Parasite-associated growth enhancement in a fish-cestode system

Parasites impose an energetic cost upon their hosts, yet, paradoxically ins tances have been reported in which infection is associated with enhanced, r ather than diminished, host growth rates. Field studies of these parasite e ffects are problematic, since the pre-infection condition of the hosts is g enerally unknown. Here, we describe a laboratory experiment in which the gr owth rate and body condition of 76 laboratory-reared three-spined stickleba ck fishes were examined before, during and after each fish was fed the infe ctive stage of the parasitic cestode Schistocephalus solidus. Twenty-one of these fishes went on to become infected by the cestode. Fishes were indivi dually housed and provided with an abundant food supply to eliminate the po tentially masking effects of variable competitive ability. Infection occurr ed independently of fish gender, size, body condition or pre-exposure growt h rate. After exposure to the cestode, infected fishes grew faster (excludi ng parasite weight) and maintained a similar or better body condition compa red with uninfected fishes, despite developing enlarged spleens. The accele rated growth could not be explained by reduced gonadal development. This re sult, one of few demonstrations of parasite-associated growth enhancement i n fishes, is discussed with respect to other such parasite systems.