Fish are a varied group of aquatic organisms with skulls and, in most cases, backbones. Their bodies are streamlined and swimming-specific, and they have fins that let them move quickly through the water. Fish are broadly categorized into freshwater or saltwater, but what differences separate the two types?
There aren't many substantial structural differences between freshwater and saltwater fish. The key difference is physiological and has to do with how the fish control water and salts in their bodies. Freshwater fish are considered more adaptable and easier to care for than saltwater fish.
The primary distinction between saltwater and freshwater fish is the environment in which they live. However, there are several critical differences between saltwater and freshwater fish. Let's take a deeper look into the vast underwater fish world to understand the numerous fish species better.
How Do Freshwater And Saltwater Fish Species Differentiate?
There are about 30,000 distinct fish species, and many new ones are discovered every year; therefore, they are the most varied group of vertebrates. In addition, many freshwater fish species may be found in rivers and lakes, whereas saltwater fish live in the ocean.
Life supposedly started in the ocean billions of years ago, so early fish were all saltwater fish. Over time, some of these sea creatures could move into freshwater rivers and lakes. Moving into freshwater bodies meant they'd have to adjust to their new surroundings, and some of these adaptations became permanent, resulting in species differentiation.
While marine waters cover more than 70% of the earth's surface, freshwater covers only 1%. Yet, surprisingly, 40% of fish species live in freshwater. According to this data, there are one fish species for every three cubic miles of freshwater and one fish species for every 23000 cubic miles of marine waters. Given the small area that bodies of freshwater cover, this is a very high proportion of species density.
Freshwater fish have a high degree of variety due to constantly changing habitats and the ease of geographical separation. On the other hand, saltwater fish have flourished in a somewhat more stable habitat in a broader ocean setting. Therefore, freshwater fish have grown more adaptable to various water conditions than saltwater fish.
Freshwater Vs. Saltwater Fish: Habitats And Size Differences
Saltwater fish live in various temperatures, from the icy Antarctic and Arctic oceans to the warmer tropical seas. Some freshwater species can live in water with temperatures of 75 degrees Fahrenheit, while others flourish between 41 to 59 degrees Fahrenheit.
Coral reefs, salt ponds, mangroves, seagrass beds, and the deep sea are all ideal habitats for saltwater fish.
Catfish, charr, cisco, mooneye, trout (apache, blueback, brook, brown, and cutthroat), sunfish, pike, salmon (pink, coho, chum, Chinook, and Atlantic), and whitefish are a few examples of freshwater fish. Albacore, some bass species, bluefish, butterfish, flounder, cod, marlin, mackerel, herring, shark, snapper, tuna, and yellowtail are some saltwater fish.
Freshwater fish vary in size, from less than an inch long to around 400 pounds. Saltwater fish also range in size and can be as small as 0.47 inches or as big as the whale shark (which averages 41 feet long and weighs more than 23 tons).
Structural Variations In Saltwater Vs. Freshwater Fish
Sturgeon and catfish use whisker-like feelers to taste and touch their food before swallowing it. Swordfish, marlin, and sailfish use their distinctive bills to shock their prey before feasting. The paddlefish feeds by stirring up bottom-dwelling creatures with its paddle-shaped snout.
The goosefish (or angler) has an attractive appendage on the top section of its snout. It attracts prey by writhing the appendage like a worm, drawing prey to itself.
Saltwater fish have structural variations that enable them to find food. Some fish have pharyngeal teeth (teeth in the throat), others have palatine and vomerine teeth (on the roof of the mouth and tongue), and others have teeth around the margins of their mouths (maxillary and premaxillary). Predators' stomachs are sac-like, with solid walls that pulverize food.
Surprisingly, no substantial structural changes exist between freshwater and saltwater fish. However, there is a significant physiological difference between freshwater and saltwater fish. The difference between saltwater and freshwater fish is how they regulate water and salts in their interior cells, which is known as osmoregulation.
How Freshwater And Saltwater Fish Regulate Water And Salts
Freshwater fish have gills that distribute water (preventing contaminants from entering) while keeping bodily fluids inside the fish. In addition, freshwater fish have huge, well-developed kidneys capable of processing massive amounts of water.
Due to osmosis, saltwater fish lose considerable amounts of internal bodily fluids through their gills. As a result, they replenish lost water by ingesting a significant amount of saltwater. Because saline water is less dilute than the fish's internal fluids, it rushes in to replace internal fluids in an attempt to form an equilibrium.
Freshwater fish and saltwater fish control water and salts differently in their interior cells. For example, saltwater fish lose water containing salt through their skin and gills, but freshwater fish absorb it.
Water is always striving for homeostasis. Because ocean water is extremely salty, saltwater fish have lower salt content in their bodies than the water in which they swim. As a result, the salt migrates from high to low concentration.
Most saltwater fish lose water (containing a high salt level) continuously via their gills and skin. On the other hand, freshwater fish will constantly absorb water (having a low salt level) through their gills and skin.
Most freshwater and saltwater fish have a blood salt content of around ten parts per thousand (ppt). Because freshwater fish swim in water containing approximately 0.5 ppt of sodium, calcium, and chloride, the chloride cells in their gills are engineered to pump sodium, calcium, and chloride into the fish.
Because saltwater fish live in water with a salinity of about 35 ppt, the chloride cells in their gills pump salt out of the fish. Osmoregulation is how fish manage the flow of water (containing salt) across their bodies.
Water Salinity Levels For Freshwater And Saltwater Fish
Hypotonic cells of saltwater fish maintain the salt inside the cells. Therefore, water rushes into their cells when saltwater fish are put in a salt-deficient freshwater habitat. The fish will perish as a result of this.
Freshwater fish contain more salt in their bodies than the water surrounding them. Therefore, they continuously strive to keep salt in their bodies and water out of their bodies.
When freshwater fish are put in a saltwater environment, a high quantity of salt floods their bodies. This would dehydrate the fish and eventually kill them.
Are There Fish That Can Live In Freshwater And Saltwater?
Some fish are capable of living in both freshwater and saltwater. Here are some examples of these fish:
- Bull Shark
- Sockeye Salmon
- Atlantic Stingray
- Barramundi
- American Eel
- Gulf Sturgeon
- Green Sawfish
Euryhaline fish are fish that can survive in both freshwater and saltwater. These very adaptive fish may travel between the ocean and rivers. There are two varieties of euryhaline fish:
- Anadromous fish are euryhaline fish that breed in freshwater and migrate to the ocean. Salmon is a perfect example of an anadromous fish
- Catadromous fish breed in the ocean and move to freshwater bodies. Catadromous fish include North American and European eels.
While euryhaline fish can adapt, they require time to allow their body to adjust to the various salinity levels. These fish's bodies may progressively acclimatize in estuaries, which are areas where freshwater and saltwater mingle.
Stenohaline species are fish that can only survive in a restricted salinity range. The vast majority of fish species are stenohaline. Stenohaline fish, unlike euryhaline fish, cannot adjust to changing salinity levels.
Goldfish, for example, can only survive in freshwater, but clownfish can only survive in saltwater. Both are stenohaline fish.
Freshwater Vs. Saltwater Tanks And Aquariums: Difficulty And Expense
It is essential to know the difference between difficulty level and the amount of care required when deciding between a freshwater or saltwater tank setup. The ocean's salinity levels and water parameters do not fluctuate as often as in rivers or ponds. Therefore, even if they are stenohaline, most freshwater fish can tolerate a broader range of water conditions than saltwater fish.
While the market value of fish varies based on the season and distance from the shore, most marine species are substantially more expensive than freshwater fish. A saltwater tank setup may frequently be twice as expensive as a freshwater aquarium setup due to the additional equipment required and the cost of the fish itself.
Are Freshwater Aquariums Good For Beginners?
Most new fishkeepers begin with a freshwater tank because freshwater fish are less expensive and easier to care for. In addition, there are numerous bright and appealing freshwater fish, such as cardinal tetras, fancy guppies, killifish, bettas, and cichlids, to mention a few. While they are not suitable for novices, discus fish are another attractive freshwater fish with various colors.
Even within freshwater aquariums, a wide range of tank configurations is available. Freshwater tropical community tanks are popular aquarium configurations for novice and experienced fish keepers.
While it is very affordable to put up and maintain, a single aquarium may house a wide diversity of fish. Just remember that it is critical to understand the needs of each species in the community and make sure they are compatible. Generally, 1 gallon of water for 1 inch of freshwater fish is recommended.
Filters, aquarium lights, and aquarium heaters are the main components of a freshwater aquarium system. However, coldwater fish are appropriate for freshwater aquariums without a heating system. Goldfish and white cloud mountain minnows, among other tiny river species, are examples of coldwater fish.
Different Types Of Saltwater Aquariums
There are several colorful saltwater fish with eye-catching patterns and body forms. While various species have varied needs, saltwater aquarium systems can be divided into three categories.
The most basic marine arrangement is a fish-only (FO) saltwater aquarium. This aquarium layout is ideal for novice saltwater fish keepers. Instead of live rocks, FO tanks are ornamented with coral replicas. Damselfish and other tank-bred marine fish are excellent choices for FO aquariums.
Fish-only with live rock (FOWLR), saltwater aquariums are another form of marine tank layout. Live rocks, fragments of mature coral reefs, allow diverse marine species, such as invertebrates and sponges, to colonize.
Because living rocks contain an abundance of beneficial microorganisms, they help with filtration and maintain optimum water parameters. However, FOWLR saltwater aquariums require more care since live rock necessitates particular acclimatization procedures, lighting specs, and supplements.
Reef aquariums are the most challenging to maintain. The primary emphasis is on reef aquariums' corals, invertebrates, and anemones. While fish can be included in a reef aquarium, they are seen as an addition to the overall arrangement.
Reef aquariums have specific water parameters, lighting conditions, and water flow that must be maintained regularly. Therefore, reef aquariums are often reserved for the most experienced aquarists. Finally, it is recommended to provide at least 5 -10 gallons of water per 1 inch of saltwater fish in any aquarium setup.
Aquariums For Saltwater And Freshwater Fish
Brackish water aquariums can be thought of as a mixture between saltwater and freshwater and accommodate Euryhaline fish. The difficulty of keeping a brackish aquarium depends on the fish held in it.
Hardy fish such as Mollies are suggested for beginner fishkeepers with brackish aquariums. Like in a saltwater aquarium, a hydrometer is required to check the salinity in brackish aquariums. Any adjustment in salinity should be introduced gradually to allow the fish to adjust appropriately.
Freshwater Vs. Saltwater Fish: Which Is Better For Farming?
Freshwater aquaculture has gradually developed over 30 years, with Asian nations leading. Freshwater species such as carp, catfish, or tilapia are often herbivorous or omnivorous. Therefore, they don't require much animal protein input to develop.
Generally, freshwater species can attain slaughter weight on a low-cost or plant-based diet. Tilapia and catfish may be cultured in biofloc systems or without the need for any feed. This makes the fish less costly and more accessible to many low-income farmers.
Atlantic salmon are carnivorous and require high-quality aquafeeds to flourish. Growing saltwater fish in the ocean can be risky and expensive compared to small ponds to raise freshwater species.
Though technical developments have made ocean farming more sustainable and cost-effective, it still has initial costs. For example, investing in high-tech ocean cages to grow marine species like cobia and seabass may be too expensive for underdeveloped countries.
Therefore, researchers believe we overestimate marine aquaculture's potential for food security. They also express worries regarding ocean farming's environmental sustainability. Instead, they urge the global community to shift its attention to freshwater aquaculture.
A Quick Overview Of Freshwater Vs. Saltwater Fish
Both freshwater and saltwater fish have a wide range of species in terms of size. However, freshwater fish have a high degree of variety, especially considering freshwater only makes up 1% of the earth's surface. Even though there aren't many structural differences, fish species have evolved and adapted to their environments.
The main difference between freshwater and saltwater fish is how they manage the flow of water (containing salt) across their bodies. Freshwater fish are considered more adaptable, easier to look after, and more economical than saltwater fish. There are various types of aquarium setups (ranging in difficulty level) available for fishkeepers.
Sources:
Freshwater Versus Saltwater Aquariums: What You Need to Know | PetMD
Why Can't Freshwater Fish Survive In Salt Water? » Science ABC