The Mystery of a Marine Monster: Morphological and Performance Modifications in the World’s only Marine Lizard, the Galápagos Marine Iguana

Reptiles have repeatedly invaded and thrived in aquatic
environments throughout history, however fewer than 8% of the 6000 extant
species are primarily aquatic. The Galápagos
Marine Iguana (Amblyrhynchus
cristatus), the world’s only marine lizard, may have had one of the most
unique and challenging transitions to aquatic life. Curiously, previous studies
have identified relatively few physiological adaptations in Marine Iguanas,
however, little is known about the extent of morphological specialisation and
performance trade-offs associated with the marine environment. By examining the
morphology and locomotory performance of the Marine Iguana in comparison to
their closely related mainland ancestors, the Black Spiny-tailed iguana (Ctenosaura
similis) and Green Iguana (Iguana iguana), we found variation
reflected specialisation to ecological niches. However, variation was more
pronounced among subspecies of Marine Iguana, suggesting that little
morphological or performance modification is required for iguanids to
successfully invade aquatic environments, thus raising the question why there
are so few extant aquatic reptilian lineages. We found that specialisation for
the marine environment resulted in a trade-off in sprint speed in a terrestrial
environment, similar to that seen in extant crocodilians. Reduced performance
in a terrestrial environment likely poses little risk to large-bodied apex predators,
whereas in iguanids, a performance trade-off would likely incur increased
predation. As such, we suggest that this may explain why iguanids and other
ancestral lineages have not undergone transitions to aquatic life.