Seasonal biochemical composition and energy sources of Sagitta hispida

There are few available data on the biochemical composition of warm-water zooplankton in general, and chaetognaths in particular. Unlike populations from higher latitudes, many species probably breed to some extent throughout the year, with life cycles measured in weeks rather than months or years. Analysis of protein, fat, carbohydrate and ash in the chaetognath Sagitta hispida Conant over 1 year showed that, although protein is always the largest component, averaging 53% of the dry weight, it fluctuates widely. The non-protein fraction of the total nitrogen also fluctuates and averages over a third of the total. Experimentally fed and starved animals showed no such protein variability, which was ascribed, there-fore, to changes in environmental parameters other than food availability (e.g. salinity). Starved animals used up body protein, and the O:N ratio in freshly-caught animals also indicated a protein-based metabolism. Periods of starvation of at least 1/4 the length of its life cycle could be tolerated. S. hispida may be added to the list of a variety of planktonic groups over a range of latitudes and feeding habits, which appear to utilize protein as a normal energy source and reserve material.