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Difference Between Freshwater and Saltwater Fish

Freshwater vs Saltwater Fish

Fish live in water, and the water is of two basic types known as freshwater and saltwater depending on the salinity levels. In freshwater, the salinity is less than 0.5 parts per thousand while it is more than 30 parts per thousand in saltwater. That means freshwater and saltwater have different conditions, and the fish species in the two environments should have different characteristics. This article summarizes the important and interesting differences between fish live in those two main water bodies.

Freshwater Fish

Freshwater fish species live most of their lifetime in freshwater, and that is why they are so called. The main freshwater habitats are the rivers, lakes, and streams. According to the latest calculations, 41% of the total number of fish species is freshwater fish. This value is highly significant when the volume ration of freshwater to saltwater in the world is compared.

A very significantly high number of fish species have been evolved in freshwater because the speciation takes place rapidly in those scattered habitats. In other words, freshwater habitats are highly scattered and more or less isolated, and that allows fish species to evolve into many different species, unlike the continuous oceans and seas. The saline condition is much low in freshwater, which demands the fish species to retain the salts inside their bodies. Their scales are broad and strong, and those cover the entire body to help maintain their osmotic regulation. In addition, freshwater fish are able to save salts while they push water through their gills. Furthermore, their kidneys play a major role in maintaining the salt concentration in blood.

Saltwater Fish

All the fish species living in the sea are collectively called as saltwater fish. However, some of the saltwater fish species would prefer to live in freshwater as well, but the majority of their lifespan is spent at sea or ocean where the salinity of the environment is more than 35 parts per thousand. Since the great majority of the earth surface is covered with water, and that being saltwater, it is not surprising to observe that most of the fish have made their home as the saltwater environment. The tropical waters are much higher than temperate waters in the density of fish species. That is mainly due to the distribution of their food sources such as algae are more common in the tropics than in colder environments. Additionally, it would be worthwhile to state that fish started to evolve on the earth in saltwater.

Saltwater being saltier than freshwater, the fish live here do have to conserve water and prevent salts being added to their body; their gills are adapted for that aspect, in addition to extract oxygen out of water. Scales of the saltwater fish are small and sometimes the whole body is not covered with those. Oceans and seas are always exposed to the atmosphere, since there are no trees or mountains to limit the access of predatory birds. Therefore, the life risk of a saltwater fish is high.

What is the difference between Freshwater and Saltwater Fish?

• The two types live in two different environments as they are so called freshwater and saltwater.

• The number of fish species is higher in saltwater than in freshwater. However, the fish species richness in a unit freshwater volume is significantly higher than in the same volume of saltwater.

• Freshwater fishes have large and broad scales while saltwater fishes have small scales.

• Freshwater fishes have their whole bodies covered by scales while saltwater fishes sometimes cover only a portion of their body with scales.

• Freshwater fishes are adapted to conserve salt, but saltwater fishes are adapted to conserve water.


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