Ontogenetic determinism of colour polymorphism in a coral reef fish, Chrysiptera brownriggii (Pomacentridae)

The determinism of ontogenetic colour changes induced by environmental factors is poorly understood in marine fishes, especially in coral ecosystems. The present study, conducted at the Rangiroa Atoll (French Polynesia) explored the effects of fish density and brightness/darkness condition (type of background) on colour determination during the ontogeny of a territorial damselfish Chrysiptera brownriggii (Bennett 1828). In this species, larvae always colonize the reef (settlement) in a yellow morph, while juveniles and adults can display two distinct colour-patterns: yellow and dark brown. Our experiments in aquaria showed that a significant higher proportion of C. brownriggii larvae turned into the brown morph in a dark condition during a period of 5 and 15 days (70-100% of brown morph induction) just after reef settlement. A significant positive effect of fish density inducing a brown colour morph was also highlighted. After a first colour induction, reversibility experiments illustrated that juveniles can change their colour morph anew after a 5-day period. Although a shift from brown to yellow morph seemed to be more limited. In the dark condition, yellow adults did not change their colour after a 5-day period. Our results showed that the colour dimorphism in C. brownriggii should be density-dependent. The period of sensitivity seems to last throughout the post-settlement period. We suggest that the yellow morph in C. brownriggii can be viewed as a paedomorphic trait. Overall, our results reveal that a darkness/lightness environment and fish density are environmental cues related to colour determinism in the polyphenetic C. brownriggii.