Pirate perch (Aphredoderus sayanus) are a small freshwater fish commonly found east of the Mississippi River. They are known predators of many aquatic organisms, including aquatic beetles and their larvae, mosquito larvae, and treefrog eggs. However, when these prey organisms are looking for a habitat patch to colonize or lay eggs in, they do not avoid pirate perch. This is particularly strange, as the prey organisms actively avoid many other fish species. Previous studies have shown that prey species detect potential predators through chemical cues, leading to the idea that pirate perch may be chemically camouflaging themselves.
These experiments were designed to test the hypothesis that ammonia is the detectable chemical cue of predators. If so pirate perch are able to either greatly reduce the amount of ammonia they excrete, or camouflage their ammonia production. Several different methods were used to delineate if ammonia could be the chemical detected, and if either of these strategies could be occurring.
Ammonia solutions were used to directly test the effects of ammonia on mosquito oviposition preferences. Mesocosm arrays were used to test the effect of increased density of pirate perch on ovipostition preferences in prey species. Cattle tanks were used to test ovipostition and colonization preferences in treefrogs and beetles between three treatments: none, two pirate perch, and twelve pirate perch. Separate arrays were used to test mosquito oviposition preference. Wading pools containing increasing pirate perch densities were used, along with other species, golden topminnows and mosquitofish, to test density effects on mosquito oviposition, as well as potential chemical camouflage of other species.
Results/Conclusions
Increased density of pirate perch had a marginally significant effect on oviposition preferences in mosquitoes, and there were no signs of pirate perch being able to camouflage other fish species’ presence. In treefrogs, increased density had a significant effect on oviposition preferences; low densities seemed to attract ovipositing frogs, while high densities of the fish were avoided. In addition, ammonia solutions did not have a detectable effect on mosquito oviposition preferences.
Ammonia is likely not the detectable chemical cue for prey species. Pirate perch have a reduced chemical signature, but in high enough densities, are still detectable by prey species. This may be a form of chemical camouflage, but in a more passive sense of the term. More testing is needed to determine why pirate perch are undetected and seemingly attractive in low density by prey species.