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 deposit feed as adults are more susceptible to food limitation than are conspecific adults. To quantify ontogenetic variations in the nutritional condition of the depositfeeding polychaete Pseudopolydora kempi japonica , 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 scaled to body size. For individual P. kempi japonica ranging in length from 2 to 12 mm, I quantified the florescence from each lipid class and analyzed sizedependent variations in the neutral/polar lipid ratio. P. kempi japonica showed a size‐dependent increase in the neutral/polar lipid ratio up to a body length of −6–8 mm; little size‐dependent change in the ratio occurred beyond this size. Ontogenetic changes in both particle‐size selection and the assimilated diet of P. kempi japonica display size‐dependent patterns that are similar to the lipid data. Together, the three lines of evidence suggest that P. kempi japonica juveniles experience a food‐related recruitment bottleneck until they grow to −6–8 mm. Once juveniles grow beyond this size, they appear to forage like adults and achieve adult levels of neutral lipid. Because deposit‐feeding benthos ingest a low‐quality and nutritionally dilute diet that will be difficult for small juveniles to digest, ontogenetic niche shifts and food‐related recruitment bottlenecks during the juvenile stage may be common in the population ecology of species that deposit feed as adults.