Histological Basis of Substrate Color-Induced Melanization and Reversal of Melanization in Painted Turtles (Chrysemys picta marginata)

Crypsis can be facilitated by dynamic color changes that are mediated by chromatophores of the integument. The histological bases of dynamic color changes are well understood for some reptiles, such as lizards, but less so for turtles. We used pixel counts from digital images of tail-tip sections to study histological changes that occur during substrate color-induced melanization, and reversal of melanization, in Midland painted turtles (Chrysemys picta marginata) during 2 companion studies. Melanization in C. picta marginata can be induced in light-skinned individuals when placed on a dark substrate and can be reversed in dark-skinned individuals when placed on a white substrate but without the shedding of scutes. Therefore, we predicted that color change would involve variations in intracellular melanosome concentrations within melanophores, in the deepest living epidermal cells, or both. At hatching, and in individuals that were reared on a white substrate, mean pixel density was relatively low in the stratum spinosum and corneum. In contrast, melanosome densities of the melanophore layer, stratum spinosum, and stratum corneum were relatively high in turtles that were reared on a black substrate. In a second experiment, mean pixel counts of the epidermal layers of turtles reared on a black substrate and then switched to a white substrate were relatively low when compared with turtles that were reared on a white substrate and then switched to a black substrate. Contrary to our expectations, melanosomes were deposited or degraded in both the living and nonliving cells of the epidermis.