Drinking behaviors and water balance in marine vertebrates

Acquisition of fresh water is important to animals, and is both difficult and critical for species residing in marine environments. Adaptive radiations to fully marine habitats were constrained by the need for fresh water and the capacity of various taxa to adapt physiology to reliance on sources of water other than free drinking water. Here, we review the water relations of marine vertebrates, with an emphasis on drinking and the need to procure fresh water. Numerous marine teleost fishes drink seawater, but some do not, and drinking is more variable and complex than suggested by textbooks. The mechanisms by which fishes and other vertebrates regulate water balance involve the renin-angiotensin and aldosterone endocrine systems, but plasma osmotic and ionic concentrations as well as other chemical signals can also be involved. Multiple mechanisms for stimulation of drinking are operative and diverse among species. Clearly, evolutionary adaptations to environmental salinities can alter drinking behaviors. Marine elasmobranchs do not characteristically drink seawater, but euryhaline species drink upon returning to more concentrated seawater, as with teleosts. Hagfish are osmoconformers, and there is no evidence for drinking. In general, marine reptiles and most marine mammals and seabirds do not drink seawater. Exceptions include sea turtles, cetaceans, and some pinnipeds. Some marine species (e.g., sea snakes) require fresh water that can be acquired from ephemeral rainwater lenses, while others are adapted to utilize dietary and metabolic water. Regardless of drinking behaviors, numerous forms have evolved varied strategies for conserving water while reducing its losses to the surrounding sea.