Vertebral evolution and ontogenetic allometry: The developmental basis of extreme body shape divergence in microcephalic sea snakes

Snakes exhibit a diverse array of body shapes despite their characteristically
simplified morphology. The most extreme shape changes along the precloacal axis
are seen in fully aquatic sea snakes (Hydrophiinae): “microcephalic” sea snakes
have tiny heads and dramatically reduced forebody girths that can be less than a
third of the hindbody girth. This morphology has evolved repeatedly in sea snakes
that specialize in hunting eels in burrows, but its developmental basis has not
previously been examined. Here, we infer the developmental mechanisms
underlying body shape changes in sea snakes by examining evolutionary patterns
of changes in vertebral number and postnatal ontogenetic growth. Our results
show that microcephalic species develop their characteristic shape via changes in
both the embryonic and postnatal stages. Ontogenetic changes cause the
hindbodies of microcephalic species to reach greater sizes relative to their
forebodies in adulthood, suggesting heterochronic shifts that may be linked to
homeotic effects (axial regionalization). However, microcephalic species also have
greater numbers of vertebrae, especially in their forebodies, indicating that
somitogenetic effects also contribute to evolutionary changes in body shape. Our
findings highlight sea snakes as an excellent system for studying the development
of segment number and regional identity in the snake precloacal axial skeleton.