Aspects of the Biology of the Northern
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Aspectsof the Biologyof the NorthernQuahog, Mercenaria mereenaria, with Emphasison Growth and Survival during Early Life History V. MONICA BRICELJ Marine Sciences Research Center StateUniversity of New York Stony Brook, NY 11794 Abstract.Key features of thebiology of Mercenaria mercenaria are reviewed with emphasis onearly life historyprocesses. Predatory mortality during juvenile stages of thenorthern quahog is identifr ed as a primaryfactor controlling recrui tment of natural populations. Predation rates are shown to be strongly modulatedboth by substrate preference and prey-size selectivity of majorpredators crabs and carnivorous gastropods!.Smaller xanthid crabs prefer heterogeneous substrates gravel and shell bottoms!, and consumequahogs ata higherrate in thesesubstrates, whereas larger, portuni d crabsprefer and forage mostelectively in homogeneoussubstrates. In contrast topredictions ofoptimal foraging theory, even largercrabs preferentially consume smaller quahogs, when a widerange of prey sizes is available, thus increasingpredation pressure on smallerquahog size classes. Underfield conditions, atnear-optimum temperatures, j uvenile M. rnercenaria exhibit mean shell growthrates of 0.8 mm week- t maximum= 1 mmwk ~!. Native populations along the east coast exhibit comparativelylowerand higher than average lifetime growth rates at the species' northern Prince Edward Island,Canada! and southern Florida! distributional limits, respectively. These extremes correlate with thelength of the gro~ing season, which is strong Ly temperature-dependent. Thus,the time to attain Legal market-sizeranges from 1.9 to ! 6yearsand averages three to four yearsin themid-portion of the northernquahog 'slatitudinal ranges Massachusetts toVirginia!. Up to a two-to three-fold variation in growthrates is typicallyobserved within a singleestuary, Three toxic/noxious algal species are identifr ed aspotentially harmful to M. mercenariaunder bloom conditions: the chrysophyte Aureococcus anophagefferens,thechlorophyte Nannochloris atomus, and the dinoflageLLate Alexandrium fundyense. Managementimplications and suggested fruitful directions for future research are discussed throughout the text. Introduction The biology of northernquahogs, Mercenaria stocks.Processes operating during early life mercenaria,has been the subjectof severalearlier history stages Figure 1! are emphasized, e,g,Belding, 1931! and more recent literature becauserecruitment successinto the fishery reviews Pmtt et al., 1992;Rice and Pechenik, appearsto be largely predeterminedduring the 1992!.Therefore this paperdoes not attemptan clams'first one to two yearsof life Malinowski, exhaustivereview, but rather,will highlight 1985;Wallace, 1991!.Poorly understoodaspects somesalient features of this species'life history of thespecies' biology will alsobe stressed, in which areof significancein managingwild orderto suggestavenues for futureresearch. Fecundity,as determined by repeated spawninginduction of matureindividuals in the laboratory, is positively correlated with body size,but highly variableamong individuals of the saine size Table 1!. M. tttercenaria shows no evidenceof reproductivesenility, or declinein reproductiveoutput or gameteviability with agelsize Bricelj andMalouf, 1980!,since older clamsproduce gametes at a levelpredicted by the powercurve relating gonadmass to body sizein youngerindividuals isometricgrowth! Peterson,1986!. Bricelj and Malouf 980! showedthat matureeggs spawned at one time by Figure 1. Schematicdiagram of major factors a singlefemale are characterized by a bimodal controlling MercenariamercerLaria recruitment to a size size-frequencydistribution, with modalpeaks at shel'Ilength of 20-25 mm! whenhard clams attai n size 67 and 81 panin diameter range= ca.50 to 97 refugefrom many of their commonpredators. See text pm!, This was confirmedby Gallagerand Mann for discussion. 986!, who found that quahog eggs separated Reproduction into thee distinct bands following density Mercenaria mercenaria is a relatively slow- gradientcentrifugation. The significanceof this growing, long-lived, dioeciousbivalve, wide rangein egg sizeshas not beendetermined. characterizedby iteroparity multiple Sinceegg sizein M mercenanais known to be reproductionsover its lifespan!, high fecundities, positivelycorrelated with egglipid content productionof planktotrophiclarvae that typically Gallager and Mann, 1986!, eggs of different remainin the planktonfor one to two weeks sizesmay be characterizedby different Camker, 1961!,and high juvenile relative to developmenttimes Clarke, 1982!or differential adult survival Malinowski and Whitlach, 1988!. viability. Thus Kraeuteret al. 982! found that Importantlife history characteristicsof this sinallereggs < 35 pm! had significantly lower speciesare summarizedin Table 1. Agmg survival than eggs > 44 pm. techniquesrely on the presenceof annualgrowth Spawningof quahogpopulations is less checks in the shell, which are typically produced synchronousand startsearlier in the yearwith during the winter in the northernand central decreasing latitude Table 3.3 in Eversole, 1989!. portion of the northernquahog's geographic The length of' the spawningseason and the range,and in the summerand early fall in frequencyof peakspawning periods also tendto southeasternstates North Carolina, Georgia, increase with decreasinglatitude. A single, and Florida! Fritz and Haven, 1983; Grizzle and annualspawning peak, occurring in the summer, Lutz, 1988; and references therein!. Longevity is characteristic of northern and rnid-Atlantic estimatesfor thespecies range widely between waters e.g. Connecticut, New York, and 23 and46 yearsbecause of the difficulty in aging Delaware!, whereas two spawning peaks in the older specimens,which show crowding of spring and fall! occur in North and South growth rings and numerousspurious growth Carolina reviewed by Eversole, 1989!, and a checks. Maxiinum size ranges between 110-111 third winter spawningmay occur in Georgia inm in shell length Rice et al., 1989;Jones et Heffernan et al., 1989! and in Florida al., 1989! and 135 mm Walker and Tenore, Hesselman et aL, 1989!. Quahogs may retain a 1984!.A long lifespan,and the coexistenceof relatively high condition index after spawning multiple year classes,will tend to buffer hard Ansell et ai., 1964; Keck et al., 1975!, and clam populationsfrom suddenpopulation crashes consequently do not experience the large causedby sporadic recruitment failure. fluctuations in meat quality and marketability associatedwith changesin the reproductivecycle chemicallymediated Keck et al., 1974;Ahn, which aretypically observedin oysters, 1990!.In the field, larval settlementand/or Crassostreaspp. e.g. Purdueet al., 1981!. retentionof postlarvaemay be enhanced in shell- Settlementof quahoglarvae is highly coveredsediment, which could providea suitable gregarious,and is stimulatedby the presenceof attachment substrateand/or refuge from predators conspecifics e,g. 3 mmjuveniles! or otherclam Carriker, 1961!,but this effect hasnot been speciessuch as Gemmagemma, which often rigorously testedunder field or laboratory ocul at high densitiesin Mercenariahabitat conditions. Flume studies show that selection Ahn, 1990!.This attractionappears to be capabihtiesof quahoglarvae for a suitable 31 settlement substrate i.e. preference for sand vs. Natural Mortality: Predation mud! areaffected by flow conditions Butmanet Predation is often considered the most al., 1988!, but the relevance of this finding to significantsource of naturalmortality, and field conditions has not been demonstrated. therebythe dominant factor controlling Studies of settlement successand post-settlement recruitment successof naturally occurring survivalof quahogshave been hindered primarily bivalves,including Mercenariamercenaria e,g. by the difficulty in efficiently segregating Virnstein, 1977; Malinowski, 1985!. postlarvaefrom sediinentgrains of comparable Vulnerability to predationis known to be size. Differential settlement was successfully strongly size-dependent,with smallestquahogs usedby Ahn 990! in small-scaleexperiments, < ca. 20 rum in shell length! suffering greatest but may not be practicalfor large-scalesampling mortalities MacKenzie, 1977; Malinowski, of a patchynatural environment. 1985!.Furthermore, modeling efforts by Interactions between adult, benthic Malinowski and Whitlach 988! demonstrated populations,through suspension-feeding activity that populationgrowth ratesof quahogswere or reworking and destabilizationof sediinents, two to four ordersof magnitudemore sensitiveto andquahog larvae are poorly understood. changesin juvenile survivorship,than to thosein Kurkowski 981! demonstratedthat adult adult survival or fecundity. These authors quahogscan readily consumeyoung veliger therefore suggestedthat stock enhancement larvae< 120 elmin laboratoryexperiments, and measures would be most effective when directed that larvaedo not surviveentrapment in towards enhancing juvenile survival e.g. pseudofeces.A negativeinteraction between throughpredator control!. In this context, adult Mercenaria stocks and settlement was also Peterson990! recentlyargued convincingly for suggestedby Rice et al. 989!, who the needto apply experimentaldata on size documentedmuch higher densities of juvenile selectivityand habitat preference of bivalve quahogsin areasof NarragansettBay, Rhode predatorsto fishery managementand resource Island, with low adult densities. enhancement efforts, Successful metamorphosis and post- settlement recruitment