Interactions between red tide microalgae and herbivorous zooplankton:: the noxious effects of Gyrodinium corsicum (Dinophyceae) on Acartia grani (Copepoda: Calanoida)

Gyrodinium corsicum is a dinoflagellate responsible for recurrent water discolourations during winter in Alfacs Bay (Ebre River delta, NW Mediterranean). Since first detected in 1994, episodic mortality of mussels and fish was attributed to this organism, although no direct evidence was obtained. In order to establish the direct role of the dinoflagellate in the mortality of marine fauna, we have studied the effects of G.corsicum on a potential predator, the co-occurring planktonic copepod Acartia grani. Female A.grani were exposed to different concentrations of intact cells, and <5 mu m and <0.2 mu m filtrates of the dinoflagellate. At concentrations of greater than or equal to 3100 cells ml(-1), G.corsicum reduced feeding and egg production rates, and caused paralysis and death of A,grani. Low dinoflagellate concentrations (630 cells ml(-1)) or the <5 mu m and <0.2 mu m filtrates were harmless, at least for the duration of the experiment. Our study demonstrated that a possible poisoning effect due to the ingestion of the dinoflagellate by the copepod must be disregarded, and the toxic effects of soluble substances excreted by the dinoflagellate, or accompanying free-living toxic bacteria, were not causes of copepad paralysis and death. Scanning electron microscopy of female A.grani exposed to G.corsicum indicated a clear tendency of the dinoflagellate to adhere to the copepod cuticle at preferential zones like the urosome and the antennae. Apparently, direct contact between the copepod and the dinoflagellate is a prerequisite for the noxious effects. Amongst other ecological consequences, the development of G.corsicum blooms represents a drastic reduction of grazing pressure in the system through the control of the herbivorous populations. Whether mechanical or chemical effects are the primary reasons for the observed effects is unknown, and to elucidate this will require further research.