Marine iguanas like the one to the right often live in the Galapagos Islands spending much of their time in a marine environment. They therefore are hypertonic to their surroundings so when they consume salt water, they have too high a concentration of salt inside them. They also gain salt through their main food source, algae in seaweed. They need isotonic conditions where they have the right concentration of each substance. Unlike some animals they cannot produce a concentrated urine containing salt to let them conserve water and relieve them of all the sodium chloride inside them. Instead they have adapted to their environment allowing them to regulate the high concentrations of salt. The primary adaptation that lets them do this involves glads at the top of their heads, above their eyes and noses. These glands excrete sodium chloride decreasing the concentration of it inside them and releasing salt into the outside environment. Another gland sorts the salt from the food and water consumed by the iguanas and sends it to the excretion gland. When they "sneeze" out the salt through their glands, it often lands on the end of their snouts and dries there forming a gross white crust. This salt sneezing allows marine iguanas to eat and drink what they do without gathering a high sodium chloride concentration inside their bodies. This regulatory adaptation is what allows them to live in a marine environment because without these excretion glands, a high concentration of salt would build up in the iguanas bodies causing them troubles and maybe even bringing about death.
Below, there are several videos showing marine iguanas using the excretion glands to get rid of salt.
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