Conspecific alarm cue sensitivity by the estuarine calanoid copepod,Paracartia longipatella

Sensitivity to chemical cues associated with predation threat has been well observed in many freshwater zooplankters, yet few studies have highlighted such sensitivity in eury‐ and stenohaline metazoans. We aimed to assess sensitivity to conspecific chemical alarm cues in the estuarine copepod, Paracartia longipatella. Alarm cues associated with predation have been shown to have population level effects on certain zooplanktonic species. As such, we assessed the occurrence of such effects on population dynamics of P. longipatella over a 12 day period. Using experimental in situ mesocosms, we compared P. longipatella adult, copepodite and nauplii numbers between three treatments; one inoculated with conspecific alarm cues, one containing direct predation pressure (zooplanktivorous fish), and a control treatment containing no predation threat. Trends in population abundances were similar between the direct predation and alarm cue treatments for the six days of the experiment, decreasing in abundance. During the latter half of the study, however, P. longipatella abundances in the alarm cue treatment increased, while those in the presence of direct predation continued to decrease. In the treatment absent of any predation threat, P. longipatella abundances increased consistently over time for the duration of the study. We suggest that P. longipatella are indeed sensitive to conspecific alarm cues associated with predation threat. Furthermore, we propose that prolonged exposure to conspecific alarm cues in the absence of any real threat results in a reduction in sensitive to these cues.