Antagonistic effects of predator colour morph abundance and saliency on prey anti-predator responses

The colour polymorphisms of prey species are often maintained by apostatic selection. In particular, rarer morphs are thought to be at an advantage because attentional constraints result in predators forming search images, which are based on the most abundant prey morph. Predatory species can also be polymorphic and predator morph abundance may be maintained by a similar mechanism, given prey are also likely to form search images to ensure fast and appropriate anti-predatory responses. Alternatively, given that the predator polymorphism may be driven by other ecological factors (e.g., niche divergence or sexual selection), prey may instead be highly sensitive to the relative visual saliency of different predatory morphs, which in turn could impact predator morph abundance. Here, by combining empirical observations with a field experiment, we assessed how the relative abundance and saliency of different colour morphs of the predatory trumpetfish (Aulostomus maculatus) influenced the behavioural responses of a typical prey species, the bicolor damselfish (Stegastes partitus). We found that more abundant predator colour morphs were less salient in damselfish vision (relative to the background) than less abundant colour morphs. By presenting 3D models of each morph to damselfish, we found that they did not respond differently to more abundant or more salient morphs. Our results suggest that both the relative abundance and saliency of predator morphs could contribute towards the search images used by prey. Specifically, each morph could have relatively equal detectability if their abundance and saliency have antagonistic effects on search-image formation in prey.