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There are many different species of fish, and they can be broadly categorized into freshwater fish and saltwater fish. Many freshwater fish can be found in rivers and lakes. Saltwater fish are found in the ocean. Together, there are over 30,000 different species of fish, and many new species are being discovered each year. This makes them the most diverse group of vertebrates.
It is believed that life began several billion years ago in the ocean. Therefore, early fish were all saltwater fish. Over time, some of these ocean-dwelling creatures were able to take advantage of untapped resources like freshwater rivers and lakes. Moving into bodies of freshwater meant they would have to adapt to their new environments. Some of these adaptations became permanent and led to species differentiation.
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While marine waters cover more than 70 percent of the earth’s surface, freshwater covers only 1 percent. Surprisingly, 40 percent of the 30,000 species of fish dwell in fresh waters.
This is a very high proportion of species density given the small area that freshwater bodies cover. This data indicates that there are approximately 1 species of fish for every 15 cubic kilometers of freshwater and 1 species of fish for every 100,000 cubic kilometers of marine waters. Constantly changing environments and ease of geographical separation in small bodies of freshwater habitat have resulted in a high degree of diversification of freshwater fish. The constantly changing environments have also forced freshwater fish to become more adaptive to their environment. In comparison, saltwater fish have been able to thrive in a relatively more stable environment in a larger ocean environment. Therefore, freshwater fish are generally more adaptable to a wider range of water parameters than saltwater fish.
Freshwater Fish vs. Saltwater Fish
Surprisingly, there are no significant anatomical differences that universally distinguish freshwater and saltwater fish. However, there is a key physiological difference between freshwater and saltwater fish. The difference between saltwater and freshwater fish can be found in how they regulate water and salts in their internal cells, also known as osmoregulation.
Osmoregulation in Fish
Freshwater fish and saltwater fish regulate water and salts in their internal cells differently. Saltwater fish loses salt through their skin, while freshwater fish tend to absorb it.
Water is always trying to reach equilibrium. Therefore, the salt moves from high-concentration areas to low-concentration areas. Since ocean water is very salty, saltwater fish will have a lower concentration of salt in their body compared to the water they swim in. As a result, most saltwater fish constantly lose water through their gills and skin. In contrast, freshwater fish constantly absorb water through their gills and skin.
Most freshwater fish and saltwater fish maintain a salt concentration in their blood of approximately 10 parts per thousand (ppt) or 10 grams of dissolved salt per liter of water. Since freshwater fish swim in water with approximately 0.5 ppt, the chloride cells in their gills are designed to pump sodium, calcium, and chloride into the fish. On the other hand, since saltwater fish swim in water with approximately 35 ppt, the chloride cells in their gills are designed to pump salt the out of the fish. This process of controlling water flow across the body is called osmoregulation.
What Happens if you put Saltwater Fish in Freshwater?
Saltwater fish have hypotonic cells, which keep the salt inside the cells. Therefore, water will rush into their cells when saltwater fish are placed in a salt-deficient freshwater environment. This will cause the fish to die.
What Happens if you put a Freshwater Fish in Saltwater?
Freshwater fish have a higher salt concentration in their body than the water around them. Therefore, they are constantly working to keep the salt in their body and expel the water from their body.
If the freshwater fish is placed in a saltwater environment, the salt floods into their body at a high concentration. This would dehydrate the fish and kill it.
Which Fish Can Live in Freshwater and Saltwater?
Some fish are able to live in both freshwater and saltwater. Some examples of these fish are:
- Sockeye Salmon
- Bull Shark
- Barramundi
- American Eel
- Atlantic Stingray
- Green Sawfish
- Gulf Sturgeon
Euryhaline Fish
Fish that can live in both fresh and saltwater are called euryhaline fish. These highly adaptable fish are able to migrate back and forth between the ocean and rivers.
There are two different types of euryhaline fish.
Anadromous Fish
Euryhaline fish spawn in freshwater and migrate to the ocean are called anadromous fish. An example of anadromous fish is salmon.
Catadromous Fish
Euryhaline fish that spawn in the ocean and migrate into bodies of freshwater are called catadromous fish. Examples of catadromous fish are North American eels and European eels.
While euryhaline fish are adaptive species, they do require an acclimation period to allow their bodies to adjust to the different salinity levels. These fish may gradually acclimate their body in estuaries, which is a regions where freshwater and saltwater mix.
Stenohaline Fish
Fish that are only able to live in a narrow range of salinity are known as stenohaline species. The majority of fish are stenohaline species. Unlike euryhaline species, stenohaline fish are unable to adapt to different salinity levels. For example, goldfish can only live in freshwater, and clownfish can only live in saltwater. Both fish are stenohaline species.
Freshwater Aquarium vs. Saltwater Aquarium
When choosing between a freshwater aquarium setup and a saltwater aquarium setup, it is important to realize the difference in the level of difficulty and level of care required.
Difficulty of Care
In general, freshwater fish are hardier due to their adaptive nature. Even as a stenohaline species, most freshwater fish will accept a wider range of water parameters than saltwater fish. The salinity level and water parameters in the ocean do not change as often as in rivers and ponds. Therefore, they require more specific and stable water conditions.
Cost of Fish and Equipment
While the market value of fish can vary depending on the season and distance from the coastal lines, marine fish are much more expensive than most freshwater species. Due to the additional equipment involved in a saltwater aquarium and the price of the fish itself, a saltwater aquarium setup can often be twice as expensive as a freshwater aquarium setup.
Freshwater Aquarium Setups
Since freshwater fish of often cheaper and easier to take care of, most beginner fishkeepers start the hobby with a freshwater aquarium. Just like saltwater fish, there are many colorful and attractive freshwater fish, including cardinal tetras, fancy guppies, killifish, bettas, and cichlids, just to name a few. While they are not recommended for beginners, discus fish is another attractive freshwater fish with various colorations.
Even within the scope of freshwater aquariums, a great variety of tank setups is available. Freshwater tropical community tanks are one of the most popular aquarium setups for novice and advanced fish keepers. While it is relatively inexpensive to set up and maintain, one can also appreciate a great variety of fish in a single aquarium. However, it is important to understand the requirements of each community species to ensure compatibility. Since different species have different swimming patterns, combining top-level swimmers, middle-level swimmers, and bottom dwellers can be an attractive effect in a community tank.
Basic components in a freshwater aquarium setup include aquarium filters, aquarium lights, and aquarium heaters. For freshwater aquariums without a heating mechanism, coldwater fish are suitable. Coldwater fish includes goldfish and white cloud mountain minnows, among many other small river species.
As a general rule of thumb, providing at least 1 gallon of water for 1 inch of freshwater fish is advisable.
Saltwater Aquarium Setups
There is abundant colorful marine fish with striking patterns and body formations. While different species have varying requirements, saltwater aquarium setups can be categorized into three basic types.
The fish-only (FO) saltwater aquarium is the most basic marine setup. This type of aquarium setup is well-suited for novice saltwater fishkeepers due to its simplicity. FO aquariums are decorated with coral replicas instead of live rocks. Hardy marine fish such as Damselfish and other tank-bred species are great candidates for FO aquariums.
Another type of marine tank setup is a fish-only-with-live-rock (FOWLR) saltwater aquarium. Live rocks, which are pieces of mature coral reefs, allow the colonization of various marine life, such as invertebrates and sponges. Since live rocks house an abundance of beneficial bacteria, it aids in filtration and maintaining desirable water parameters. Since live rock requires specific acclimation techniques, lighting specifications, and supplements, FOWLR saltwater aquariums require a higher degree of care than FO saltwater aquariums.
The most challenging type of aquarium, more so than FOWLR aquariums, are reef aquariums. In reef aquariums, the primary focus is placed on the corals, invertebrates, and anemones. While fish can be present in a reef aquarium, they are considered an accessory of the entire setup. Reef aquariums require very specific water parameters, lighting conditions, and water flow that must be maintained regularly. Thus, reef aquariums are generally reserved for the most advanced aquarists.
Finally, as a general rule of thumb, providing at least 5 -10 gallons of water for 1 inch of saltwater fish is advisable.
Brackish Aquarium Setups
Brackish water aquariums lie between freshwater and saltwater aquariums. Difficulty in maintaining a brackish aquarium depends on the type of fish that is being housed. For novice fishkeepers, hardy fish such as Mollies are recommended for brackish aquariums. Just like a saltwater aquarium, a hydrometer is necessary in order to monitor the salinity in brackish aquariums. Whenever there is a change in salinity, it should occur gradually in order to allow the fish to acclimate properly.
