RESOURCE-ALLOCATION, DEMOGRAPHY AND THE RADIATION OF LIFE-HISTORIES IN ROUGH PERIWINKLES (GASTOPODA)

Applicability of life-history theory to higher levels of comparison (f rom populations, through ecotypes to sibling species) was investigated in rough periwinkles, whose life histories have diversified since col onization of the North Atlantic by an oviparous ancestor in the upper Pliocene. Comparisons were made among populations of the ovoviviparous Littorina saxatilis, between L. saxatilis and its ecotype, L. neglect a (with an annual life history) and between the sibling species L, sax atilis and L. arcana, the latter of which retains the ancestral ovipar ity. Resource-allocation priority, reproductive effort and related tra de offs were compared between the ecotypes and the sibling species by measuring changes in flesh mass and reproductive output in snails subj ected to different degrees of food deprivation, and by measuring morta lity rate of snails stressed by desiccation, high temperature and low salinity. Body size had a marked effect on all parameters, but after s tatistically removing this effect there remained no significant differ ences in allocation among ecotypes or species. Published demographical data were reviewed for correlations between habitat, mortality regime and life-history characteristics. Populations of L, saxatilis varied principally in size at birth and in adult size. Theoretical premises b ased on density-dependent versus density-independent mortality regimes could not explain these trends. Instead, size at birth may have refle cted the mechanical, physiological or biological nature of mortality r isk rather than its density dependence or independence. Adult size ref lected the available sizes of crevices used for shelter and perhaps al so the quality of feeding conditions. Radiation of life histories with in the rough periwinkles is interpreted as a series of adaptations to a progressively wider range of habitats. The transition from oviparity to ovoviviparity allows colonization of estuaries, saltmarshes and pe bble beaches too hazardous for naked egg masses. The transition from a perennial to an annual life history in barnacle ecotypes follows from allometric re-scaling of morphological and physiological parameters, enabling reproduction and brooding to occur at the small body size nec essary for life within empty barnacle tests. This suite of adaptations allows exploitation of a relatively benign microhabitat that occurs a lmost ubiquitously on exposed rocky shores of the temperate North Atla ntic. The persistence of oviparous forms, presumably in the face of co mpetition from sympatric ovoviviparous forms, remains unexplained.