Reference: Biol. Bull 175: 361-371. (December. 1988) Intraspecific Variation in Growth and Reproduction in Latitudinally Differentiated Populations of the Giant Scallop Placopecten magellanicus (Gmelin) B. A. MACDONALD AND R. J. THOMPSON Marine Sciences Research Lahoraion; Memorial University of Newfoundland, St. John's, Newfoundland A1C 5S7, Canada Abstract. The giant scallop, Placopecten magellanicus, exhibits a discrete gametogenic cycle which varies be-tween populations. In our study, spawning occurred later in scallops from New Jersey than in those from New-foundland, but there is no latitudinal trend when data from the literature are considered. Reproduction is prob-ably controlled by local environmental factors. There was high intraspecific variation in shell and so-matic growth rates, and in the production of somatic and germinal tissue. Reproductive output in particular showed great plasticity. Variation in these traits along a depth gradient on a micro-geographical scale was equal to or greater than variation on a latitudinal scale, al-though reproductive output in New Jersey scallops ex-ceeded that of scallops from Newfoundland. Enhanced reproductive output was associated with reduced lon-gevity. Introduction Many species of marine ectotherms are distributed over a wide latitudinal range and often display intraspe-cific variation in physiological characteristics and life-history strategies (Levinton, 1 983). Such species are ideal candidates for determining which environmental fac-tors, such as water temperature, that vary with latitude in a predictable manner may influence the growth and reproduction of individual animals. Causal relationships between water temperature and growth or reproductive output have proved difficult to establish unequivocally, owing to local variations in environmental conditions, such as food availability and temperature (Newell el ill.. Received 21 July 1988: accepted 20 September 1988. 1982; MacDonald and Thompson, 1985a). There is a need for studies in which intraspecific variation on a mi-cro-geographic scale is examined for a number of charac-ters and related to observations on latitudinally sepa-rated populations. With some exceptions, the general consensus in the literature is that bivalve molluscs from low latitudes grow more rapidly at ambient temperature, attain a smaller maximum size, and have a shorter lifespan than do conspecifics from higher latitudes (Newell, 1964). This view is supported by studies on several species of bivalves, including Silic/uci pcilula (Weymouth el ai, 1931) and Mytilits eilnlis (Seed, 1 976), but clear latitudi-nal trends have not been observed in others, e.g., Mya arenaria (Brousseau, 1979) and Placopecten magellani-cus (Posgay, 1979). In Macoma halthica from North America, however, maximum size is greatest in popula-tions from low latitudes, whereas in M. halthica from Eu-rope growth is faster at intermediate latitudes (Gilbert, 1973; Bachelet, 1980; Beukemaand Meehan, 1985). There is an extensive literature on the gametogenic cy-cle and the timing of spawning in many bivalve species (Giese and Pearse, 1974; Sastry, 1979; Newell el al., 1982). For several species in the northern hemisphere, spawning occurs at higher temperatures and later in the year in southern populations than in northern ones (Sas-try, 1970, 1979; Seed, 1976; Barber and Blake, 1983), and is often more synchronized at higher latitudes (Oc-kelmann, 1 95 8; Bricelj <?//.. 1987). Unfortunately, there is very little information on intraspecific variation in re-productive output and reproductive effort in latitudi-nally separated populations (Bricelj cl ai, 1987), yet these quantities are often more sensitive to environmen-tal change than is shell growth, the most commonly mea-sured variable (MacDonald and Thompson, 1985a, b). 361
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