Life Aquatic: A Review of Adaptations in Non‐Avian Reptiles

Abstract

Nonavian reptiles are among the many tetrapod lineages that have secondarily evolved semi‐aquatic and fully aquatic lifestyles. As thermal conformers occupying varying depths of the marine and freshwater environment, reptiles are great models for studying aquatic adaptations. The approximately 600 crocodilians, turtles, lizards and snakes enter the water for numerous reasons including to forage, mate, lay eggs and give live birth, seasonally hibernate and seek refuge from predators. Those that feed in the aquatic environment can be separated into two categories, obligate and facultative foragers, that contribute to different degrees of aquatic dependence. Morphological traits such as protruding nostrils and tentacles or chin barbels have evolved repeatedly in aquatic lineages. Aquatic nonavian reptiles must also overcome challenges associated with submersion similar to endotherms, including voluntary apnea, hydrostatic pressure at depth, and thermo‐ and osmoregulation. Some notable morphological, behavioural and physiological adaptations that facilitate movement through the water or prolonged submergence times include streamlined morphology, cutaneous respiration and depressed metabolic rates. Key Concepts Nonavian reptiles that forage for prey in the water are either facultative foragers or obligate foragers. In terms of aquatic representation, all crocodilians are semi‐aquatic, with turtles having the next largest proportion of aquatic individuals, followed by snakes and lizards, despite snakes being the only group with fully aquatic species. There are a myriad of behavioural, morphological and physiological adaptations observed in aquatic nonavian reptiles that are taxon‐specific but also others that are convergent across disparate groups. Despite the striking differences in body shapes between nonavian reptiles, aquatic members of all groups have modified their bodies to take on more streamlined shapes. Valved or protruding nostrils and piscivorous feeding habits are commonly observed across all aquatic nonavian reptile clades. Many marine inhabitants have the ability to osmoregulate and get rid of excess salt. Nonavian reptiles are great models for studying aquatic adaptations as they perform a combination of behaviours in the water including foraging, mating and seasonal hibernation despite homeostatic challenges pertaining to the aquatic environment.