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Population Dynamics and Adaptive Sexual Strategies in a Brackish Water Crustacean, Gammarus duebeni

Caroline Naylor, J. Adams and P. Greenwood
Journal of Animal Ecology
Vol. 57, No. 2 (Jun., 1988), pp. 493-507
DOI: 10.2307/4920
Stable URL: http://www.jstor.org/stable/4920
Page Count: 15
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Population Dynamics and Adaptive Sexual Strategies in a Brackish Water Crustacean, Gammarus duebeni
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Abstract

(1) Gammarus duebeni is a brackish water shrimp with a sex-determining system cued by daylength. Males are produced in long day photoperiods and females in short days. Populations are characterized by fluctuating sex ratios and marked sexual size dimorphism. Males are always larger than the female they carry in precopula. (2) The life cycle of G. duebeni in northern England is annual and is restricted by temperature. Overwintering adults breed early in the year and die by August. Generations do not overlap. (3) Sex ratio during breeding is female biased but then becomes male biased due to massive recruitment of young males early in the year. Recruitment late in the year is, by contrast, exclusively female. (4) Field and laboratory data confirm that sex ratio changes are due to sex differential production rather than differences in growth or mortality. The seasonal pattern of male and female production is precisely in accord with the pattern of photosensitive sex determination described in the laboratory. (5) Fitness increases with size for both sexes: larger females produce more eggs and larger males are paired with larger females. However, because of the mating pattern, males benefit relatively more from large size than do females. (6) Early produced animals have a predictably long growth period and will be large in the next year; they maximize fitness by becoming male. Late animals are growth restricted and minimize loss of fitness by becoming female. (7) In populations where generations overlap, season no longer determines future sex-specific size-related fitness. Environmental sex determination will not then be adaptive and sex ratio fluctuations should not occur.

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