Filial cannibalism in spottail darters (Etheostoma squamiceps) includes the targeted removal of infected eggs

Abstract

The ubiquity of freshwater molds makes them important sources of disease in aquatic organisms and their eggs and has selected for defense mechanisms in fishes and amphibians. Previous studies have shown that, among fishes, the presence of nest-guarding males can reduce egg losses due to infection; however, how much infection males encounter and whether they actively target infected eggs or consume them during cannibalism for other reasons has not been investigated. In a 2-year laboratory study, I artificially raised broods of spottail darter (Etheostoma squamiceps) eggs to determine whether the potential rate of egg loss to Saprolegnia infection was significant enough to select for the targeting of infected eggs. Broods were inspected daily and infected eggs were removed; infections occurred across the broods and throughout the 11–13 days of development. Egg loss due to infection was significant and varied between years, with concomitant reductions in fry production. In a separate laboratory study, I found that 10 male spottail darters consumed both healthy and Saprolegnia-infected eggs, but half of the males consumed infected eggs at rates significantly greater than expected by chance. The results show that Saprolegnia infection is significant and temporally variable, and that guarding males can respond by non-randomly consuming infected eggs. In many species of fishes in which males guard their eggs, they also eat some of those eggs (filial cannibalism). One possible explanation for this behavior in freshwater fishes is that males are consuming eggs that have become infected with water mold, reducing its spread to other eggs. This hypothesis predicts that (1) egg loss due to infection is potentially significant if not controlled and (2) egg-guarding males can detect and target infected eggs for consumption. I confirmed both of these predictions in laboratory experiments using the spottail darter, a freshwater fish found in midwestern North America. This result supports an adaptive explanation for filial cannibalism, a seemingly nonadaptive behavior.