A study of 78 species of cardinal fishes (Apogonidae) revealed that 22 of them had black guts, 5 species had partly black guts and, in 51 species, the digestive tube was unpigmented or had dispersed melanophores in the external tunic. The black pigmentation is caused by melanization of the submucosal connective tissue which is situated between the muscularis and the basal lamina of the internal epithelium. This phenomenon was previously observed in moray eels and some pelagic fish. In nocturnal predators, it appears to serve to conceal bioluminescent prey in the stomach cavity.
- Melanization of guts