Two heads are not always better than one: Craniofacial and axial bifurcation in cheloniid embryos and hatchlings (Chelonia mydas and Caretta caretta).

Abstract

Morphological data on craniofacial and axial bifurcation in sea turtles is not well documented in the literature. Here, we use micro-computed tomography (μ-CT) imaging to describe the body, skull, and vertebral morphology in axially-bifurcated cheloniid sea turtle embryos and hatchlings (Chelonia mydas and Caretta caretta) from south Florida beaches. We describe three types of craniofacial and axial bifurcations: bifacial, bicephalic, and bicephalic with biaxial duplication ranging from facial bones to the sacrum. We predicted smaller body dimensions in bifacial and bicephalic embryos and hatchlings compared with their normal counterparts. In addition, we hypothesized that bicephalic individuals would have greater rostral deviation angles than bifacial animals, and that vertebral dimensions would vary between the control and anomalous embryos and hatchlings. Among hatchlings (developmental Stage 31), we found that maximum curved carapace length and curved carapace width were greatest in the control specimens when compared with the anomalous specimens. Overall, we found that rostral deviations were smaller in bifacial animals compared with their bicephalic counterparts. Right and left rostral deviations were symmetrical or nearly symmetrical in all bifacial and bicephalic specimens. Among C. caretta, we found that bicephalic animals had greater standardized vertebral measurements than their bifacial conspecifics. In bifacial animals, bifurcation extended to either the frontal or parietal skull bones, while duplication extended to C5 vertebrae and T8 vertebrae in bicephalic animals. This study provides an in-depth description of anatomical alterations associated with these abnormalities. Prognosis of these organisms is poor; however, understanding the prevalence of these malformations can allow for better assessments of population health, as numerous environmental factors are known to lead to these changes.