INTRASPECIFIC VARIATIONS IN DELTA-C-13 INDICATE ONTOGENIC DIET CHANGES IN DEPOSIT-FEEDING POLYCHAETES

Many species change diets during development. Often, these ontogenetic changes are discrete and coincide with metamorphosis (e.g., amphibian s), but more gradual niche changes can occur during growth. Identifyin g nondiscrete changes in diet and understanding their implications at the population and community levels are especially difficult for ecolo gists who study detritivores and other species that have poorly charac terized diets. Theory and several lines of evidence suggest, however, that benthic juveniles of species that deposit feed as adults may be u nable to meet their nutritional demands by deposit feeding. To reject the null hypothesis that both juveniles and adults of deposit-feeding species assimilate the same diet and to infer ontogenetic changes in d iet, I used stable carbon isotopes as a natural diet tracer. I quantif ied body-size-dependent variations in the delta(13)C of four species o f tentaculate, surface-deposit-feeding polychaetes: the ampharetid Hob sonia florida and the spionids Pseudopolydora kempi japonica, Polydora cornuta, and Pygospio elegans. In addition to worm tissues, I measure d the isotopic compositions of the most likely primary producers at ea ch field site (benthic diatoms, macroalgae, and saltmarsh grasses) to predict the worms' carbon sources. All species showed significant size -dependent variations in delta(13)C. Furthermore, populations of P. ke mpi japonica at two different sandflats had similar ontogenetic trends despite isotopic differences in available foods at each site. Individ uals fed a fixed diet in the laboratory, however, did not show signifi cant size-dependent variation in delta(13)C, leaving ontogenetic chang es in diet as the most parsimonious explanation of the field data. Reg ression analyses indicated that the gradual change in delta(13)C(= wit h body size was nonlinear, with most of the change in delta(13)C occur ring before individuals reach sexual maturity. The complex life cycle of these species, therefore, includes both a radical change in niche w hen larvae metamorphose to juveniles and a gradual niche shift as bent hic juveniles grow. The isotopic data indicate that the smallest juven iles assimilated much of their carbon from benthic diatoms (delta(13)C approximate to -20 parts per thousand), while adults assimilated most of their carbon from detritus derived from macroalgae (delta(13)C app roximate to -9 parts per thousand) or saltmarsh grasses (delta(13)C ap proximate to -14 parts per thousand). Because abundances of benthic di atoms or other high-quality components of sediment are more variable a nd more likely to be in limiting supply than detritus or bulk sediment ary organics, populations of species that deposit feed as adults may e xperience food-related recruitment bottlenecks during the juvenile sta ge.