Ritchie andMenzel 1969). More oysters settle inthe subtidal zone than elsewhere inDelaware Bay (Hidu 1978).

Oyster larvae usually set inestablished oyster beds or where shellsubstrate is present. Crisp (1967)postulated that larvae are attractedto the proteinaceous surface of theperiostracum of adult shells andobserved that larvae did not settle on

4

shells that had been treated withbleach. Hidu (1969) demonstrated,however, that a waterborne factor,perhaps a pheromone, stimulates larvaeto settle on oyster shells. Currentsalso influence setting patterns:settlement in Delaware Bay is heaviestwhere tidal currents cut through saltmarshes (Keck et al. 1973). Althoughhigh salinities stinulate settlement,mortality increases because oysterpredators are more numerous at highsalinities (Ulanowicz et al. 1980).However,the number of adult spawnersis not correlated with the density ofspat produced (MacKenzie 1983).Reduction in the quantity of freshshells and widespread siltation limitsthe habitat suitable for setting inmost oyster beds in the mid-Atlanticregion (MacKenzie 1983).

The time of settlement variesamong locations and is generallyshorter than that of spawning. In theNiantic River estuary, Connecticut,most larvae settled in July and a fewin August (Marshall 1959). In LongIsland Sound, settlement is frommid-July to early August and againfrom late August through September(Loosanoff 1966). Large numbers oflarvae sometimes die in mid-Sumner,perhaps because of blooms ofdinoflagellates. In Delaware Bay,larvae settle from around July 4 toearly September (Maurer et al. 1971).Peaks in setting in the fivetributaries studied occurred mostly inJuly. In Chesapeake Bay, Maryland,setting is from May to October with atwo-week peak, usually in July (Beaven1955). Different tributaries inChesapeake Bay have peaks at differenttimes: July in the St. Mary's River,Island Bar and Holland Straits; July,August and September in bars in theopen bay: August and September atWreck Shoal; and July and September atYorktown Fish Pier and Page's Rock(Andrews 1951). In the James Riverestuary, Virginia, settlement was frommid-June to mid-October with peaks inmid-August and mid-September (Loosa-noff 1932).

Adults

Because adult oysters aresessile, their distribution depends onwhere the larvae set and on subsequentsurvival of the spat. Oysters typi-cally live in clumps called reefs orbeds, in which they are the dominantorganisms. The mass of shells some-times alters the currents, andincreases deposition of particulatesso that the local environment is modi-fied.

Adults are often dioecious, butalso often change genderprotandrous hermaphrodites (Bahr a::Lanier 1981). The gender and theprocess of sex inversion are geneti-cally determined by perhaps three loci(Haley 1977). Typically the youngadults are predominately males;subsequent sex inversion with age in-creases the number of females. Sexratios in the James River Estuary,Virginia, change from 90% males at 1year of age to 80% females in olderoysters (Andrews 1979a).

GROWTH CHARACTERISTICS

Oysters grow fastest during theirfirst 3 months of life (Bahr 1976).In their second year, juveniles inDelaware Bay that were 11 to 14 mmlong on April 3 were 18 to 22 mn onMay 7, 23 to 27 mm on June 5, and 26to 32 mm on July 2 (Carriker et al.1982). Whole body weight increasedfrom 0.23 g to 4.0 g in those months.In a review of growth rates in theAmerican oyster, Ingle and Dawson(1952) cited the following sizes atcorresponding age: 35 to 75 mn in 6mo and 100 to 125 mm at 4 years inLong Island Sound; 95 to 106 mn at 1year in New Jersey; 21 mm at 44 days,40 mn at 12 mo, and 90 mn at 23 mo inChesapeake Bay. Oysters in the deepcool waters of Chesapeake Bay growslower than those in shallow warmerwaters (Merrill and Boss 1966). Inthe Virginia part of Chesapeake Bayoysters weigh 24 g at the end of the

5

first year, 90 g at year 2, 105 g at3 years, and 190 g at year 4 (Andrewsand McHugh 1957). Instantaneousmonthly growth coefficients range from0.42 to 0.84 (Gillmor 1982). In themid-Atlantic region, the minimummarketable size of 90 mm is attainedin 2 to 5 years.

The growth rate of the oyster islargely governed by telrperature,salinity, intertidal exposure,turbidity, and food. Growth ceasesduring winter, except in Florida,where growth is continuous throughoutthe year (Butler 1952). Growth ofVirginia oysters is slow and the con-dition factor is low during spawningbecause energy is used for gameteproduction instead of production ofbody biomass (Haven 1962). Oystersexpend 48% of their annual energybudget in reproduction (Dame 1976).After spawning, Florida oysters gainweight before they increase in length(Butler 1952). Growth of oysters inLong Island is greatest in August andSeptember after spawning, whenglycogen reserves are restored(Loosanoff and Nomejko 1949; Price etal. 1975).

Environmental conditions affectgrowth. Oysters in a salt pond onLong Island in fluctuating salinitygrow faster than those underrelatively uniform salinity (Pierceand Conover 1954). Oysters in Maineexposed for a relatively short timeduring the tidal cycle grow at aboutthe same rate as those continuouslysubmerged (Gillmor 1982). Long expos-ure to the atmosphere, however,reduces growth; those exposed 20% ofthe time grow twice as fast as thoseexposed 60% of the time. Growth rateis directly related to phytoplanktondensity, and some of the observeddifferences in rates of growth likelyare caused by changes in phytoplanktoncomposition and abundance. In SouthCarolina, oysters grow faster innutrient-rich salt ponds than in tidalcreeks where primary productivity islower (Manzi et al. 1977). A Walford

plot predicts that oysters in SouthCarolina would cease growing when 140mm long (Dame 1971); however, oysters200 mn long occur.

COMMERCIAL SHELLFISHERIES

The American oyster has tradi-tionally supported a valuable industryalong the eastern seaboard (MacKenzie1983). Today, the commercial areas inthe Mid-Atlantic region (Figure 2) arein Long Island Sound, bays along theNew Jersey coast, Delaware Bay, baysalong the coast of Maryland andVirginia, Chesapeake Bay, andAlbemarle Sound of North Carolina.Landings have decreased from about 100million lb during the 1920's(Matthiessen 1969) to about 25 millionlb since the 1960's (Table 1). Oysterlandings have remained relativelystable in Chesapeake Bay from1950-1982, whereas landings in therest of the mid-Atlantic region havefluctuated more than 10 fold (Table1); oyster production declinedI;;;e;p

Chesapeake Bay ’ - .AIryland and Virginia arlnthe leadingproducers of oysters followed by NewYork (Table 2). Annual landings inNew York and New Jersey havefluctuated as much as 50%, but inConnecticut the annual landings havefluctuated from 136,000 lb to 1million lb, and in Delaware from 8,000to 501,000 lb. In Long Island Sound,persistent set failure is responsiblefor the decline in landings since 1920(Matthiessen 1969). A reduction inthe spreading of shells due to anincrease in the sale of oysters in theshell is partially responsible for thedecline in abundance of oysters inLong Island Sound (MacKenzie 1983).

Oysters are taken by handpickingof clumps from reefs (Bahr and Lanier1981), hand and patent tonging fromboats, and dragging and dredging fromboats (Korringa 1976). Dredging hasmade capture more efficient but italso increases the potential foroverharvest and depletion of oyster

d

d

6

.

MARYLAND ,ii.d

.7 BALTlMORE$

I,‘-^?_

ASHINGTON,D.C. \ ’

ATLANTIC OCEAN

MILES

OV100KILOMETERS

IORTH CAROLINA

Figure 2.mid-Atlantic.

Commercial production areas for the American oyster in the

7

Table 1. Oyster landings (thousands of pounds meat weight) bygeographical region for the years 1950-1982. U.S. Dep. Commerce,NOAA, Natl. Mar. Fish. Serv., Natl. Fish. Statistics Program. Annualsummaries of oyster landings, 1950-1982.

Year New England Mid-Atlantic Chesapeake South Gulf ofAtlantic Mexico

1950 4,727 18,170 29,954 3,033 12,2921951 1,970 17,410 29,598 3,783 11,5191952 2,209 16,767 34,418 4,111 14,6371953 1,038 14,462 36,946 4,019 12,8361954 735 13,377 41,587 3,811 11,4431955 619 9,848 39,227 2,260 13,8811956 506 8,466 37,064 3,656 13,5131957 405 7,981 34,234 3,069 14,3071958 276 4,296 37,530 2,651 10,4081959 387 1,392 33,322 3,516 13,7211960 500 1,154 27,111 4,119 16,0981961 453 1,921 27,500 3,984 18,2401962 294 2,362 19,939 3,850 18,8381963 452 951 18,274 4,837 24,1391964 195 1,356 22,098 3,527 23,3851965 340 757 21,188 4,082 19,1561966 408 917 21,232 3,657 17,1821967 323 1,190 25,798 3,160 21,7471968 195 1,538 22,679 2,965 26,7391969 152 1,322 22,157 1,830 19,7651970 190 1,413 24,668 1,626 17,7141971 190 1,965 25,557 1,846 20,2661972 129 3,335 24,066 1,868 18,2601973 181 3,181 25,400 1,656 14,9141974 644 2,739 25,021 1,841 14,8781975 600 3,274 22,640 1,585 19,2951976 201 3,566 20,964 1,704 21,5691977 905 2,412 18,014 1,861 19,6701978 887 2,415 21,531 2,138 l 18,2121979 1,129 3,038 20,428 2,441 15,2891980 200 2,635 21,777 2,279 16,5481981 26 2,396 22,153 2,786 17,0791982 1,195 2,352 18,134 2,650 24,158

beds. Harvesting by divers has become U.S. oyster landings. Seed oystersimportant in Maryland waters. are also harvested for transplanting

The Americandominant shellfishthe United States.of cultured oyster5valued at $37production is equal

to water with insufficient naturaloyster is the setting. About 99% of the U.S. seed

in mariculture in production comes from Virginia watersIn 1980, the yield (Alford 1975).was 23 million lbmillion. This The equivalences between differ-to 55% of the 1980 ent values reported for harvest are: 1

8

.I

L

i

Table 2. Landings (thousands of pounds) of American oyster meats in themid-Atlantic States 1977-1982. U.S. Department of Commerce, NOAA, Natl.Mar. Fish. Serv., Natl. Fish. Statistics Program. Annual summaries ofoyster landings.

State Mid-Atlantic

Year CT NY NJ DE MD VA total U.S. total

1977 136 1,059 1,225 128 13,028 4,986 20,562 46,0171978 852 798 1,553 64 13,470 8,061 24,798 50,9681979 1,058 1,355 1,675 8 12,290 8,138 24,524 48,0711980 175 1,378 756 501 14,944 5,833 23,587 49,0701981 - - - 1,635 446 315 16,546 5,607 24,549 50,0401982 1,000 1,474 632 246 11,545 6,589 21,486 54,317

bu = 34 1 = 32 kg total weight = 7.8pints = 3.4 kg meat weight (Pruder1975).

The market quality of oystersdepends on the meat size and taste,which vary with the season. InFlorida, the yield of oyster meats pershell is best in March just beforespawning and lowest in the summermonths during spawning (Rockwood andMazek 1977). In the lower ChesapeakeBay, meat yields are best in June andagain in October and November (DexterS. Haven, Virginia Inst. of Mar. Sci.,pers. comm.). The reduced yields insummer correspond to a reducedcondition index following spawning(Hopkins et al. 1954; Lawrence andScott 1982). Glycogen, which givesoysters their sweet taste, is highestin flesh of Maryland oysters in Apriland May just before spawning (Sidwellet al. 1979).

Population Dynamics

The vast majority of the eggs andlarvae produced by American oystersperish before setting. Followingspawning, oyster larvae are abundantplankters; densities have ranged from2,000 to 5,50O/kl in Virginia coastalwaters (Andrews 1979a; Seliger et al.1982), 100 to 17,00O/kl in Long IslandSound (Loosanoff and Nomejko 1951),

and 24,00O/kl in Gardiners Bay of LongIsland Sound (Carriker 1959) and inthe deeper waters of Delaware Bay(Hidu and Haskin 1971). The concen-tration of larvae near shore inDelaware Bay was only l,OOO/kl.Abundance was greater after high tidein the James River Estuary in Virginia(Andrews 1979a). In the St. Mary'sRiver Estuary in Maryland the numbersper kiloliter were 2,000 to 5,000straight-hinge larvae, 100 to 1,000early umbo larvae, and 50 to 200 lateumbo larvae (Manning and Whaley 1955).

The daily mortality of larvae inCanada was about 10% (Drinnan andStallworthy 1979), and for spat inMassachusetts was about 90% monthly(Krantz and Chamberlin 1978).

The density of newly set spatfluctuates greatly from year to yearand is different even in adjacentareas. Reported densities range from0.35-500 spat/shell (Andrews 1949,1955; Loosanoff and Nomejko 1951;Carriker 1959; Webster and Shaw 1968;Kennedy 1980). Typically there havebeen 60 spat/bu of natural shells(range 0.6 to 72/bu) in the Marylandportion of Chesapeake Bay (Krantz andMeritt 1977).

Survival of spat in Long IslandSound ranged from 0 to 14% in their

9

first summer of life (Loosanoff andEngle 1940). Spat mortality of 50% to70% in Delaware Bay was reduced to 30%to 40% when spat were protected byscreens from larger predators (Tweed1973). Survival of spat to seed (3 to4 months) in Eastern Bay, Maryland,was less than 10% (Engle 1956).Survival in estuaries of the ChoptankRiver, Maryland, ranges from 1 to 27%through the first season (Webster andShaw 1968). Of 314 spat per shellthat settled, only 14 were still aliveat the end of the season in the JamesRiver estuary, Virginia (Andrews1949). Spat survival was less indense sets than in sparse sets in theJames River, Virginia (Andrews 1955)and in Chesapeake Bay (Webster andShaw 1968). The number of seed persquare meter in Long Island Sound was200 to 10,000; for l- to 2-year-oldsit was 300; and for 3- to 4-year-oldsit was 75 (MacKenzie 1981).

The population density of oystersin Delaware Bay was about 70 bu/acrein natural beds and from 10 to 375bu/acre in planted beds (Maurer et al.1971). The density of 70 bulacre wasabout 80 kg/ha, including shell, orabout 800 oysters/ha, assuming anaverage weight of 100 g. InChesapeake Bay, the density was about70 bu/acre in natural beds and 500bu/acre in planted beds (D. S. Haven,pers. comm.).

American oysters that survivetheir first year of life are subjectto relatively little mortality exceptthat inflicted by shellfishermen orcaused by disease outbreaks. In LongIsland Sound the annual naturalmortality of adults was only 4%, e.g.,77% survived after 4 years (MacKenzie1981). Adult survival in South Caro-lina was 85% in a salt pond and 40% inan estuary (Manzi and Burrell 1977).Mortality was size dependent. Mortal-ity of adults 25 to 50 mm long was 19%per month in summer (28% in July),but those 50 to 75 mm long werelost at the rate of 5% per month (near0% in July) and those greater than

75mm long died at a rate of less than1% per month (Dame 1976). Survivalwas 100% in areas protected from heavywaves in North Carolina, and 50% permonth if exposed to relatively heavywaves (Ortega 1981). Mortalities weremuch higher in the areas of DelawareBay and lower Chesapeake Bay whereoysters were infected with MSX disease(Andrews 1968).

ECOLOGICAL ROLE

Oyster larvae feed largely onplankton, particularly small, nakedflagellates (Chrysophyta), accordingto Guillard (1957). At moderatetemperatures larval growth is bestwith a diet of naked flagellates,whereas at temperatures above 27 "Cnaked algae are scarce and chloro-phytes are much more abundant as food(Davis and Calabrese 1964). Thelarvae, unlike adults, do not consumebacteria (Davis 1953). Oyster larvaeare food for a wide variety of filterfeeders (Andrews 1979a).

Adult oysters filter largequantities of brackish water andremove naked flagellates. They mosteffectively filter particles in the 3-to 4-urn size range (Haven and Morales-Alamo 1970). For each gram of dryweight of tissue, an oyster held at21 'C filters 1.5 l/hr (Palmer 1980).At somewhat higher temperatures about8 l/hr are filtered (Langefoss andMaurer 1975). The volume of waterfiltered per hour was 1,500 times thevolume of the oyster's body (Loosanoffand Nomejko 1946). The filtrationrate is independent of the availablefood supply, the stage of tide, ortime of day. If food is absent,however, the valves are closed most ofthe time (Higgins 1980). InChesapeake Bay, oysters ingested thepredominate diatom plankton whichchanges seasonally (Morse 1945).Dinoflagellates, ostracods, smalleggs, and terrestrial pollen were alsoingested.

10

The oyster is the dominantspecies of a diverse community inbrackish waters. Over 40 macrofaunalspecies or groups live in oyster beds(Bahr and Lanier 1981) and the numberof species in an oyster communitysometimes exceeds 300 (Wells 1961).Oysters were responsible for 88% ofthe respiration of an oyster reef(Bahr and Lanier 1981).

Oysters have a variety of dis-eases and parasites and are preyedupon by several carnivores (Galtsoff1964). The bacteria Vibrio andPseudomonas sometimes kiliters.In the laboratory Vibrio exotoxin at47pg/l kills oyster embryos-andRoland 1984). The protozoan pathogenPerkinsus marinum infects oysters fromDelaware to Mexico. Thehaplosporidian protozoan Minchinianelsoni is responsible for the disease-and Minchinia costalis for SSO(seaside organism) (Andrews 1979b;1982a). Minchinia nelsoni, commonfrom North Carolina to Massachusetts(Krantz et al. 1972), caused extensiveoyster mortality in the Delaware Bayin 1957 and in Chesapeake Bay in 1960(Andrews 1968). It also killed largenumbers of oysters in the bays alongthe coast of Maryland and Virginia in1960 (Rosenfield 1971).

Predators often limit theabundance of oysters, especially ifsalinities are above 15 ppt (MacKenzie1983). In the mid-Atlantic region,the gastropod oyster drills

d& southern(Urosal inx cinerea and Eupleura 4pu

(Thais haemastoma), the w~~ltker(Bu~~:~~canaliculatuIm),he starfish (Asterias

.r)Callinectes

and the crab (Cancer irror--' andCarcinus maenas) des

sapidus,-

of ovsterZ7GZTtsofftroy large numbers1964). All sizes

of oysters are killed but small sizesare affected most by oyster drills,which bore through the shells with acombination of chemical dissolution ofthe shell and radular rasping. Smalloysters are preyed on by crabs andstarfish. The widespread boring

sponge Cliona weakens the shell andthus lowers quality (Schlesselman1955). Spat are preyed upon by theflatworm Stylochus(MacKenzie 1970: Christensen

elli;;;;;:

The bay anemone (Diadumene leucolena)consumed 0.6 to 4.9 oyster larvae perminute in the laboratory and alsofeeds on larvae in the naturalenvironment1979).

(Steinberg and KFnnedyOver 100 bay anemones/m have

been known to occupy oyster beds inChesapeake Bay (MacKenzie 1977). Thesouthern oyster drill consumes 2.4spat/day under optimum conditions(Garton and Stickle 1980). In astudy of oyster mortality in Long Is-land Sound, MacKenzie (1981) estimatedthat starfish inflicted 25% mortalityon spat in 5.5 months, mud crabs 12%,and oyster drills 5%. For adultoysters near Norwalk, New York,starfish cause 3.2% of the annualmortality, oyster drills 0.296, andsuffocation 0.2%. In New Haven, Con-necticut, starfish had no impact, oys-ter drills killed 0.5%/year, and suf-focation killed 2.9%/year. The oysterdrills are the primary predators ofoysters1973).

in Chesapeake Bay (LippsonIn the Gulf of Mexico, oysters

are preyed on by a variety of otherpredators (Cake 1983).

Major competitors of the oysterfor space on substrate include theslipper shells (Cre idula .spp.) andthe jingle shells Anomla spp.) as---+well as barnacles (Balanus eburneusand Chthamalus fragilis)nd otheroysters that set on adult shells (Mac-Kenzie 1970; 1983). Shells with heavyfouling by barnacles have only about25% as many spat as clean shells(Manning 1953). Heavy sets ofbarnacles reduce the hard surfaceavailable for oyster spat and thusreduce oyster settlement (Ingle 1951);therefore oyster spat are limited toareas relatively free of barnacles andbryozoans (Beaven 1955). The musselBrachiodontes exustus may also competewith oysters (Ortega 1981). Youngoysters may be smothered by excreta ofpolychaete worms of the genus

11

Polydora, or by excreta of adultoysters (Stenzel 1971). Blooms of redtjde (Cdchlodinium heterolobatum) atconcentrations of 5nlledoyster larvae (Ho and Zubkoff 1979).Competition by the slipper shells andbarnacles may limit numbers of oystersin Long Island Sound (MacKenzie 1981;1983). Tunicates and encrustingsponges are also major competitors forspace (D. S. Haven, pers. comm.).

ENVIRONMENTAL REQUIREMENTS

The American oyster typicallylives in shallow, well-mixed estuar-ies, lagoons, and nearshore bays, andtolerates widely fluctuating watertemperatures, salinities, andsuspended solid concentrations(Andrews 1979a). Because of thetolerance of extreme fluctuations inenvironmental conditions, the environ-mental requirements of oysters aredifficult to define.

Temperature

Differences in thermal require-ments of oysters from different areashave led to the postulation that racesmay be separated on the basis ofdifferent temperature requirements(Ahmed 1975). Approximate spawningtemperatures for three distinct raceswere 16 'C for the northern race (NewEngland), 20 Y for the mid-Atlanticrace, and 25 'C for the Gulf of Mexicorace (Stauber 1950). Menzel (1955)found that ciliary activity continuedat 0 'C in the New England oysters butceased at 6 'C in oysters from themid-Atlantic. Andrews (1979a)suggested there are other races aswell, but genetic studies did notclosely support the existence ofphysiological races (Buroker et al.1979). All oysters studied by Burokeret al. (1979) were geneticallyidentical, except those from NovaScotia and Florida. These populationswere 82% similar, about the level of

similarity between the American andthe mangrove oyster, which can suc-cessfully hybridize (Menzel 1968).According to Groue and Lester (1982)American oysters in Laguna Madre,Texas, are genetically distinct fromfour other gulf populations. Measure-ment of isozymes in genetic studies,however, may not validate these races.

In the mid-Atlantic coastalwaters, oysters spawn when watertemperatures are somewhat above 20 'C.Gonads do not develop at watertemperatures below 10 "C, and 16 'C isneeded for gonadal maturation (Loosa-noff and Davis 1952). Exposure to a35 'C water temperature acceleratedgametogenesis and spawning, but subse-quent spawning in the same season wasprevented (Quick 1971). In laboratorytests, embryos developed normally atwater temperatures between 20" and30 "C but abnormalities increasedprogressively when water temperaturesdeclined to 15 'C or rose to 35 'C(MacInnes and Calabrese 1979). Thepercentage of abnormal embryosincreased from 2% at 25 'C to 12% at30 'C (Dupuy 1975). The growth oflarvae was impaired by water temper-atures of 30 'C or more and even abrief exposure for 10 min at 40 'Cretarded growth (Hidu et al. 1974).In contrast a temperature range of27 to 32 'C is optimum for fastestgrowth and highest survival of larvaein Long Island Sound (Davis andCalabrese 1964).

Adults tolerate water tempera-tures from a low of -2 "C in NewEngland to a high of 36 'C in the Gulfof Mexico. At low tide, oysters maybe exposed to and survive air temper-atures ranging from well belowfreezing to above 49 'C (Galtsoff1964). High temperatures sometimesincrease the mortality rate; tempera-tures above 35 'C for an entire tidalcycle may kill some or all oysters(Tinsman and Maurer 1974). The criti-cal thermal maximum for the Americanoyster is 48 'C (Henderson 1929).

Oysters can tolerate freezing of theirtissues, and sometimes revive afterthawing (Loosanoff 1965a).

Optimum water temperatures forcarrying out various life functions inthe adult American oyster are 20 to 30"C. Optimum temperatures for maximumpumping rates were 20 to 25 'C(Collier 1951) or at 28 to 32 “C(Loosanoff 1958). Growth stops inwaters with temperatures below about 8'C (Price et al. 1975). Oysters at 2to 7 OC are inactive. The thresholdfor feeding is 3 'C (Haven and Moral-es-Alamo 1966). Exposure tounseasonably high temperatures inwinter stimulates growth if food isavailable (Ruddy et al. 1975). Growthis possible between 6 and 32 "C butthe optimum is about 26 "C (Galtsoff1964).

Salinity

Oysters prefer waters ofrelatively high salinity. Whensalinity is above about 20 ppt, marinepredators flourish and destroy largenumbers of oysters. Oysters usuallylive in brackish waters or in areas ofunstable salinity that are unsuitablefor marine predators. In upperChesapeake Bay, for example, spatdensity was positively correlated withhigh salinity, whereas oyster harvestwas negatively correlated with highsalinity (Ulanowicz et al. 1980).

Salinities above 7 ppt are re-quired for spawning (Loosanoff 1948).Embryos developed normally atsalinities of 16 to 30 ppt (Maclnnesand Calabrese 1979). Larvae toleratedsalinities of 3 to 31 ppt (Carriker1951)) but grow fastest and survivedbest at salinities above 12 ppt (Davisand Calabrese 1964). In Virginia,salinities of 20 to 35 ppt wererequired for normal embryo developmentand the optimum was 28 ppt (Castagnaand Chanley 1973). In the laboratory,almost no embryos of Long Island Soundoysters developed below 15 ppt; thepercentage progressively increased to

a salinity of 22.5 ppt (Davis 1958).As salinity increased further to 35PPt, abnormalities increased, and alldied above 40 ppt. Differences insalinity tolerance of larvae are ex-plained by the acclimation of theadults before spawning. Larvaeproduced from the spawning of Marylandoysters acclimated to the salinity ofLong Island Sound (26 ppt) had thesame salinity tolerance as larvae fromLong Island Sound oysters. Marylandoyster larvae, however, were much moretolerant of low salinities if theparents were acclimated to 9 ppt water(similar to Chesapeake Bay). Thelarvae from parents adapted to lowsalinity developed normally at 10 ppt,and most survived at 7.5 ppt.Development stopped at 22.5 ppt, whichwould be the optimum salinity if theparents had been held at highersalinities.

Most larvae in a New Jersey es-tuary were in the halocline at salin-ities above 5 ppt (Carriker 1951).Larvae of Virginia oysters of parentsin high salinity water did notmetamorphose below 17.5 ppt (Castagnaand Chanley 1973), a finding obviouslynot applicable to populations livingin low saline waters. Optimum salini-ties for the growth of spat in lowerChesapeake Bay were 15 to 22 ppt(Chanley 1957) and in Long IslandSound, 17.5 to 22.5 ppt (Davis 1958).Oysters in Chesapeake Bay did not growat salinities below 5 ppt (Abbe 1982).

Adult oysters tolerate a salinityrange of 5 to 32 ppt. Outside ofthis range of salinities they dis-continue feeding and reproducing. Theoptimum salinity range in Long IslandSound is 10 to 28 ppt (Loosanoff1965a) and in Delaware Bay, 14 to 28ppt (Maurer and Watling 1973). Loosa-noff (1965b) found that many oysterssurvive 3 ppt for 30 days. Large num-bers, however, die during prolongedfreshwater inflow from the JamesRiver, Virginia (Andrews et al. 1959;Andrews 1973). Similar observationswere made in Mobile Bay, Alabama (May

13

1972), and in the Santee River, SouthCarolina (Burrell 1977). Salinitiesduring high freshwater inflow werebelow 2 ppt in the Santee River. Manyoysters died in the Beaufort Inlet,North Carolina, after exposure to asalinity near 5 ppt for about a month(Wells 1961). Oysters in Louisianadied after 14 days at 6 ppt (Andersonand Anderson 1975). In Delaware Bay,oysters survived salinities as low as2 ppt (Maurer et al. 1971). Thelength of time oysters survive lowsalinities evidently depends on theabruptness of the changes in salinity.

Low salinities inhibit gonadalmaturation in oysters in ChesapeakeBay (Butler 1949) and Long IslandSound (Loosanoff 1953). Reproductivefailure may be a direct effect of sa-linity or might be caused by inade-quate feeding at low salinity.

Substrate and Current

The preferred habitats in shallowestuarine waters are mud flats andoffshore bars (Hidu 1968), and oysterreefs (Bahr and Lanier 1981). Maximumdensity of spat is on horizontalsurfaces (Clime 1976).

Oysters grow equally well onshells, rocky bottoms, or on thickmud, capable of supporting theoysters' weight. Soft muddysubstrates may be improved by addingclam or oyster shells. Oyster shellsfrom muddy substrates are more slenderthan those from hard substrates(Galtsoff 1964). In Delaware Baymariculture sites, oysters preferredto set on the bottom rather than onpanels suspended in the water column,and preferred subtidal waters (Hidu1978).

Currents are particularlyimportant to the larvae and adults ofthe American oyster. Larvae aretransported by currents. Theyposition themselves in the ebb andflow of tidal currents to remain inestuaries. Excessive currents,

however, may prevent settlement (Cake1983). Since the volume of water im-mediately above an oyster bed must becompletely flushed about 72 timesevery 24 hr for maximum feeding, oys-ters require currents (Galtsoff 1964).Tidal flows of 156 to 260 cm/set orhigher are needed for optimum growthin Mississippi (Veal et al. 1972).Currents over oyster bars in BeaufortInlet and the Newport River estuary inNorth Carolina were 11 to 66 cm/set(Wells 1961). Currents of 150 to 600cm/set were measured above oyster barsin Delaware Bay (Hidu and Haskin1971)) and 180 cm/set in MilfordHarbor, Connecticut (Prytherch 1929).Turbulent currents that carry sand andpebbles, however, can damage shells byeroding shell surfaces (Galtsoff1964). A velocity of 150 cm/setcaused unattached oysters to tumblealong the bottom of Long Island Sound(MacKenzie 1981). In Delaware Bay,oyster abundance was greatest in areasof scour where current kept the bedsfree of sediments (Keck et al. 1973).Oysters died in one week if coveredwith sediment at a water temperature *\of 20 "C, and in 2 days if the watertemperature was 25 'C (Dunnington1968). Currents are also necessaryfor removal of pseudofeces and feces(Lund 1957).

Oxygen

In one study, the hourly oxygenconsumption was 39 ml/kg for a wholeanimal including the shell or 303ml/kg of wet tissue (Hammen 1969).Oxygen consumption increases withincreasing temperature; QlO values(the factor by which a reactionvelocity is increased by an increasein temperature of 10 "C) ranged from1.2 to 2.3 for gill tissue and 2.7 to4.2 for mantle tissue (Bass 1977).

The rate of oxygen consumption byoysters increases as temperatureincreases and salinities decrease(Figure 3). Oysters exposed toprolonged periods of low salinitiesclosed their shells and died of anoxia

14

for over a decade, and high harvestpressure.

Figure 3. Oxygen consumption in Ameri-can oyster adults as a function ofsalinity and temperature (Shumway1982).

in the James River and RappahannockRiver estuaries (Andrews 1982b). Oys-ters are facultative anaerobes and areable to survive daily exposure to lowoxygen. They also are known tosurvive anaerobically for 3 days afterspawning (Galtsoff 1964). Oxygenconsumption is zero when the valves

Lare closed (Hammen 1969). In a studyin Delaware Bay, oxygen concentrationsover oyster reefs ranged from 1 to 12mg/l (Maurer et al. 1971). Thedecline in oyster harvest in Chesa-peake Bay in recent years may becaused by decreased concentrations ofoxygen, although more establishedfactors include the outbreak of MSXdisease in 1981-1983, poor spat set

Turbidity and Sedimentation

Oysters tolerate water with largeamounts of suspended solids, but thepumping rate decreases with increasingconcentrations of suspended solids.Pumping is reduced 70% to 85% over therange 0 to 1 g/l, depending on thenature of the suspended sediment(Loosanoff and Tommers 1948). In thelaboratory the growth of larvae wasreduced at concentrations ofparticulates above 0.75 g/l (Davis andHidu 1969). In natural environments,oysters apparently develop and growbetter in waters with more suspendedsolids in oyster beds than in waterswith less suspended particulates(Rhoads 1973). Storms and hurricanesmay destroy oyster reefs by coveringthem with sediment.

Acidity

Oyster embryos develop normallywithin a pH range of 6.8 to 8.8 anddevelop abnormally at a pH above 9.0or below 6.5 (Calabrese and Davis1966; Calabrese 1972). Larvaetolerate the same pH range as embryosbut growth is fastest at a pH of 8.2to 8.5.

15

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19

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20

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21

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Veliger

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22

L

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* ’

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24

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25

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L Abstract (Limit: ZOO word*)

Species profiles are literature summaries of the taxonomy, morphology, range, life historyand environmental requirements of coastal aquatic species. They are designed to assist inenvironmental impact assessment. The American oyster (Crassostrea virginica)is an impor-tant commercial and mariculture species. It is the dominant species in many bays andoyster shells form extensive reefs that modify sedimentation and local currents.. Spawningoccurs repeatedly during warmer months with millions of eggs released. Embryos and larvaeare carried by currents throughout the estuaries and oceanic bays where oysters occur.The few surviving larvae cement themselves to solid objects for the remainder of life.

Unable to move, they must tolerate changes in the environment that range from -1.7'to 49 OC, 5 to 30 ppt salainity, and clear to muddy water. The distribution and abundanceof adults are limited by marine predators, so that oysters are limited largely to brackishwaters.

_.- -~ ___ -_ -.

Estuaries Sediments OxygenOysters Feeding habits CompetitionGrowth Life cyclesFeeding

b. Idw,llfi,n/Oc..n.End.d TumrAmerican oysterCrassostrea vjrginicaSalinity requirementsTemperature requirements

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DEPARTMENT OF THE INTERIORU.S. FISH AND WIlDLIfE SERVICE

As the Nation’s principal conservation agency, the Department of the Interior has respon-sibility for 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 thaenvironmental and cultural values of our national parks and historical places,and providing for the enjoyment of life through outdoor recreation. The Department as-sesses our 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.