THE EFFECT OF SCHISTOCEPHALUS-SOLIDUS (CESTODA, PSEUDOPHYLLIDEA) ON THE FORAGING AND SHOALING BEHAVIOR OF 3-SPINED STICKLEBACKS, GASTEROSTEUS-ACULEATUS

In this paper we review recent experimental work on the effects of the parasite Schistocephalus solidus (Cestoda: Pseudophyllidea) on the fe eding behaviour of three-spined sticklebacks (Gasterosteus aculeatus L .). We also discuss how increased feeding motivation and subsequent al tered foraging behaviour may be a mechanism for parasite-associated ch anges in the shoaling behaviour of infected sticklebacks. The presence of S. solidus plerocercoids in the body cavity constricts the stomach , increases the handling time for large prey and consequently reduces the profitability of such prey for infected fish. This is reflected in a switch in dietary preference from large to small prey in the labora tory and in altered stomach contents and impaired nutrient reserves in the wild. By altering their hosts' nutritional slate by direct compet ition for nutrients from digested food (and possibly indirectly by alt ering diet and reducing competitive ability) and also by altering the fishes' appearance, such parasites have the potential to alter the cos ts and benefits involved in joining a shoal of conspecifics. Experimen tal work on the shoaling decisions of S. solidus-infected sticklebacks supports this hypothesis, and such behavioural modification is discus sed in the context of the manipulation hypothesis of parasite transmis sion.