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.
Ethogram of surfacing behaviors. Ambiguous and basking behaviors were excluded from statistical analyses.
(A) Time spent in surface postures by yearling and hatchling Malaclemys terrapin. Crosshairs indicate the means. Horizontal posture is defined as less than 30° from the surface of the water, and vertical posture is more than 60° from the surface of the water. Yearlings spent more time at the surface in the vertical posture than in the horizontal posture. Hatchlings spent more time at the surface in the horizontal posture than in the vertical posture. (B) Because the yearlings spent longer at the surface than the hatchlings did, we calculated the proportion of time the turtles spent at the surface in each posture. We defined a breathing instance as the time between the moment when a terrapin's nose was out of the water to the moment when the nose went back down. The length of time for each instance was noted as well as the length of time within that instance that the turtle spent in each posture. The proportions were calculated by dividing the time the turtle spent in each posture by the total length of the instance. Yearlings spent more of their surfacing instances in the vertical posture, and hatchlings spent more of their surfacing instances in the horizontal posture.
Upper: Dissected cloacae in situ from Malaclemys terrapin (MCZ R-178448) and Trachemys scripta (MCZ R-19883), with arrows showing lack of internal apertures to cloacal bursae in M. terrapin and the apertures clearly visible in T. scripta. Lower: Dissected cloacal bursae from same specimens, removed from carcass. Organs are arranged with posterior (vent, V) being at bottom of image, and the large usually anteriorly positioned urinary bladders (UB) have been reflected to an unnatural posterior position to accommodate photography.
Contributor Notes
Associate Editor: W. L. Smith.