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 lusitanicu m on the reproductive success of the calanoid copepod Acartia clausi was ex amined in the laboratory. Experiments were carried out to investigate the f unctional response of feeding and fecundity of copepod females at increasin g concentrations (200 to 1600 mu g C l(-1)) of either the toxic A. lusitani cum or the nontoxic Rhodomonas baltica as food sources. Additional experime nts were performed to determine if prolonged exposure to A. lusitanicum aff ects copepod survival and fecundity. Results demonstrate that A. clausi fed on toxic cells at high rates without lethal effects and was able to produc e eggs. Survival of females was similar with both diets. Depending on the f ood source, different functional responses were found. Feeding and fecundit y of A. clausi on a diet of R. baltica followed simultaneously a typical sa tiation 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. W hereas feeding rates increased linearily with increasing food concentration s, 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 c alculated gross growth efficiencies for females feeding on both algae indic ated 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 adverse ly affected. It is suggested that ingested toxins probably interfere with d igestive processes or cause an enhanced energy expenditure due to detoxific ation because copepods could cope with toxic algae. As a result, less energ y is available and this might explain the reduced fecundity in females.