Reference: Biol. Bull. 162: 299-310. (June, 1982) TEMPORAL VARIATION IN THE REPRODUCTIVE CYCLE OF MYTILUS EDULIS L. (BIVALVIA, MYTILIDAE) FROM LOCALITIES ON THE EAST COAST OF THE UNITED STATES 1 ROGER I. E. NEWELL 2 , THOMAS J. HILBISH, RICHARD K. KOEHN, AND CHRISTINE J. NEWELL 2 Department of Ecology and Evolution, State University of New York, Stony Brook, New York 1 1 794 ABSTRACT The reproductive condition of seven latitudinally separated populations of the mussel Mytilus edulis on the east coast of the United States was determined using histological analysis and stereology. Differences in the timing of various phases of the gametogenic cycle among populations did not have any discernible latitudinal trend. Two populations at the same latitude on Long Island, N. Y. had the greatest temporal differences observed in gametogenic cycle, with summer reproduction maxima separated by a 3-month interval. There was no difference in the water temperature regime between these two habitats and thus the rate of gametogenic development was not a constant function of temperature. The observed differences in the gametogenic cycle were attributed to temporal and quantitative differences between habitats in the energy content of the mussel's food supply. INTRODUCTION Marine benthic bivalves have a cyclical pattern of reproduction which can be divided into three phases: gametogenesis and vitellogenesis; spawning and fertil-ization; larval development and growth. Each species has a variety of adaptations, both genetic and nongenetic, which coordinate these reproductive events with the environment so as to maximize reproductive success. It has been well documented (for review see Sastry, 1979) that the duration of each stage of the cycle may be variable among species and the gametogenic cycle may either be annual, semian-nual, or continuous depending upon the species and environment. Although the reproductive cycles of numerous bivalve species have been described (reviewed by: Giese, 1959; Sastry, 1975, 1979; Seed, 1976; Andrews, 1979) there is still only partial understanding of the complex interactions between exogenous (e.g. food availability, temperature, salinity, etc.} and endogenous (nutrient reserves, hor-monal cycle, genotype, etc.} variables that determine the initiation and duration of the various phases of the cycle, and thus ensure synchrony of gamete development within a population. This is of prime importance for dioecious species having ex-ternal fertilization, which requires synchronized liberation of gametes. Gametogenesis in Mytilus edulis is, in common with other boreo-temperate marine bivalves, a complex succession of events that eventually results in the for-Received 14 September 1981; accepted 17 February 1982. Abbreviation: GVF, gamete volume fraction. ' Contribution number 1293 from the Center for Environmental and Estuarine Studies, University of Maryland, and 401 in Ecology and Evolution at the State University of New York at Stony Brook. 2 Present Address: Horn Point Environmental Laboratories, University of Maryland, Box 775, Cam-bridge, Maryland 21613. 299
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