Abstract
The brackish-water mysid Neomysis mercedis in tidal marshes of the Fraser River produced overwintering and summer generations that differed in life-history traits. Summer generation females matured at a smaller size and produced fewer and larger eggs than the overwintering generation. Size-adjusted clutch weights were identical for summer and overwintering females. Reproductive effort was slightly lower for the overwintering females. Both generations were iteroparous, but the average frequency of breeding was higher for the overwintering generation. Seasonal variations in reproductive traits were strongly linked to fluctuations in the relative mortality rates of neonates and adults. Overwintering adults that bred in late spring had lower mortality rates than neonates, while mortality rates for summer adults were higher than those for neonates. Rearing suggested that changes in adult body size were a phenotypic response to temperature. Food availability had little additional effect on adult body size. A positive correlation between ambient water temperatures and the increase in mortality with increasing adult size provided a possible mechanism through which temperature-dependent phenotypic variation in adult body size could be selected.
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