Recruitment in the predacious isopod Saduria entomon (L.): alternative prey reduces cannibalism

Abstract

Time series data (1983–1995) from the sublittoral zone of the northern Bothnian Sea, northern Sweden, were analysed to investigate the effects of the density of the cannibalistic isopod Saduria entomon and its alternative prey Monoporeia affinis (Lindström) (Amphipoda) on the recruitment success of Saduria entomon. In addition, a laboratory experiment was performed to investigate the effects of large S. entomon at six different densities of Monoporeia affinis on survival and growth of young-of-the-year (0 +) Saduria entomon. The effects of Monoporeia affinis density on the survival of large Saduria entomon was also analysed. In the time series analysis the density of 0 + and offspring survival of S. entomon was positively correlated with Monoporeia affinis density. There was no positive correlation between the density of 0 + Saduria entomon and the reproductive output of the isopods. The survival of S. entomon offspring showed no negative correlation with large S. entomon density. Thus, the recruitment of S. entomon was mainly dependent on Monoporeia affinis density, rather than low density of large Saduria entomon, or high reproductive output of the isopods. In the laboratory experiment there was a high mortality of 0 + S. entomon due to cannibalism from large conspecifics. There was no impact from Monoporeia affinis on survival or growth of the 0 + isopods. The mortality of large Saduria entomon did not depend on Monoporeia affinis density and was mainly restricted to the smaller isopods. It is suggested that the mortality is due to size-specific cannibalism. Thus, successful recruitment in Saduria entomon should be facilitated by high Monoporeia affinis density because of the high cannibalism on 0 + isopods at low M. affinis densities, particularly since M. affinis has no negative effects on 0 + isopods even at high amphipod densities.