Feeding, reproduction and toxin accumulation by the copepods Acartia bifilosa and Eurytemora affinis in the presence of the toxic cyanobacterium Nodularia spumigena
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Feeding, reproduction and accumulation of cyanobacterial toxins by the calanoid copepods Acartia bifilosa and Eurytemora affinis were studied during a cruise in the northern Baltic Sea. The experiments were carried out using both mixtures of natural plankton communities, mixtures containing the toxic Nodularia spumigena, and diets containing only the toxic cyanobacterium. Both copepod species had a high survival and fed actively on N. spumigena, both as a single food source and when offered in mixtures. Feeding on N. spumigena resulted in the detection of nodularin equivalents in the animals. However, there was a negative relationship between the gross growth efficiency and accumulated toxins, which indicates that the food quality was not ideal, possibly related to a high metabolic cost to cope with ingested toxins. Overall low egg production rates by both species and low egg hatching success by A. bifilosa in natural seawater suggested that the copepods were food-limited in the environment. The presence of Brachiomonas submarina offered in combination with N. spumigena enhanced A. bifilosa egg production, but not egg hatching success. Egg hatching success was not affected by increasing concentrations of N. spumigena in the diet. Instead, lack of food seemed to be a more important factor. Similar responses by E. affinis populations from sites with different history of toxin occurrence suggest that tolerance to cyanobacterial toxins has evolved in the Baltic Sea. This has possibly been guaranteed by genetic exchange between the 2 populations. These results suggest that N. spumigena is not directly harmful to copepods if an alternative food source is available, even though reproduction is not sustained if the species is offered as a single diet. Moreover, even if both copepods might act as a link transporting toxins to higher trophic levels, a very small fraction of the estimated ingested toxin was found in the animals, therefore the relative importance of this pathway seems limited.