Underwater caustics disrupt prey detection by a reef fish.

Natural habitats contain dynamic elements, such as varying local illumination. Can such features mitigate the salience of organism movement? Dynamic illumination is particularly prevalent in coral reefs, where patterns known as 'water caustics' play chaotically in the shallows. In behavioural experiments with a wild-caught reef fish, the Picasso triggerfish (Rhinecanthus aculeatus), we demonstrate that the presence of dynamic water caustics negatively affects the detection of moving prey items, as measured by attack latency, relative to static water caustic controls. Manipulating two further features of water caustics (sharpness and scale) implies that the masking effect should be most effective in shallow water: scenes with fine scale and sharp water caustics induce the longest attack latencies. Due to the direct impact upon foraging efficiency, we expect the presence of dynamic water caustics to influence decisions about habitat choice and foraging by wild prey and predators.

Conspicuous visual signals do not coevolve with increased body size in marine sea slugs.

Many taxa use conspicuous colouration to attract mates, signal chemical defences (aposematism) or for thermoregulation. Conspicuousness is a key feature of aposematic signals, and experimental evidence suggests that predators avoid conspicuous prey more readily when they exhibit larger body size and/or pattern elements. Aposematic prey species may therefore evolve a larger body size due to predatory selection pressures, or alternatively, larger prey species may be more likely to evolve aposematic colouration. Therefore, a positive correlation between conspicuousness and body size should exist. Here, we investigated whether there was a phylogenetic correlation between the conspicuousness of animal patterns and body size using an intriguing, understudied model system to examine questions on the evolution of animal signals, namely nudibranchs (opisthobranch molluscs). We also used new ways to compare animal patterns quantitatively with their background habitat in terms of intensity variance and spatial frequency power spectra. In studies of aposematism, conspicuousness is usually quantified using the spectral contrast of animal colour patches against its background; however, other components of visual signals, such as pattern, luminance and spectral sensitivities of potential observers, are largely ignored. Contrary to our prediction, we found that the conspicuousness of body patterns in over 70 nudibranch species decreased as body size increased, indicating that crypsis was not limited to a smaller body size. Therefore, alternative selective pressures on body size and development of colour patterns, other than those inflicted by visual hunting predators, may act more strongly on the evolution of aposematism in nudibranch molluscs.

Conspicuousness is correlated with toxicity in marine opisthobranchs.

Aposematism is defined as the use of conspicuous colouration to warn predators that an individual is chemically or otherwise defended. Mechanisms that drive the evolution of aposematism are complex. Theoretical and empirical studies show that conspicuousness can be either positively or negatively correlated with toxicity as once aposematism is established, species can allocate resources into becoming more conspicuous and/or increase secondary defences. Here, we investigated the evolution of conspicuousness and toxicity in marine opisthobranchs. Conspicuousness of colour signals was assessed using spectral reflectance measurements and theoretical vision models from the perspective of two reef fish signal receivers. The relative toxicity of chemicals extracted from each opisthobranch species was then determined using toxicity assays. Using a phylogenetic comparative analysis, we found a significant correlation between conspicuousness and toxicity, indicating that conspicuousness acts as an honest signal when signifying level of defence and provides evidence for aposematism in opisthobranchs.

Cleaner gobies evolve advertising stripes of higher contrast.

Elacatinus gobies of the Caribbean have undergone rapid speciation along ecological axes, and particular species from this genus act as 'cleaners' that remove ectoparasites from larger coral reef fish, termed 'clients'. Evolutionary shifts in habitat use, behavior and lateral body stripe colors differentiate cleaners from ancestral sponge-dwelling lineages. High-contrast stripe colors associated with cleaning behavior on coral reefs may have evolved as a signal of cleaning status. We asked whether cleaner gobies with blue stripes are more conspicuous than ancestral yellow- and green-stripe phenotypes to a diverse set of potential client visual systems in the tropical reef environment where cleaning stations are commonly observed. Using spectrophotometric measurements of cleaners with blue and yellow stripes and their F1 hybrid, we tested the contrast of each color stripe to both potential dichromatic and trichromatic reef fish visual systems, against typical coral and sponge microhabitat background colors. Blue stripes provide the highest average chromatic contrast across a range of possible microhabitat colors to the majority of fish visual systems tested. The contrast provided by yellow and hybrid green stripes are comparable across habitats to dichromatic visual systems. The green stripe is less contrasting than both blue and yellow to many potential trichromatic visual systems. We suggest that the evolution of blue stripes in Elacatinus gobies could be a result of natural selection for signals of high color contrast, driven by the sensory biases and visual systems of diverse reef fish clients.