SPECTROFLUOROMETRIC QUANTIFICATION OF NEUTRAL AND POLAR LIPIDS SUGGESTS A FOOD-RELATED RECRUITMENT BOTTLENECK FOR JUVENILES OF A DEPOSIT-FEEDING POLYCHAETE POPULATION

A growing body of evidence suggests that juveniles of species that dep osit feed as adults are more susceptible to food limitation than are c onspecific adults. To quantify ontogenetic variations in the nutrition al condition of the deposit-feeding polychaete Pseudopolydora kempi ja ponica, I modified existing spectrofluorometric assays of neutral and polar lipids. The hydrophobic fluorochrome Nile red fluoresces yellow- orange when bound to neutral lipids and red-orange when bound to polar lipids. The ratio of neutral lipids (energy storage) to polar lipids. (cell membranes) provides an index of nutritional condition that is sc aled to body size. For individual P. kempi japonica ranging in length from 2 to 12 mm, I quantified the florescence from each lipid class an d analyzed size-dependent variations in the neutral/polar lipid ratio. P. kempi japonica showed a size-dependent increase in the neutral/pol ar lipid ratio up to a body length of similar to 6-8 mm; Little size-d ependent change in the ratio occurred beyond this size. Ontogenetic ch anges in both particle-size selection and the assimilated diet of P. k empi japonica display size-dependent patterns that are similar to the lipid data. Together, the three lines of evidence suggest that P. kemp i japonica juveniles experience a food-related recruitment bottleneck until they grow to similar to 6-8 mm. Once juveniles grow beyond this size, they appear to forage like adults and achieve adult levels of ne utral lipid. Because deposit-feeding benthos ingest a low-quality and nutritionally dilute diet that will be difficult for small juveniles t o digest, ontogenetic niche shifts and food-related recruitment bottle necks during the juvenile stage may be common in the population ecolog y of species that deposit feed as adults.