Vertical migration of the demersal copepod Psevdodiaptomus as a means of predator avoidance

Abstract

We tested the hypothesis that mortality due to visual predation is reduced by vertical migration of the demersal copepod Pseudodiaptomus (P. cornutus Nicholls and P. colefaxi Bayly) in Westernport Bay, Australia. Late copepodites and adults of Pseudodiaptomus remain near the bottom by day, rising into the water column at night or on cloudy days. Vertical migration was most pronounced in ovigerous females while early copepodites did not migrate. In aquarium experiments, the yellow-eye mullet, Aldrichetta forsten (Cuvier & Valenciennes), a common visual planktivore, preyed most heavily on ovigerous female Pseudodiaptomus and least on early copepodites. Furthermore, predation was heavier on adults of Pseudodiaptomus than on those of Acartia tranteri, Bradford an abundant copepod that is not demersal. Pseudodiaptomus does not feed while on the bottom by day, although its feeding is not inhibited by light. The rate of egg production by Pseudodiaptomus with a continuous food supply was not different from that of females given an intermittent supply. In contrast, egg production by Acartia tranteri was significantly lower with a discontinuous supply of food than with a continuous supply. This apparently relates to lipid storage, since the proportion of lipid in dry wt of Pseudodiaptomus females was 32 ± 9% compared to 17 ± 2% for Acartia tranteri. The demersal behaviour of Pseudodiaptomus therefore imposes no penalty, and is a suitable way of avoiding visual predators.