Fish inhabit diverse aquatic environments, from oceans to rivers and lakes. The fundamental difference between them lies in their physiological adaptations for maintaining internal water and salt balance. This process, known as osmoregulation, is crucial because a fish’s internal salt concentration differs significantly from its surrounding water.
The Challenge of Saltwater
Marine fish live in water saltier than their internal fluids. This creates an osmotic challenge: water tends to move out of the fish’s body into the saltier seawater through osmosis. Without specific adaptations, these fish would continuously lose water and accumulate excess salts. This occurs across permeable surfaces like gills and skin.
The concentration of salts in seawater is typically around 35 parts per thousand (ppt), much higher than the internal salt concentration of most marine fish. This osmotic gradient drives water loss, forcing saltwater fish to actively counteract this tendency. The gain of excess salts, particularly sodium and chloride, also presents a significant problem that requires specialized mechanisms to manage.
How Saltwater Fish Adapt
To counter water loss, saltwater fish actively drink large amounts of seawater. This ingested water helps replenish what is lost through osmosis. However, drinking salty water introduces a substantial amount of excess salt into their bodies.
To manage this salt influx, marine fish possess specialized chloride cells in their gills. These cells actively pump excess sodium and chloride ions out of the fish’s blood and into the surrounding seawater, a process that requires energy. Their kidneys also play a role by producing very small amounts of highly concentrated urine. This minimizes further water loss while effectively excreting divalent ions like calcium, magnesium, and sulfate.
The Challenge of Freshwater
Freshwater fish face a different osmotic challenge. Their internal body fluids are saltier than the surrounding freshwater. This means water tends to constantly move into the fish’s body from the less salty environment through osmosis, primarily across the gill membranes.
Without specific physiological responses, freshwater fish would continuously absorb excess water, leading to their cells swelling and potentially bursting. Simultaneously, essential salts within their bodies tend to diffuse out into the more dilute surrounding water. This constant influx of water and loss of vital ions necessitates precise regulatory mechanisms to maintain internal balance.
How Freshwater Fish Adapt
Freshwater fish do not drink significant amounts of water, as they are already gaining water osmotically. Instead, they manage the continuous influx of water by producing large volumes of very dilute urine. Their kidneys are highly efficient at filtering blood and excreting excess water.
To counteract the loss of essential salts through diffusion and urine, freshwater fish have specialized cells in their gills. These cells actively absorb ions, such as sodium and chloride, from the surrounding water into their bloodstream. This active transport mechanism ensures that despite constant water excretion, the fish can maintain the necessary internal salt concentrations for proper bodily function.
Fish That Navigate Both Worlds
Some fish, known as diadromous species, possess the ability to migrate between saltwater and freshwater environments during their life cycles. Examples include salmon, which hatch in freshwater, mature in saltwater, and return to freshwater to spawn, and eels, which do the opposite.
These fish undergo significant physiological transformations to switch their osmoregulatory mechanisms. When moving from freshwater to saltwater, they must activate the adaptations of marine fish, such as drinking more water and excreting salt through their gills. When transitioning from saltwater to freshwater, they switch to the freshwater adaptations, like producing dilute urine and actively absorbing salts. This adaptability allows them to exploit the resources of both aquatic habitats.