Effects of Cannibalism and Alternative Prey on Population Dynamics of Saduria Entomon (Isopoda)

Abstract

Field sampling and laboratory experiments were performed to investigate the importance of cannnibalism in the isopod Saduria entomon from the Gulf of Bothnia. In the experiments the vulnerability to cannibalism was high for small individuals. The vulnerable phase lasted °9 mo in the deep zone and 3 mo in the shallow zone. Large S. entomon individuals caught small conspecifics more efficiently than they caught the alternative prey Pontoporeia affinis (Amphipoda), and Neomysis integer (Mysidacea). Both relative and absolute densities affected the number of small conspecifies eaten. The preference for conspecifics increased with decreased with decreasing density of P. affinis. In a behavioral experiment a small but significant proportion of the small S. entomon specimens actively sensed and avoided aggregations of large conspecifics. In contrast to the findings of the laboratory experiments, only a small amount of S. entomon were found in the guts of large conspecifics from the field samples. Pontoporeia affinis was the dominant prey. Due to differences in the life history of S. entomon, conditions for cannibalism differ between shallow and deep zones, favoring higher rates of cannibalism in the deep zone. In the deep zone there were lower densities of alternative prey per predator, the vulnerable phase was three times as long, and the cannibals reached larger sizes. Calculations using information from laboratory data showed that cannabalism has the potential to cause substantial mortality in S. entomon in the deep zone also when inflated mortality rates were accounted for. In the field, the density of small S. entomon was negatively correlated with the density of large conspecifics, but positively correlated with the density of P. affinis. The size distributions of the field samples indicated a high mortality in deep—living young S. entomon. These patterns were consistent with the results of the laboratory predation experiments, which are consistent with the hypothesis that cannibalism produced these patterns.