Independent Loss of Cloacal Bursae and Age-Dependent Surfacing Postures in Diamond-Backed Terrapins (Chelonia: Emydidae: Malaclemys terrapin)

Malaclemys terrapin is a species of turtle that lives in brackish waters along the eastern coast of the United States from Cape Cod to Texas. We documented distinctive underwater postures during surfacing in hatchlings versus individuals after one year (yearlings). Hatchlings approach the water's surface in horizontal postures, while yearlings approach in a more vertical posture. Because cloacal bursae play a role in controlling buoyancy in freshwater turtles, we investigated their potential role in determining surface postures. We discovered that cloacal bursae are absent in M. terrapin, and we attribute this absence to the osmoregulatory challenges presented by the brackish habitats of this species. We posit that the horizontal postures in the hatchlings create a broader visual target to both aerial and aquatic predators and that the younger turtles likely do not have the strength, muscle mass, lung-volume coordination, or sufficient bone density in the shell to adopt the more visually streamlined vertical posture at the surface.