Effects of animated images of sympatric predators and abstract shapes on fear responses in zebrafish

Abstract

Previously, zebrafish were shown to respond to the presence of a live predator, the sight of a live predator, moving computer images of a predator, and images of different moving objects in a context and stimulus specific manner. For example, zebrafish showed a robust antipredatory reaction towards a sympatric predator, the Indian leaf fish (Nandus nandus) but not towards an allopatric predator or sympatric or allopatric harmless fish species. In a subsequent study, however, we found zebrafish not to show avoidance in response to the image of another sympatric predator, Xenentodon cancila. The results implied that zebrafish may possess genetic predispositions that allow them to effectively evade at least certain sympatric predators without any prior exposure to them. However, it is not known whether zebrafish can respond to other sympatric predators with antipredatory reactions without any prior exposure to such predators. It is also not known whether such responses may be dependent upon, or idiosyncratic to, predator species. To answer these questions, we examined the effect of computer animated (moving) images of five different predator species all sympatric with zebrafish (Nandus nandus, Channa maculata, Xenentodon cancila, Mystus bleekeri, Chitala chitala) and of two abstract shapes (star and rectangle, also animated) on zebrafish behaviour. The results confirmed that the sight of N. nandus is an aversive stimulus and that the image of X. cancila is not. However, the results also revealed that images of three other species (C. maculata, M. bleekeri, C. chitala) and that of a rectangle induce even more robust aversive reactions than what is elicited by N. nandus. At this point we do not know what features of the images may be perceived as aversive by zebrafish or whether the observed behavioural responses would be adaptive or effective in nature. Nevertheless, we argue that image-presentation based fear paradigms will be useful in future translational research. We also conclude that it may be premature to decide whether zebrafish can recognize their sympatric predators.