Differential growth of body segments explains ontogenetic shifts in organ position for the Diamondback Water Snake (Nerodia rhombifer)

Abstract

As snakes grow, their organs move anteriorly relative to body size. We explored a developmental explanation for the ontogenetic shift in the relative position of internal organs for snakes using the Diamondback Water Snake (Nerodia rhombifer (Hallowell, 1852)). With age, this water snake’s heart, liver, small intestine, and right kidney move anteriorly by 2.5–5.0 percentage points of snout–vent length. The number of precaudal vertebrae did not vary due to size or sex. The anterior edge of the heart, liver, small intestine, and right kidney were typically aligned within a span of 4–8 vertebrae that likewise did not differ as a function of size or sex. Snakes exhibited a positive relationship between the number of precaudal vertebrae and the vertebra number aligned with each organ. Total length, centrum length, centrum width, ball width, height, and mass of eight vertebrae sampled at consistent vertebral number revealed that vertebrae in the middle region of the body grow at a greater rate than vertebrae at the anterior or distal ends of the body. For N. rhombifer, the observed forward shift in relative organ positions is the product of regional differences in the growth of body segments. Predictably, these differences arise from a developmental program generated by the differential expression of Hox genes.