INTRASPECIFIC VARIATIONS IN δ13C INDICATE ONTOGENETIC DIET CHANGES IN DEPOSIT-FEEDING POLYCHAETES

Abstract

Many species change diets during development. Often, these ontogenetic changes are discrete and coincide with metamorphosis (e.g., amphibians), but more gradual niche changes can occur during growth. Identifying nondiscrete changes in diet and understanding their implications at the population and community levels are especially difficult for ecologists who study detritivores and other species that have poorly characterized diets. Theory and several lines of evidence suggest, however, that benthic juveniles of species that deposit feed as adults may be unable 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 diet, I used stable carbon isotopes as a natural diet tracer. I quantified body-size-dependent variations in the δ13C of four species of tentaculate, surface-deposit-feeding polychaetes: the ampharetid Hobsonia florida and the spionids Pseudopolydora kempi japonica, Polydora cornuta, and Pygospio elegans. In addition to worm tissues, I measured the isotopic compositions of the most likely primary producers at each field site (benthic diatoms, macroalgae, and saltmarsh grasses) to predict the worms’ carbon sources. All species showed significant size-dependent variations in δ13C. Furthermore, populations of P. kempi japonica at two different sandflats had similar ontogenetic trends despite isotopic differences in available foods at each site. Individuals fed a fixed diet in the laboratory, however, did not show significant size-dependent variation in δ13C, leaving ontogenetic changes in diet as the most parsimonious explanation of the field data. Regression analyses indicated that the gradual change in δ13C with body size was nonlinear, with most of the change in δ13C occurring before individuals reach sexual maturity. The complex life cycle of these species, therefore, includes both a radical change in niche when larvae metamorphose to juveniles and a gradual niche shift as benthic juveniles grow. The isotopic data indicate that the smallest juveniles assimilated much of their carbon from benthic diatoms (δ13C ≈ −20‰), while adults assimilated most of their carbon from detritus derived from macroalgae (δ13C ≈ −9‰) or saltmarsh grasses (δ13C ≈ −14‰). Because abundances of benthic diatoms or other high-quality components of sediment are more variable and more likely to be in limiting supply than detritus or bulk sedimentary organics, populations of species that deposit feed as adults may experience food-related recruitment bottlenecks during the juvenile stage.