Short-term starvation effects on nitrogen and phosphorus excretion by the chaetognath Sagitta enflata

Abstract

Freshly captured Sagitta enflata exhibited specific excretion rates of ammonium and phosphate (expressed as percentage body content of N or P per hour) that were not significantly related to the size of individual animals. The degree of crowding in experimental vessels was positively correlated with specific excretion rates of ammonium. Excretion rates, under conditions that precluded feeding, decreased sharply during the first several hours' incubation time, approaching the rates exhibited by animals starved overnight. The practice of holding freshly captured zooplankton for a time before determining excretion rates may seriously affect those rates, if the animals are unable to feed. Animals captured during the day in Kaneohe Bay, Hawaii, having no food items in their guts, had mean specific excretion rates (±s.d.) of 0·81±0·51% body content of N h−1 for ammonium, and 1·29±1·24% body content of P h−1 for phosphate. Minimal estimates of natural excretion rates, made from the first hour of incubation in further experiments, were 1·19±0·47% h−1 for nitrogen and 3·8±3·95% h−1 for phosphorus. Sagitta is not a large contributor to nutrient regeneration in Kaneohe Bay.