Factors extending gastropod embryonic development and their selective cost.

Pre-hatching developmental times for prosobranch gastropods are greatly influenced by temperature and taxonomic affinity. If the data used here (including all available data from the Muricacea) are a representative sample, then reasonably accurate estimates of developmental time can be obtained for most prosobranchs knowing only temperature and taxon. Times are also significantly affected by egg or hatching size. Correlations between developmental time and hatching form are probably accounted for by egg size. Prehatching periods are little, if at all, longer for metamorphosed hatchlings than for swimming hatchlings; in any event, differences are small relative to typical free swimming periods. Therefore, the planktonic period is a substantial addition to the total pre-juvenile period. Many embryos die before hatching. More would survive if development were faster; development is, therefore, prolonged at a measurable selective cost. Factors promoting extended developmental periods should be evaluated with these costs in mind. For example, providing much of the yolk as nurse-eggs may allow a species to have a large hatching size and at the same time a relatively brief developmental time.