One of the broadest generalizations used to group fish is whether a given species occurs in freshwater or saltwater. When anglers draw this distinction, they are thinking of what fish species will be available for them to catch where they’ve decided to go fishing. When biologists compare the two, however, their thoughts will run to fish biology and fish excretory (or waste elimination) systems.
One of the most basic biological principles is that different substances dissolved in a solution tend to move from an area of higher concentration to one of lower concentration, until the distribution is even. Put a few drops of blue food coloring into a glass of water without stirring it, and eventually the water will be one color (instead of clear in one part and blue in the other). This process is called diffusion.
A substance in solution will eventually become evenly distributed throughout the solution through the process of diffusion.
Since all fishes actually live in a solution — water — the same basic principle is extremely important to them. In freshwater, the concentration of salts and minerals important for life is much higher in the fish’s own body than it is in the surrounding water. These salts tend to move from the fish’s body (high concentration of salts) to the surrounding water (low concentration of salts). Similarly, the nearby water itself has a tendency to move from the surrounding environment (high concentration of water) into the fish’s body (lower concentration of water). As a result, the excretory system of a freshwater fish must get rid of excess water that enters the fish, while at the same time keeping important salts from escaping from the fish’s body into the watery environment. A freshwater fish’s urine is very dilute, containing excess water that the fish must rid its body of, but few of the salts that the fish needs to retain.
For a freshwater fish, water tends to enter the body while salts tend to leave.
A saltwater fish has just the opposite problem! The concentration of salts in the saltwater around it is higher than within its own body, and excess minerals tend to travel into the fish’s body. At the same time, pure water itself is at a higher concentration within the fish’s body than outside it, and the fish must keep this water from leaving its body. A saltwater fish’s excretory system works to rid the body of excess salts, while simultaneously preventing the loss of water. The urine of saltwater fishes therefore contains high concentrations of minerals, but low amounts of water.
For a saltwater fish, salts tend to enter the body while water tends to leave.
Some species of fish can tolerate a very wide range of salinities (salt concentrations) in the surrounding water. Their excretory systems are highly adaptable, and are actually able to adjust to freshwater or saltwater conditions; fishes with this ability are called euryhaline (say YOUR-EEE-HAY-LINE). Blue tilapia (a freshwater species; see above) can live in saltwater, while snook and tarpon (saltwater species) can live in freshwater. This adaptability is just one factor contributing to the diversity of fishing available in our Florida fresh and salt waters!