Interspecific variation in organ position in hydrophiine snakes is explained by modifications to the vertebral column

Interspecific disparities in the position of the internal organs of snakes have been associated with evolutionary history and cardiovascular performance, as influenced by habitat use. For snakes, the positions of internal organs are typically determined as a linear measurement relative to body length. Therefore, interspecific variation in organ position could be explained either as heterotopic shifts in organ position or by modifications to the vertebral column. Using vertebral counts from radiographs, I determined the positions of the atria and pyloric sphincter relative to the cloaca in hydrophiine sea snakes. I found interspecific variation in the number of pre-atrial vertebrae to be labile, whereas the number of vertebrae in the atria to pyloric sphincter region and in the pyloric sphincter to cloaca region was relatively constrained. Furthermore, the number of pre-atrial vertebrae was dissociated from the number of vertebrae between the atria and cloaca, indicating that these two regions of the vertebral column can evolve independently. I conclude that variation in organ position among hydrophiine sea snake species is attributable, in part, to differences in the number of vertebrae among regions of the vertebral column rather than to heterotopic shifts in the positions of the internal organs.