Ontogenetic differences in swimming behavior of fish exposed to the harmful dinoflagellate Cochlodinium polykrikoides

Blooms of the ichthyotoxic dinoflagellate Cochlodinium polykrikoides present both lethal and sublethal threats to coastal marine organisms. Because prior studies of this harmful algal bloom (HAB) species have focused on its acute toxic effects on fish, there remains a limited understanding of the sublethal effects on the swimming behavior of fish when exposed to these blooms. We conducted a video-based laboratory assessment of the effects of C. polykrikoides exposures on larval and juvenile fish swimming behavior. Juvenile and larval life stages of three forage species common to the U.S. East Coast were examined: wild Atlantic Silversides Menidia menidia, hatcheryreared Inland Silversides M. beryllina, and hatchery-reared Sheepshead Minnow Cyprinodon variegatus. Results showed that juvenile Atlantic Silversides and Inland Silversides exposed to C. polykrikoides concentrations of 102 cells/mL swam significantly further distances in comparison with their baseline (control) behavior. In an ecosystem context, the results of this research support field-based studies demonstrating that when blooms of this HAB species are present, the relative abundance of fish declines (i.e., the fish presumably detect the blooms and swim away after exposure). Conversely, juvenile Sheepshead Minnow exhibited no increased swimming behavior when exposed, supporting previous studies indicating that this species is more resistant to C. polykrikoides toxicity. Importantly, no behavioral changes were found in experiments with larval conspecifics (i.e., Inland Silversides and Sheepshead Minnow), suggesting that younger life stages, which lack developed gills and olfactory systems, may be unable to detect the dinoflagellate. This clear ontogenetic difference implies that for certain fish species, reaching later life stages may provide a refuge to HAB toxicity.