The effect of epibiont ciliates on the behavior and mating success of the copepod Eurytemora affinis

• Epibionts were able to colonize the entire copepod body inducing high mortality.
• Epibionts affected swimming activity of copepod males and reduced mating success.
• The swimming activity of copepod females was less affected than males.
• The surviving copepods were able to recover their reproductive activity.

Epibiotic relationships in aquatic ecosystems usually involve planktonic micro-crustaceans (e.g. copepods) as hosts, and a variety of other microorganisms (e.g. bacteria, algae, ciliates) as epibionts. Several studies have reported the presence of epibionts on various species of calanoid copepods. However, the consequences of copepod behavior have not been extensively studied. Epibionts can impede reproduction and mating and increase predation pressure on copepods by limiting their swimming and escape capabilities.In late winter of 2009, we collected the copepod Eurytemora affinis from the Seine estuary and maintained them in the laboratory. We cultured copepods infested by the ciliate epibiont Zoothamnium sp. separately from uninfested copepods for one month under laboratory conditions. We then recorded the behavior of both the heavily infested and the clean copepods using 2D infrared video techniques, and analyzed the trajectories obtained using appropriate statistical methods. The trajectories revealed that the presence of large numbers of epibionts could negatively affect the behavior of the host, in terms of swimming activity (break, cruise, sink, and jump). Females were less affected than males. The high proportion of infestation also dramatically affected mating success. Mating was rare and practically nonexistent after one week in culture.This study indicates some of the difficulties that E. affinis could face during critical periods of its life span. In late winter, the population is mainly composed of old copepods, in low temperature water. Limited food resources and the presence of epibionts on their bodies eventually weaken the copepods, thus impairing their mating success and decreasing their survival rates.