Estimation of predatory impact of Pleurobrachia rhodopis (cydippid ctenophore) in the northwestern Mediterranean Sea: in situ observations and laboratory experiments

The predation potential of Pleurobrachia rhodopis in the northwestern Mediterranean was studied using two methods. In laboratory experiments, adult copepods ( Acarria clausi ) were used as prey in a large range of concentrations and in two fixed volumes of 1 and 5 l beakers. We found, for three defined size classes of predators, that the clearance rate was independent of the food availability. In all prey concentrations used, there was no satiation of P.rhodopis . In 1 l beakers, P.rhodopis spent twice as much time searching for prey as in 5 l beakers and feeding rates in the larger volume were always superior to those of the smaller one. In situ observations of P.rhodopis and copepod populations were monitored during 1994 and 1995 in the bay of Villefranche-sur-Mer. Predation impact of the ctenophore on copepods was calculated by combining experimental water volume cleared of copepods with in situ abundances of prey and predators. The maximum predation impact was 0.6% of the copepod populations per day; thus, the predation pressure of P.rhodopis in the bay is not signifi cant for the establishment or maintenance of the copepod populations.