Marine iguanas face a unique challenge among lizards because they must cope with specific osmoregulatory  obstacles. Although marine iguanas have a lower cutaneous water flux than sea snakes, terrestrial water-going snakes, and sea turtles, they still must find ways to excrete water that enters through the skin and, more significantly, through ingestion. Also, marine iguanas ingest high amounts of salt during feeding and must find a way to excrete these ions as well. So, how does the marine iguana tackle these obstacles?

• First, cloacal excrement in marine iguanas has a very high water content
• Second, evaporative water loss is much higher than other lizards of its size
• Finally, sea water accounts for close to 40% of the marine iguana’s water intake. To rid itself of Na and Cl, marine iguanas are equipped with a nasal salt gland that excretes large amounts of these ions (Shoemaker and Nagy, 1984).

Salt Gland

Sea algae are much richer in electrolytes than most terrestrial plants. For marine iguanas, about one-third of NaCl and almost all of K+ ions come from algae. This, along with ingestion of salt water during feeding, results in high amounts of electrolyte intake. Luckily for the marine iguana, the salt gland has a particularly high secretory capacity. The NaCl concentration in nasal secretions is four times that of food content and 6 times that of sea water. The salt gland also excretes nearly 80% of K+ ions (Shoemaker and Nagy, 1984). This amazing feature has helped the marine iguana survive in its specialized lifestyle.

Marine iguanas are equipped with a nasal salt gland in order to excrete excess salt and other ions. Here you can see the salt residue where iguanas "sneeze" out extra salt.

Photo used with permission from Rob Stewart