Highly defended nudibranchs escape to visually distinct background habitats.

The escape and radiate hypothesis predicts that once species have evolved aposematism, defended species can utilize more visually diverse visual backgrounds as they escape the need to be well camouflaged. This enables species to explore new ecological niches, resulting in increased diversification rates. To test this hypothesis escape component, we examined whether the background habitats of 12 nudibranch mollusk species differed among species depending on the presence and strength of chemical defenses. We obtained a rich array of color pattern statistics using quantitative color pattern analysis to analyze backgrounds viewed through the eyes of a potential predator (triggerfish, Rhinecanthus aculeatus). Color pattern analysis was done at viewing distances simulating an escalating predation sequence. We identified 4 latent factors comprising 17 noncorrelated color pattern parameters, which captured the among-species variability associated with differences in chemical defenses. We found that chemically defended species, indeed, were found on visually distinct backgrounds with increased color and luminance contrast, independent of viewing distance. However, we found no evidence for increased among-species background diversity coinciding with the presence and strength of chemical defenses. Our results agree with the escape and radiate hypothesis, suggesting that potent chemical defenses in Dorid nudibranchs coincide with spatiochromatic differences of visual background habitats perceived by potential predators

Visual detection of a cryptic predator by its prey fish

Animal interactions based on visual signals have been one of the oldest and most interesting research topics for early naturalists and biologists, that led to a better understanding of animal behaviour and its evolutionary implications. This study underlines the importance of considering the perspective of the species of interests when investigating visual communication in prey-predator interactions. Indeed, inaccurate conclusions are often made when we interpret animal behaviour basing ourselves on human vision, which outperforms that of most animals. Built on this premise, this dissertation focuses on the visual interaction between a small marine fish, the yellow black-faced triplefin, and one of its common cryptic predators, the black scorpionfish. The research approach first aims at better understanding the visual perspective of a triplefin when facing its predator (chapter 1 and 2), and then behaviourally and theoretically tests a new form of active sensing in the context of this prey-predator interaction (chapter 3). The first chapter describes the contrast sensitivity function of triplefins, later combined with other known visual features to estimate the amount of information that this species can visually perceive from natural scenes where predators might be concealed. The second chapter investigates how triplefins perceive the eye of a scorpionfish, and focuses on the unusual daytime eyeshine featured by this predator. This study describes, quantifies the phenomenon and tests its potential role for pupil camouflage in the context of visual inspection by triplefins under different light scenarios. The last chapter finally introduces and tests “diurnal active photolocation”, a new mechanism of active sensing that redirects ambient light (rather than emitting sound waves or electric fields) to detect reflective targets. By combining behavioural experiments with theoretical visual modelling, this final study provides first evidence for the functionality of diurnal active photolocation in triplefins by means of light redirected from their iris. This process supplements regular vision by increasing the chances of detection of a cryptic predator by exploiting its daytime eyeshine, and may have strong implications for the evolution of fish eyes

Scorpionfish adjust skin pattern contrast on different backgrounds

The two scorpionfish species Scorpaena maderensis and S. porcus are well camouflaged ambush predators that rapidly change body colouration to adjust to background colour in less than 1 min. We tested whether individuals of both species also adjust body pattern to that of the background. We placed fish on backgrounds of different pattern granularity and quantified the change in fish body pattern over 1 min. We used calibrated image analysis to analyse the patterns from the visual perspective of a prey fish species using a granularity (pattern energy) analysis and an image clustering approach. In our experiment, fish did not change their most contrasting pattern components as defined by the dominant marking size, but changed their average marking size. Moreover, fish responded with a change in pattern in contrast to the different experimental backgrounds, especially when compared to the acclimation phase. These results indicate that scorpionfish have one main pattern that can be adjusted by modulating its internal contrast. A reduction in pattern contrast could thereby improve background matching, while an increase could promote camouflage via disruptive colouration

The marbled goby, Pomatoschistus marmoratus, as a promising species for experimental evolution studies

Breeding and rearing the offspring through successive generations are mandatory in order to study evolutionary responses to anthropogenic impact in marine organisms. However, fish offer a limited number of marine model species that allow performing multigenerational experimental approaches. Here, we propose a novel breeding and rearing experimental model based on the marbled goby Pomatoschistus marmoratus (Risso 1810) which is representative of small (up to 65 mm total length), benthic species with a short life cycle. We devised a \u2018full-sib/half-sib\u2019 breeding design, and the resulting offspring were reared in captivity using a complex feeding protocol and a creative design of the tanks. Three families survived up to 160 days post-hatching (dph); one was reared at 24 \ub0C and two at 18 \ub0C. The families reared at 18 \ub0C reached sexual maturity and spawned. The size range at sexual maturity of individuals reared in captivity was consistent with the one observed in nature. The possibility to complete the entire life cycle, from hatching to sexual maturity and spawning in P. marmoratus offers great perspectives for experimental evolution and quantitative genetics studies aimed at understanding the role of evolutionary processes in response to global change