Marine and freshwater animals maintain their water balance differently depending on their habitat. Let’s take a look at how osmosis regulation works.
All animals must maintain a certain level of fluid consistency in order for their physiological systems to function properly. The process of regulating the balance between fluid intake and loss is called osmoregulation. When there is a difference in habitat and fluid consistency, especially in salinity, animals must maintain a certain level of fluid consistency in order to survive as the osmotic phenomenon causes fluid intake and loss.
Osmosis is the movement of a solvent from a lower concentration solution to a higher concentration solution across a semi-permeable membrane. In saltwater, the salt dissolved in the water is the solute and the water is the solvent. If there is salt water of different concentrations on both sides of the semipermeable membrane, the water on the side with the lower concentration will move to the side with the higher concentration. If the concentrations on both sides are the same, there is no net movement of the solvent. This phenomenon of osmosis is one of the basic principles of mass transfer and plays an important role in several biological processes.
Animals are classified into osmosis-adaptive and osmosis-regulating types according to how they maintain their water balance in response to this phenomenon of osmosis. First, all osmotic animals are marine animals, and since their body fluids and seawater have the same salt concentration, or salinity, there is no net movement of water. This includes crabs, mussels, and nudibranchs. Since the salinity of their body fluids and seawater is the same, they can maintain their physiological balance without osmotic regulation.
In contrast, osmoregulators live by adjusting osmotic pressure to keep the salinity of their body fluids constant, even though the salinity of their habitat is different from that of their body fluids. Most osmoregulators that live in seawater have body fluids that are less salty than seawater, allowing water to escape from their body fluids. Therefore, their epidermis is impermeable, but they easily lose water through the epithelial cells of their gills. To replenish the moisture lost through osmosis, they continue to drink seawater. As a result, 70 to 80% of the seawater is absorbed from their intestines into the blood vessels, and salt also enters the blood vessels. Then the salt-secreting cells in the epithelial cells of the gills are activated to expel excess salt.
The osmoregulation problem faced by freshwater animals is the exact opposite of that faced by marine animals. The body fluids of freshwater animals are more saline than those of marine animals, allowing water to continue to enter through the gills. Therefore, freshwater animals solve the problem by excreting large amounts of urine instead of drinking water. Their impermeable epidermis prevents moisture from entering. Freshwater fish, for example, rarely drink water and instead release large amounts of diluted urine to eliminate excess water. This helps them maintain the salt content of their body fluids.
On the other hand, terrestrial animals also lose water in a variety of ways. They lose water through urine, feces, skin, and moist surfaces of gas exchange organs. Therefore, terrestrial animals replenish their water deficit by drinking water, eating food, and producing water through cellular respiration. In particular, many terrestrial animals have developed various physiological and behavioral adaptations to conserve water. For example, animals living in deserts minimize water loss by avoiding activity during the day and becoming active at night. These animals conserve water in a variety of ways, such as excreting highly concentrated urine or having extremely low water content in their feces.
In summary, all animals survive by adapting the concentration of their body fluids to their habitat. The mechanism of osmoregulation appears in different forms depending on the environment and allows animals to maintain their physiological balance and survive. This ability to osmoregulate for survival is an important factor that allows animals to successfully adapt and thrive in a variety of environments.
