The reproductive biology of the intertidal prosobranch Searlesia dira (Reeve, 1846) was examined with special attention given to variability in the nurse egg to embryo ratio among capsules, among clutches and among geographically isolated populations. Embryos and nurse eggs were distributed among the capsules in a manner consistent with the hypothesis that nurse eggs were genetically predetermined, that each female had a genetically defined nurse egg to embryo ratio, and that each capsule represented a random sample of that ratio. The binomial distribution of embryos and nurse eggs among the capsules resulted in some capsules receiving many more embryos per nurse egg than others. The number of nurse eggs an embryo succeeded in eating was proportional to the number of capsule-mates sharing a capsule. Embryos eating more nurse eggs hatched out at a larger size. Differences in the nurse egg to embryo ratios among capsules in the same clutch were much larger than that of the mean ratios among clutches. Among-site differences in the mean nurse egg to embryo ratios suggest that selection pressure for different mean hatching sizes may have acted on the mean nurse egg to embryo ratios. In contrast to the predictions of optimal hatching size theory, hatching size varied widely within clutches as a consequence of differences in nurse egg to embryo ratios among capsules. This variance may be adaptive for species that lay their eggs months before juveniles emerge into an unpredictable environment, or simply be a consequence of an imperfect mechanism for increasing hatching size. © 1983.
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