Fish exhibit diversity in their habitats, with some species exclusively inhabiting either freshwater or saltwater environments. However, a select group of fish possesses the ability to survive and thrive across a wide range of salinities, transitioning between freshwater and marine ecosystems throughout their lives.
Understanding Euryhaline Fish
Fish capable of tolerating a wide range of salinities are termed “euryhaline.” This physiological flexibility allows them to move between freshwater, brackish water, and marine environments. The bull shark, for instance, is a well-known euryhaline species that can move freely between rivers and the ocean. This adaptability contrasts with “stenohaline” fish, which can only survive within a narrow range of salt concentrations. Most fish are stenohaline, being restricted to either freshwater or saltwater. For example, a goldfish is a freshwater fish, while a haddock is a marine fish; neither can survive significant changes in salinity.
The Physiological Adaptations
The ability of euryhaline fish to live in both freshwater and saltwater hinges on osmoregulation, the active management of water and salt balance within their bodies. Fish constantly face an osmotic challenge because their internal salt concentration differs from their surrounding water. This means they tend to gain water and lose salts in freshwater or lose water and gain salts in saltwater.
In freshwater, where the external environment has a lower salt concentration than the fish’s internal fluids, water enters the fish’s body through osmosis. To counteract this, freshwater fish produce large volumes of dilute urine to excrete excess water, while their kidneys actively reabsorb salts. Specialized cells in their gills, known as ionocytes, also actively absorb ions to replenish lost salts. Freshwater fish do not drink water.
Conversely, in saltwater, the external environment has a higher salt concentration than the fish’s internal fluids, causing water to leave the fish’s body. To combat dehydration, saltwater fish drink large quantities of seawater. Their kidneys produce small amounts of concentrated urine to conserve water and excrete excess divalent ions like magnesium and sulfate. The primary mechanism for eliminating excess salt occurs through their gills, where ionocytes actively pump out sodium and chloride ions.
Diverse Strategies of Migratory Fish
Many fish species utilize both freshwater and saltwater environments at different stages of their lives, often involving distinct migratory strategies. These migrations are driven by biological needs like spawning or feeding. The term “diadromous” describes all fish that migrate between fresh and saltwater.
One common migratory pattern is “anadromy,” where fish spend most of their adult lives in the ocean but migrate to freshwater to spawn. Salmon hatch in freshwater streams, migrate to the sea to mature, and return to their natal freshwater environment to reproduce. Striped bass also exhibit anadromous behavior, living in saltwater but returning to freshwater rivers for breeding.
In contrast, “catadromous” fish primarily reside in freshwater but undertake migrations to saltwater to spawn. Eels, such as the American and European eel, live in freshwater before migrating to the Sargasso Sea to reproduce. Their larvae then drift back to freshwater habitats.
A third category is “amphidromous” fish, which migrate between freshwater and saltwater at various life stages, but not specifically for breeding purposes. This migration often occurs to access different feeding grounds or nursery habitats. For instance, some goby species hatch in freshwater or estuaries, with their larvae drifting to the ocean to feed and grow, before juveniles return to freshwater environments to mature.