Biogeographic effects of the Gulf of Mexico red tide dinoflagellate Karenia brevis on Mediterranean copepods

Abstract

There may be biogeographic aspects of interactions between toxic phytoplankton and their zooplankton grazers. Grazers that naturally co-occur with a given species of toxic phytoplankton may have evolved mechanisms to counteract adverse effects of phytoplankton toxins. Conversely, grazers that have no co-evolutionary experience with a toxic phytoplankter may be more susceptible to its effects. This may also be true for toxic metabolites other than neurotoxins that adversely affect copepod reproduction and development such as the well-described diatom oxylipins. In order to test this hypothesis, we investigated interactions in terms of feeding, reproductive success and survival of a copepod feeding on a potentially toxic dinoflagellate species with which the copepod does not naturally co-occur: the red tide dinoflagellate Karenia brevis from the Gulf of Mexico and the copepod Calanus helgolandicus from the Mediterranean. We additionally compared interactions of these two allopatric plankters with various co-occurring sympatric ones, including the copepod Temora stylifera and a non-neurotoxic strain of the dinoflagellate Alexandrium tamarense, both from the Mediterranean, and as a control, the non-toxic dinoflagellate Prorocentrum minimum. Both Mediterranean copepods fed, reproduced and survived on diets of K. brevis from the Gulf of Mexico, but neither copepod did as well on this diet as on diets of A. tamarense from the Mediterranean or the control diet of P. minimum. Of the two copepods, T. stylifera fared worse on a diet of K. brevis than did C. helgolandicus. Egg hatching success for both copepods feeding on K. brevis was sharply diminished over the seven days of the experiments. This was surprising, since we subsequently discovered that our culture of K. brevis contained no brevetoxins. However, our culture appeared to contain another as yet unidentified compound. Similarly, we have previously demonstrated that our culture of A. tamarense, which contained no saxitoxins or aldehydes, did contain another unidentified compound that reduced egg hatching success by inhibiting sperm from fertilizing eggs. Thus, although diets of the dinoflagellate from the Gulf of Mexico (K. brevis) caused adverse effects on both of the Mediterranean copepods, T. stylifera was also adversely affected by the Mediterranean dinoflagellate (A. tamarense). The copepod T. stylifera which co-occurs in the Gulf of Mexico with the dinoflagellate K. brevis was far more adversely affected by feeding on a sole diet of this dinoflagellate than was the Mediterranean copepod (C. helgolandicus) which never naturally co-occurs with this dinoflagellate. Thus, any putative biogeographic effects of copepods feeding on allopatric versus sympatric dinoflagellates did not clearly emerge from our data. However, since both species of dinoflagellates appear to contain additional undescribed chemicals other than the neurotoxins for which they are best known, and these compounds are potentially related to the adverse effects we recorded, it appears that understanding of the biogeography must await further defining the biochemistry of some harmful algae:grazer interactions.