Patterns of intracolumnar size variation inform the heterochronic mechanisms underlying extreme body shape divergence in microcephalic sea snakes

Abstract

Sea snakes (Hydrophiinae) that specialise on burrowing eel prey have repeatedly evolved tiny heads and reduced forebody relative to hindbody girths. Previous research has found that these ‘microcephalic’ forms have higher counts of precaudal vertebrae, and postnatal ontogenetic changes cause their hindbodies to reach greater girths relative to their forebodies. We examine variation in vertebral size along the precaudal axis of neonates and adults of two species. In the non-microcephalic Hydrophis curtus, intracolumnar patterns take the form of symmetrical curved profiles, with longer vertebrae in the midbody relative to distal regions. In contrast, intracolumnar profiles in the microcephalic H. macdowelli are strongly left-skewed curves due to the presence of numerous, smaller-sized vertebrate in the forebody (above the heart). Neonate and adult H. macdowelli specimens both exhibit this pattern, implying an onset of fore- versus hindbody decoupling in the embryo stage. Based on this, we suggest plausible developmental mechanisms involving the presence and positioning of Hox boundaries and heterochronic changes in segmentation. Tests of our hypotheses would give new insights into the drivers of rapid convergent shifts in evolution, but will ultimately require studies of gene expression in the embryos of relevant taxa.