Observations on the development of unusual melanization of leopard frog ventral skin

The ontogeny of ventral pigmentation of two species of leopard frog, Rana pipiens and R. chiricahuensis, was examined by light microscopy and transmission electron microscopy to reveal how the unusual melanistic ventral pigmentation of R. chiricahuensis is achieved at the cellular level. Ventral skin of R. pipiens is always white. Ventral skin of adult R. chiricahuensis is white when frogs are background-adapted to a white substrate, but ventral skin becomes nearly as dark colored as the dorsal skin when frogs darken in response to a black background. Skin samples from tadpoles of both species, newly metamorphosed frogs, and adult frogs were analyzed for chromatophore composition and distribution. Ventral skin of R. pipiens larvae, newly metamorphosed frogs, and adults and of R. chiricahuensis larvae was white due to abundant iridophores and no melanophores. Melanophore density in the ventral integument of R. chiricahuensis was 9.1 +/- 2.8/mm2 in newly metamorphosed frogs and 87.0 +/- 4.8/mm2 in adult frogs. Pigment within ventral melanophores migrated during physiological color change during background adaptation.