Ultrastructure of Eggshells from Wild and Captive Speckled Dwarf Tortoises, Chersobius signatus

Ultrastructures of chelonian eggshells show wide interspecific variation, which might, in part, reflect requirements to protect embryos in various incubation environments. Relationships between eggshell ultrastructures and incubation environments are poorly understood. Using scanning electron microscopy, we examined ultrastructures of eggshells from wild and captive Speckled Dwarf Tortoises (Chersobius signatus), including shells from hatched and undeveloped eggs. Speckled Dwarf Tortoises produce multiple single-egg clutches during a short breeding season in spring, and bury their eggs in shallow nests that experience high temperatures and low water potentials. In light of this harsh incubation environment, we expected thick calcareous layers to minimize water loss during incubation. However, wild and captive eggshells had thin calcareous layers (mean values ranging from 125.1 to 148.3 µm) that lacked the multiple crystallite layers and cuticles found in several other tortoise species. We hypothesize that thin calcareous layers in eggshells of Speckled Dwarf Tortoises might be related to the production of multiple clutches within a short breeding season, leaving little time for the calcification of each eggshell. The benefits of producing multiple clutches might outweigh the benefits of a thick eggshell. Hatched eggshells had porous crystallite cores, and the shell membrane of hatched eggshells was usually separated from the calcareous layer. These characteristics are consistent with calcium absorption from the eggshell by developing embryos, resulting in shorter crystallite heights in shells of captive hatched compared to undeveloped eggs.