Saltwater fish vs freshwater fish

Most fish live either in saltwater (oceans and seas) or freshwater (lakes, rivers, ponds and streams).

Angelfish
Freshwater angelfish originate from the Amazon Basin, Orinoco Basin and various rivers in the Guiana Shield in South America. Image credit
Blue Tangs are a saltwater fish commonly found in reefs throughout the Indo-Pacific.

The key difference between the two lies in their physiological adaptation to the salinity of their environment.

salinity

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.

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.

osmoregulation saltwater fish
Osmoregulation in a saltwater fish. Source

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.

osmoregulation freshwater fish
Osmoregulation in a freshwater fish. Source

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.

Siberian Sturgeon
The Siberian sturgeon (Acipenser baerii) is a freshwater species of sturgeon in the Acipenseridae family. Image © Citron via Wikimedia Commons

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.

Saltwater fish are all cold-water fish, such as clownfish, butterfly fish, eels, seahorses, and lionfish.

Golden butterflyfish
Golden Butterfly Fish is a saltwater fish commonly found on reefs throughout the world.

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.

Green Crabs can tolerate a wide range of salinity (from 4 to 52‰), and survive in temperatures of 0 to 30°C . Image source

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.

The Bull Shark adjusts its kidney function according to the salinity of its water environment. Image source

However, some fish are euryhaline because their life cycle involves migration between freshwater and seawater environments, such as salmon and eels.

Pacific Salmon
Pacific salmon are born in freshwater streams and migrate to the sea where they spend most of their lives, before returning to their natal streams to spawn and die. Image source

Here at S.E.A. Aquarium, most of our fishy residents are saltwater species. Some of our freshwater residents include the Platinum Alligator Gar and African Tigerfish which can be found at the Central and South American exhibits, located next to the Poison Arrow Frogs habitat.

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