Risk of Predation of Parasitized Sticklebacks (Gasterosteus Aculeatus L.) Under Competition for Food

Abstract

Three-spined sticklebacks which were either unparasitized, heavily infested by plerocercoids of the cestode Schistocephalus solidus, or parasitized by the sporozoan Glugea anomala were singly offered to choose between Tubifex worms at different distances from a live fish predator. The uninfested fisn preferred the worms most distant from the predator and fed at a lower rate than in its absence. Almost all sticklebacks infested by S. solidus, however, did not react to the predator; they fed at all distances equally often and at the same rate as with no predator present. Those infested by G. anomala fed at an intermediate rate between healthy fish and ones carrying S. solidus but stayed even farther away from the predator than uninfested fish. Both the increased need for food and the reduced fleeing ability of the parasitized sticklebacks do not seem to provide a sufficient functional explanation for the behavioural changes assuming that the infested fish balance differently the conflicting demands of predator avoidance and feeding. The fearless feeding of the fish infested by S. solidus which has to end up in a fish eating bird and the fearful behaviour of the sticklebacks parasitized by G. anomala for which the stickleback is the only host could both have been influenced by parasitic manipulation. During intraspecific competition for food between a stickleback infested either by S. solidus or G. anomala and an unparasitized one in the presence of the predator the healthy fish were clearly outcompeted by the parasitized ones. The overall competitive disadvantage of the parasitized fish can therefore be mitigated under the risk of predation.