Latitudinal Effects on Juvenile Size and Fecundity in Petaloconchus (Gastropoda)
Adult tube diameters, embryonic capsule dimensions, and hatching juvenile shell lengths are about 25% larger in the more northerly of two populations (Washington and California) of the sessile vermetid gastropod Petaloconchus montereyensis. The proximate cause of the size differences between the two populations is apparently the lower annual temperature in Washington waters which results in larger adult females. Their larger capsule glands produce larger capsules containing more eggs, but egg diameters are the same as in the southern population. Thus the number of eggs per capsule is significantly greater in the northern population and, since capsules typically produce a single juvenile snail with all but one egg serving as nurse yolk, this explains the greater size of San Juan Island juvenile snails. The number of capsules brooded at one time is similar in the two populations. Thus, while northern females do produce more eggs, as expected for larger animals, they sacrifice increased fecundity for significantly increased juvenile size. Plausible explanations include a wide range of tolerable variance around an optimized settling size in the species, a range in the size of selection agents paralleling the latitudinal cline of differences, and selection for increased hatching size to stabilize developmental time between hatching and sexual maturity. The proximate chain of relationships delineated here leads to very different results than are usually predicted by life-history theory: fecundity does not increase with body size; egg size does not predict developmental mode or hatching size; body size seems to have little to do with developmental mode. Clearly, different mechanisms for supplying developmental nutrition can lead to very different suites of life-history traits, and a priori assumptions of trade-offs can lead to false predictions for many species.
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Document Type: Research Article
Publication date: 1989-09-01
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