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 (<i>Amblyrhynchus cristatus</i>), 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 (<i>Ctenosaura similis</i>) and Green Iguana (<i>Iguana iguana</i>), 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.