Rapid gain and loss of evolutionary resistance to the harmful dinoflagellate Cochlodinium polykrikoides in the copepod Acartia tonsa

Evolutionary resistance of the copepod Acartia tonsa to the dinoflagellate Cochlodinium polykrikoides was investigated to establish whether zooplankton can rapidly adapt to harmful algal blooms (HABs). Copepod resistance was evaluated by egg production rates when feeding on C. polykrikoides relative to the nontoxic flagellate Rhodomonas lens. An experiment with six geographically separate A. tonsa populations demonstrated that the copepods collected during C. polykrikoides blooms within eastern Long Island bays were significantly more resistant to C. polykrikoides than their conspecifics in nearby nonbloom regions. An artificial selection experiment with a naïve A. tonsa population demonstrated that copepod resistance significantly increased when individuals were chronically exposed to C. polykrikoides (three times greater than the control after four generations). Following a two-generation relaxation of selection, however, the elevated resistance in A. tonsa was completely lost. Such rapid gain and loss of resistance suggests that copepod adaptation to C. polykrikoides in the bays of eastern Long Island may mainly result from the selection of resistant genotypes by an ongoing bloom event rather than prior HABs, which could shape trophic interactions between zooplankton and harmful algae.