Many small fish on coral reefs have body patterns that are likely to provide visual concealment from predators: either by matching an aspect of the background (background matching); or disguising their body outline (disruptive colouration). Conversely, conspicuous body patterns are often used to advertise the presence of an organism: this is useful in both sexual selection, territoriality or when advertising distastefulness or toxicity (aposematism). Nudibranchs are opisthobranch molluscs that display a range of highly contrasting and apparently conspicuous body patterns that likely advertise their toxicity to potential predators. The majority of studies into animal body patterns have quantified body patterns based on the spectral contrast of an animal and that of the background; however the contrast of other pattern components such as spatial frequency and luminance have largely been ignored. Moreover, few studies have also incorporated modelling the target and background through the eyes of ecologically relevant predators; an important component of visual ecology studies into the function of body patterns. In this study we investigated the cryptic potential of a variety of prey body patterns, from the perspective of several reef predators (fish, crustacean and human). We measured the spatial frequency contrast of the prey body pattern and that of the background, using data about the predators’ visual systems. Behavioural tests were used to determine how the spatial frequency contrast between prey and the background affects predation likelihood (Fig.1). Using image statistics, retinal wholemounts and visual modelling, we show that predator visual systems are a key selection pressure in the evolution of coral reef prey body patterns.