SELF-REGULATING MECHANISMS IN CANNIBALISTIC POPULATIONS OF JUVENILE SHORE CRABS CARCINUS MAENAS

Abstract

Juvenile cannibalism may play an important role in population regulation of marine organisms with juvenile stages that aggregate in nursery habitats. I conducted a series of field and laboratory experiments to assess whether three potential density-dependent mechanisms could regulate local recruitment of juvenile shore crabs Carcinus maenas in Swedish nursery areas: (1) intracohort cannibalism, (2) juvenile cannibals' functional response to smaller conspecific prey densities, and (3) juvenile cannibals' numerical effect on prey survival. Juvenile cannibals in mesocosms displayed a type III (sigmoid) density-dependent functional response, resulting in significantly higher proportional mortality of smaller conspecifics at high prey densities. Consistent with the laboratory result, cage-enclosed juvenile cannibals in a field experiment eliminated conspecific settlement peaks in mussel habitats and decoupled the relationship between larval and settlement abundance within three days. Density-dependent interference between cannibals in mesocosms decreased per capita predation rates, but conspecific prey mortality still increased with cannibal densities when the abundance of cannibals passed a critical threshold, suggesting that the recruitment success of consecutive cohorts may be negatively correlated. This was supported in a field experiment where the abundance of juvenile cannibals explained 71% of the variation in settlement loss from uncaged mussel habitats. Density-dependent cannibalism also occurred within juvenile cohorts even though food was provided in excess, but the resulting mortality was too low to eliminate initial density patterns over a 30-day period. The results demonstrate that cannibalism between juvenile shore crabs can cause high and strongly density-dependent mortality at natural densities, and that both a functional response and a numerical effect of larger cannibals can regulate the recruitment of new cohorts within days of settlement. A coupling of cannibal and settlement densities on a regional scale is expected to decrease recruitment variability within and between local populations. Moreover, the interaction between the type III functional response, mutual interference, and density-dependent dispersal of cannibals is predicted to reduce cannibalistic rates at low prey and high cannibal densities, respectively, and stabilize local population dynamics in relation to the abundance of juvenile habitats, consistent with field observations of shore crab populations in the study area.