Observations on carapace color change in the juvenile big-headed turtle (Platysternon megacephalum).

Dainan Cao,Shiping Gong,Yufeng Wei,Yan Ge,Haoran Duan

doi:10.7717/peerj.7331

Abstract

The carapace color of newborn big-headed turtles (Platysternon megacephalum) is polymorphic and usually consists of two phenotypes: yellowish brown and olive green. As the turtles grew, over the first year of life, its carapace gradually turned from yellowish brown to chestnut brown, or from olive green to dark brown, depending on the phenotype. Meanwhile, the turtle’s plastron remained an orange and black pattern and did not change much. In this study, we primarily used HE staining to observe the carapace color change with age in big-headed turtle juveniles. We took the carapace marginal scute tissues twice from the same turtles before and after the carapace color change. Histological observations show that in the marginal scutes of the four tested turtles with different carapace color phenotypes, melanin granules are all concentrated in the dermal layer underneath the dorsal corneous layer, but rarely on the ventral side. Melanin deposits in the dorsal corneous layer were found to increase as the corneous layers thickened, while the melanin deposits in the ventral corneous layer did not change significantly. However, there was no significant difference in melanin deposition in the epidermis and dermis of the carapace among the yellowish brown, chestnut brown, olive green, and dark brown big-headed turtles. The results of our study indicate that the carapace color darkening in big-headed turtles may not be due to changes in melanin content of the carapace, but is the result of melanin accumulation and superposition in the dorsal corneous layer.

Highlights

Body coloration is one of the major morphological features of animals and can change in response to factors such as temperature, light, habitat characteristics, and the vision and behavior of prey and predators (Roulin, 2004)
Histological observations of the marginal scute from yellowish brown, chestnut brown, olive green, and dark brown big-headed turtles consistently showed that melanin granules were concentrated in the dermis underneath the corneous layer on the dorsal side (Figs. 2A–2D), Figure 3 Analysis of melanin deposition ratio in marginal scutes of big-headed turtles with different carapace color phenotypes
Melanin deposition analysis In order to compare the relative ratio of melanin deposition in the younger yellowish brown and olive green phenotypes, as well as the older chestnut brown and dark brown carapace, the HE slices were saved as .TIFF images and the pixels of melanin spots were analyzed by Photshop

Summary

Introduction

Body coloration is one of the major morphological features of animals and can change in response to factors such as temperature, light, habitat characteristics, and the vision and behavior of prey and predators (Roulin, 2004). Body coloration change is a common way for animals to adapt to the natural environment and plays an important role in self-protection and mate selection. Observations on carapace color change in the juvenile big-headed turtle (Platysternon megacephalum). The color pattern of the shell may be permanent or may gradually change with age or environment, in a process usually taking several months (Cao et al, 2018; Rowe et al, 2014). Increasing the amount of carotenoids in the food can increase the yellow chroma in chin stripes and increase red chroma in the neck and carapace stripes on painted turtles (Steffen, Hultberg & Drozda, 2019)

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