There are three main types of osmoregulatory environments in which animals live: freshwater, marine, and terrestrial. Animals need to adapt accordingly to maintain appropriate water and ion levels (homeostasis) depending on whichever type of environment they call home.

Here are a few examples of anatomical/physiological adaptions that a variety of organisms have to maintain appropriate internal living conditions:

• Cartilaginous fishes such as sharks, rays, and skates, have plasma that is approximately isosmotic to seawater. This unusually high osmotic concentration (compared to that of other vertebrates) is maintained by high levels of urea and trimethylamine oxide (TMAO) in the blood.
• Freshwater fishes tend to take in water passively and remove it actively through the osmotic work of kidneys. They lose salts to the dilute environment and replace them by actively absorbing ions from the surrounding fluids into their bodies through the gills.

• Ticks, mites, and other terrestrial arthropods have the ability to extract water vapor directly from the air. The way they accomplish this is by producing very concentrated solutions that absorb water from air.
• Marine reptiles (iguanas, sea turtles, crocodiles, and sea snakes) drink seawater to obtain a supply of water but are unable to produce a concentrated urine that is significantly hyperosmotic to their body fluids. They compensate for this by the use of specialized glands for the secretion of salts in a strong hyperosmotic fluid. Salt glands are generally located above the orbit of the eye and nose in lizards.
• Because mammals cannot consume seawater, a different method of hydration needs to be found. They have highly efficient kidneys capable of producing very hypertonic urine. These animals also rely on metabolic water (water produced as an endproduct of cellular metabolism) and water from feeding on fishes and invertebrates.
• Elasmobranchs, marine birds, and some reptiles have a structure called a salt gland to secrete NaCl from their bodies. These animals require a lower internal NaCl concentration than the surrounding seawater, which causes a concentration gradient favoring the influx of salt. Therefore, they need a way to secrete it. The solution is provided by glands in the rectum of sharks and the skulls of marine birds and reptiles which produces a concentrated salt solution for secretion

Isn't that AMAZING!?!?!?

Here's your assignment for the weekend:

Pick an organism, either one mentioned above, or one you find out about on your own, and explain adaptations it has in place in order to assist in osmoregulation. Make sure to explain the environment it is up against as well. Videos, links, pictures, etc. might all be helpful in this write-up. Any questions can be posted in the comment section below.

Also, as I talked about in class today, here's a link to info on the kangaroo rat and how this little guy maintains appropriate internal water levels under very hot temperature conditions...

Have fun!