Inconspicuous breeding coloration to conceal eggs during mouthbrooding in male cardinalfish

The body coloration of animals has various functions, such as camouflage coloration for hiding their existence from other organisms and conspicuous coloration for appealing to their potential mates. Since the breeding colorations of males are widely considered to attract females, most previous studies on breeding coloration have mostly focused on conspicuous breeding coloration, which may have prevented the discovery of possible functions other than appealing. Here, based on a speculation that such breeding coloration might occur in species with weak sexual selection in males, we focused on Apogoninae whose sex role is considered to be reversed due to their characteristic parental behavior, paternal mouthbrooding. Through detailed morphological observations, we found that males exhibit non-conspicuous white structures, consisting of iridophores, in the lower jaw during the breeding season. Artificial implantation of eggs inside the mouth in both sexes showed that the white structure in the lower jaws, which specifically exists in males, drastically reduces the visibility of eggs during mouthbrooding. This suggested that this coloration may serve to conceal the conspicuous coloration of eggs during mouthbrooding. In addition, in vivo and in vitro hormone treatment experiments revealed that iridophore development in the lower jaw is induced by androgen through the Alkal-Ltk pathway. These results suggest that androgen-dependent breeding colorations in males, which have been considered to attract females, may serve the opposite function in these species, “inconspicuous coloration”, to increase the fitness of their specialized behavior, mouthbrooding. ### Competing Interest Statement The authors have declared no competing interest.