Most fish live either in saltwater (oceans and seas) or freshwater (lakes, rivers, ponds and streams).
The key difference between the two lies in their physiological adaptation to the salinity of their environment.
The salinity of saltwater is about 3.5% and about 0.1% for freshwater. This vastly different salt concentration affects the way fish regulate water and salts in their body, also called osmoregulation.
Body tissues in a saltwater fish contain less salt than the water in which it lives. The saltier environment draws water from its body tissues, resulting in constant water loss through its skin and gills. To compensate and prevent dehydration, saltwater fish drinks large amounts of saltwater, produces small amounts of concentrated (salty) urine, and secretes salt through its gills.
In contrast, body tissues in a freshwater fish contain more salt than the water it lives in. As such, its body continually draws in water through its skin and gills. Due to this constant water ‘intake’, freshwater fish drinks very little water and produces copious amounts of diluted urine to avoid excessive water in its body tissues.
Freshwater fish tend to be hardier
Freshwater makes up only 2.5% of all water on Earth. Surprisingly, about 40% of all fish species are found in fresh waters.
Constantly changing environments and geographical separation of small bodies of freshwater habitats have resulted in a high degree of diversification of freshwater fish. The constantly changing environments have also forced them to be more adaptive to their environment.
On the other hand, saltwater fish enjoy a relatively more stable environment in a larger ocean environment with little fluctuation in temperature, salinity, ammonia, nitrate and pH levels. Therefore, freshwater fish are generally more adaptable and hardier than saltwater fish.
Tropical or cold
Freshwater fish can be cold-water fish or tropical fish. An example of cold-water freshwater fish is the goldfish, while tropical freshwater fish include angelfish, discus and cichilds.
Best of both worlds
Some species can live in both freshwater and saltwater. Called euryhaline, they are commonly found in habitats such as estuaries and tide pools where the salinity changes regularly. An example is the Green Crab (Carcinus maenas) which can live in both saltwater and brackish water.
Another example is the Bull Shark. Unlike most other sharks, the Bull Shark can live in both ocean and freshwater estuaries and lakes. When it moves gradually into freshwater, its kidneys remove less salt and more urea from the bloodstream through urination, essentially reversing the normal marine shark method of osmoregulation. This adaptation allows it to live entirely in freshwater.
However, some fish are euryhaline because their life cycle involves migration between freshwater and seawater environments, such as salmon and eels.
Here at S.E.A. Aquarium, most of our fishy residents are saltwater species. Some of our freshwater residents include the Vampire Fish which can be found at the Central and South American exhibits, located next to the Poison Arrow Frogs habitat.