Osmoregulation in Marine Reptiles

Marine Reptiles consist of animals like crocodiles, sea turtles, marine iguanas, marine snakes. Marine Reptiles have inherited or retained the osmotic concentration of bodily fluids of about 300-350 mOsm (milliosmole), therefore they are hyporegulators.

Marine Reptiles differ from fish or amphibians because they only use their lungs. Lungs don't come in contact with water so they have developed a respiratory epithelium. The epithelium is thin and highly permeable to gasses. The skin is thick and reduces permeability to water. Nonetheless, no epithelial is entirely impermeable to water and marine reptiles still lose some water through their integument to the seawater. Thinner epithelia covering the mouth, nasal passages, and eyes are sites of osmotic water loss.

Despite the relative impermeability of skin, reptiles still face a substantial osmotic load. If they feed on fish, they have food source with an osmotic concentration isosmotic to their own blood. It is likely that they will ingest at least small quantities of seawater with he fish as the capture and eat them.  Other food sources in the sea are osmoconformers and eating them is equivalent to drinking salt water. This poses problems for crocodiles, feeding on crabs, turtles, feeding on jellyfish, and marine iguanas, feeding on algae.

Marine Reptiles maintain osmotic homeostasis by excreting a hyperosmotic, sodium chloride-rich fluid from their bodies in the surrounding waters. This is carried out by a epithelia in the head region. Crocodiles have glands in the tongue that can secrete hyperosmotic fluid. In sea turtles, it is carried out by the lacrimal (tear) glands surrounding the eye. In marine iguanas, nasal glands are the site of active salt transport. The situation with marine snakes is a bit more uncertain.