Captive rearing reduces the sensitivity of Acartia tonsa copepods to predator cues

Abstract

Captive populations are often subject to different selective pressures than their wild counterparts, which could result in trait differences between these populations. This study investigates the effect of captive rearing on the swimming behavior and escape responses of Acartia tonsa, a species of marine copepod zooplankton that use hydromechanical signal detection to aid in finding food, locating mates, and avoiding predation. As captive populations of A. tonsa experience reduced interspecific predation and higher population densities compared to wild populations, it was hypothesized that these differences may drive adaptive evolution of swimming behavior in this species. Several components of routine swimming were compared (swimming speed, number of hops, distance of hops, frequency of hops) for groups of captive-reared and wild-caught A. tonsa, revealing that wild-caught copepods swim faster and hop more frequently than captive-reared copepods. However, when the escape responses of the captive-reared and wild-caught populations were compared using an artificial predator mimic, no significant differences were found in the number of sequential hops performed during the escape response, the maximum velocity of the response, or the total distance traveled during the response. Although the escape responses performed by the captive-reared and wild-caught copepods were similar, the captive-reared individuals often showed no response to the artificial predator mimic (34 % of individuals responded to the predator mimic), whereas wild individuals almost always showed a response (96 % responded). This suggests that captive rearing may have resulted in reduced sensitivity to hydromechanical signals in captive copepods compared to wild copepods, as responding to these signals in a predator-free captive environment would impose an unnecessary energy cost. This study offers new insight into how captive-rearing may impact copepod populations and provides evidence of how predator-driven evolution and density dependent selection may influence the behavior of copepod species.