RESOURCE-ALLOCATION, DEMOGRAPHY AND THE RADIATION OF LIFE- HISTORIES IN ROUGH PERIWINKLES (GASTOPODA)
Authors: Hughes RN.,
Year of publication:1995
Journal: Hydrobiologia, , Volume: 309(1-3), Pages: 1-14.
Publisher: Kluwer Academic Publ
Applicability of life-history theory to higher levels of comparison (from populations, through ecotypes to sibling species) was investigated in rough periwinkles, whose life histories have diversified since colonization 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. neglecta (with an annual life history) and between the sibling species L, saxatilis and L. arcana, the latter of which retains the ancestral oviparity. Resource-allocation priority, reproductive effort and related trade offs were compared between the ecotypes and the sibling species by measuring changes in flesh mass and reproductive output in snails subjected to different degrees of food deprivation, and by measuring mortality rate of snails stressed by desiccation, high temperature and low salinity. Body size had a marked effect on all parameters, but after statistically removing this effect there remained no significant differences 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 based on density-dependent versus density- independent mortality regimes could not explain these trends. Instead, size at birth may have reflected the mechanical, physiological or biological nature of mortality risk rather than its density dependence or independence. Adult size reflected the available sizes of crevices used for shelter and perhaps also the quality of feeding conditions. Radiation of life histories within 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 pebble 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 necessary for life within empty barnacle tests. This suite of adaptations allows exploitation of a relatively benign microhabitat that occurs almost ubiquitously on exposed rocky shores of the temperate North Atlantic. The persistence of oviparous forms, presumably in the face of competition from sympatric ovoviviparous forms, remains unexplained.