Reduced disease in offspring: A benefit of coloniality in sunfish

Summary Increased disease and parasitism are a well-documented cost of group living for colonial birds and mammals, but we now show that disease in offspring of fish may be reduced by nesting in colonies. The aquatic fungusSaprolegnia sp., which is a common cause of egg mortality among freshwater fishes, is more prevalent in the nests of solitary than colonial male bluegill sunfish (Lepomis macrochirus). Moreover, fungal infection decreases with nest density in colonies. This may be due in part to a behavioural advantage since colonial males can devote less time to defending eggs and more time to fanning them, which reduces fungal infection. In addition, we demonstrate experimentally that solitary nests become infected at higher rates than colonial nests, even in the absence of parental males. This suggests that colonies are encountered by spores at a lower rate and/or that the large number of nests in colonies dilutes the number of fungal spores per nest. Through one or all of these mechanisms, egg mortality in colonial nests is lowered significantly. Therefore, in some cases, disease may select for group living.