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

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.