GROUP FORAGING IN A MARINE GASTROPOD PREDATOR - BENEFITS AND COSTS TO INDIVIDUALS

We studied the benefits of group foraging for individuals in the marine gastropod Stramonita (=Thais) haemastoma, which preys on the oyster Crassostrea virginica along the coast of Louisiana, USA, From 40 to 58% of the snails at 2 sites on 2 dates were feeding with at least 1 additional snail, suggesting group feeding occurs frequently in the field. In the laboratory, we studied the feeding of solitary foragers as well as groups of 4 snails, and found that, for snails feeding in groups on both small (< 70 g wet mass) and large oyster prey, per capita feeding rates actually decreased. However, the fraction of tissue removed per oyster was on average 21% greater for the snails feeding in groups, resulting in similar per capita oyster tissue consumption rates and growth rates for solitary and group foragers. Inter-feeding intervals also decreased for snails feeding in groups. Oyster-handling times were similar for snails feeding in groups and for solitary snails, indicating Little benefit in subduing prey, but handling times did increase with prey size, suggesting large oysters may be more difficult to consume. Feeding rates and percent consumption were also lower for large oysters, and snails fed in larger groups when feeding on larger oyster prey. In the group feeding treatment, snails joined oysters already under attack much more frequently than they initiated an attack, joined with others to initiate an attack, or fed alone. These trends were also more pronounced for snails feeding on larger prey, and most snails pursued a mixed strategy, both initiating and joining oyster prey. Most additional foragers joined early in the foraging bout. We suggest several explanations for the existence of group foraging: not that it increases individual feeding rates, but that it occurs instead because (1) there is no measurable cost in rates of tissue consumption for individuals feeding in groups, (2) not all oyster tissue can be removed by solitary foragers, (3) there is no mechanism available for snails to defend a prey, and (4) it may provide a mechanism for handling larger oyster prey.