Repression of fecundity in the neritic copepod Acartia clausi exposed to the toxic dinoflagellate Alexandrium lusitanicum:: relationship between feeding and egg production

The effect of the saxitoxin-producing dinoflagellate Alexandrium lusitanicum on the reproductive success of the calanoid copepod Acartia clausi was examined in the laboratory. Experiments were carried out to investigate the functional response of feeding and fecundity of copepod females at increasing concentrations (200 to 1600 mu g C l(-1)) of either the toxic A. lusitanicum or the nontoxic Rhodomonas baltica as food sources. Additional experiments were performed to determine if prolonged exposure to A. lusitanicum affects copepod survival and fecundity. Results demonstrate that A. clausi fed on toxic cells at high rates without lethal effects and was able to produce eggs. Survival of females was similar with both diets. Depending on the food source, different functional responses were found. Feeding and fecundity of A. clausi on a diet of R. baltica followed simultaneously a typical satiation response. Fecundity was high and attained maximal rates of 32 to 36 eggs female(-1) d(-1). In contrast, functional responses of ingestion and fecundity by A. clausi fed on A. lusitanicum were not closely associated. Whereas feeding rates increased linearily with increasing food concentrations, egg production was limited and stayed constant at 16 to 20 eggs female(-1) d(-1) over the range of food concentrations offered. The comparison of calculated gross growth efficiencies for females feeding on both algae indicated an inefficient utilization of ingested toxic food. High feeding rates on toxic A. lusitanicum suggest that saxitoxins do not act as allelopathic chemicals against grazing in A. clausi. Nevertheless, fecundity was adversely affected. It is suggested that ingested toxins probably interfere with digestive processes or cause an enhanced energy expenditure due to detoxification because copepods could cope with toxic algae. As a result, less energy is available and this might explain the reduced fecundity in females.