Chemical defense of the red tide dinoflagellate Karenia brevis against rotifer grazing

In order to test whether phytoplankton such as the red tide dinoflagellate Karenia brevis negatively affect rotifer grazers and what mechanisms are involved in these interactions, we conducted laboratory feeding experiments using K. brevis cultured from Gulf of Mexico coastal waters and two species of rotifers, one cooccurring and the other from an inland sea in Russia. The co‐occurring rotifer Brachionus ibericus did not ingest K. brevis as a sole diet but tolerated K. brevis in a diet mixed with Rhodomonas lens , whereas the allopatric rotifer, Brachionus plicatilis , rejected K. brevis in either diet, although B. plicatilis started feeding on the mixed diet after a delay of 4 days. The deterrence of K. brevis was mediated by its cellular organic extracts and not by exudates, with compounds other than brevetoxins PbTx‐2, PbTx‐3, and PbTx‐9 implicated in deterrence. The finding that closely related rotifers respond differently to K. brevis and to its chemical defense suggests that a shared evolutionary history may have led B. ibericus to tolerate K. brevis in a mixed diet.