Marine and freshwater animals maintain their internal water balance differently depending on their habitat. Let’s explore how osmoregulation mechanisms work.
All animals must maintain a relatively constant concentration of body fluids for their physiological systems to function properly. The process of regulating the balance between water intake and loss is called osmoregulation. When there is a difference in habitat and body fluid concentration—particularly in salinity—animals experience water gain and loss due to osmotic phenomena. Therefore, maintaining a constant body fluid concentration under these conditions is a critical survival challenge.
Osmosis is the phenomenon in which, when two solutions of different concentrations are separated by a semipermeable membrane, the solvent from the side with the lower concentration moves toward the side with the higher concentration. In a saltwater solution, the salt dissolved in the water is the solute, and the water is the solvent. If saltwater solutions of different concentrations are placed on either side of a semipermeable membrane, the water from the side with the lower concentration will move toward the side with the higher concentration. If the concentrations on both sides are equal, there is no net movement of the solvent. This phenomenon of osmosis is one of the fundamental principles of mass transport and plays a crucial role in various biological processes.
Animals are classified into osmoadaptive and osmoregulatory types based on how they maintain water balance in response to this osmotic phenomenon. First, osmo-adaptive animals are all marine animals; since the salinity of their body fluids matches that of seawater, there is no net movement of water. Crabs, mussels, and sea spiders fall into this category. Because the salinity of their body fluids is identical to that of seawater, they can maintain physiological balance without the need for separate osmoregulation.
In contrast, osmoregulatory animals live in environments where the salinity of their body fluids differs from that of their habitat, so they regulate their body fluid salinity to prevent it from changing. Among osmoregulatory animals, the body fluids of most fish living in seawater have a lower salinity than seawater, which means water can escape from their body fluids. Therefore, although their skin is impermeable, water is easily lost through the epithelial cells of their gills. To replenish the water lost through this osmotic process, they continuously drink seawater. As a result, 70–80% of the seawater is absorbed into the bloodstream from their intestines, and salt enters the bloodstream at the same time. This triggers salt-secreting cells in the gill epithelium to expel the excess salt.
The osmotic regulation problem faced by freshwater animals is the exact opposite of that faced by marine animals. Since the body fluids of freshwater animals have a higher salinity than freshwater, water can continuously enter through their gills. Therefore, freshwater animals solve this problem by drinking very little water and excreting large amounts of urine. Their impermeable epidermis serves to prevent the influx of water. For example, freshwater fish drink very little water and excrete large volumes of dilute urine to eliminate excess water. This helps them maintain the salinity of their body fluids.
Meanwhile, terrestrial animals also lose water through various pathways. This is because they lose water through urine, feces, the skin, and the moist surfaces of their respiratory organs. Therefore, terrestrial animals replenish lost water by drinking, through food, and by producing water via 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 being active at night. These animals conserve water in various ways, such as excreting highly concentrated urine or drastically reducing the water content of their feces.
In conclusion, all animals survive by regulating the concentration of their body fluids to suit their habitat. Osmoregulatory mechanisms vary depending on the environment, allowing animals to maintain physiological balance and survive. This ability to regulate osmolarity for survival is a crucial factor enabling animals to successfully adapt and thrive in diverse environments.
