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What Are the Different Types of Fish Species?

B. Schreiber
B. Schreiber

Fish species are a very diverse of set of aquatic animals adapted to living underwater. The main characteristics shared by most fish species are that they live in water, breathe with gills, have scales, and swim with their fins. A species is the most specific biological category and refers to a group of similar animals that can breed with each other, but not with other species. Most species of fish have adapted to many types of aquatic habitats and therefore can be very different from one another. One basic distinction can be made between freshwater and saltwater fish.

Fish are classified as vertebrates, meaning they have a backbone or spinal column. Vertebrates include land animals like birds, mammals, reptiles and amphibians. Fish are in fact so diverse that there are more individual fish species than all of the other vertebrates combined. The total number of species has been estimated to be about 20,000 or more.

An Atlantic salmon, a boney fish.
An Atlantic salmon, a boney fish.

There are three broad types of fish groups that evolved along similar lines. One group is the bony fishes, which includes many of the species the word "fish" usually conjures up. This group includes the goldfish, for instance, and many of the freshwater sport fish, like bass or perch. Another group is called cartilaginous because they have cartilage skeletons instead of true bones, and this group includes sharks and rays. A small group is classified as jawless, and this group includes the most primitive of all fish, with the lamprey being one example.

Bony fishes are by far the largest group of species. Estimates of their numbers range from some 20,000-25,000 species. Cartilaginous fish are made up of some 600 species. The jawless group has perhaps 45 member species.


Fish species have the largest size differences among any group of vertebrates. The largest, the whale shark, can weigh 20 tons (18 metric tons) and grow to about 50 feet (15 m), while the smallest is less than an inch long (1.5 cm). Fish come in a great variety of colors, and some are very bright like those which have adapted to colorful coral reefs. Other fish might be the dull color of a rocky stream.


The difference between freshwater and saltwater has very important implications for the biological chemistry of fish. Therefore, almost all fish species live exclusively in either one or the other. An exception is salmon, which spend their lives in the ocean and return to fresh water to spawn. Some freshwater eels reverse the process.

The first fish species appeared almost 500 million years ago, so the fossil record contains a wide range of prehistoric fishes. One type of fish, the coelacanth, was known from a variety of fossils and thought to have gone extinct millions of years ago. Scientists were greatly surprised, therefore, when one was caught in waters off South Africa in 1938.

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Discussion Comments


@Chmander - As I said before, some of the sea creatures can be very dangerous. Take puffer fish for example. Though they may seem quite harmless at first, when they feel provoked or threatened, they will inflate themselves up in an attempt to scare you. However, I think that sharks are the best example. Not only are they extremely dangerous with their razor sharp teeth and all, but they are also attracted to blood. Sharks have been known to smell blood from miles away, swarming like a pack of ferocious lions. This is why it's always a good idea to be prepared when you're out in the ocean, as the atmosphere doesn't suit those who lack preparation.


@RoyalSpyder - You make some great points. However, I've always wondered, just how dangerous is it to go to the bottom of the sea? I know you gave some specific examples, but do you think you can give a few more? I'm studying life under the ocean, and I'd really like to know more about this.


One thing that I really like about aquatic life is how much variety there is. Not just in how many fish there are, but how many sea creatures roam the deep. From puffer fish, to sharks, to sea urchins, there's a lot a more to the ocean than one can even begin to imagine. In my opinion, one thing that makes underwater life so unique is that we're basically seeing ourselves from their perspective.

As an example, though we can use scuba diving gear to reach the bottom of the ocean, we're still not made to inhabit it. It can get very dark down there, some of the creatures can be extremely dangerous, and the pressure from the atmosphere can crush us if we're not careful. On the other hand, fish will start gasping for breath when pulled out of the sea. See the contrasting differences?


@Chmander - Though whales swim in the ocean, they're mammals, not "fish" exactly. I used to consider whales to be fish when I was a little kid, but my parents corrected me on that. Also, you note that they should be considered fish because they swim in the ocean. You need to remember that there are a lot of sea creatures that are anything but fish. As an example, take the jellyfish, with its poisonous stingers or the electric eels, who will definitely put a shock to your system.


It's amazing how many species of fish there are. Who would have ever thought? In fact, this doesn't just apply to fish, but it can apply to many other animals. However, I've always wondered, if whales aren't considered fish, then what are they exactly? They swim in the ocean, and they're quite large, so shouldn't be considered aquatic sea creatures?


Do marine biologists study sawfish?


@ PelesTears- Osmosis is one of the major reasons that fish are either physiologically built for salt water or fresh water. The salinity of a fish’s blood is what prevents a fish from bloating or drying out in salt and fresh water. The bull shark however can alter the amount of urea in its blood, thus altering the salinity of its blood. When the shark is in fresh water, it urinates more than ten times as often than if it is in freshwater, effectively reducing the amount of salt in the blood, and preventing death. The opposite happens when the salt returns to salt water. This is one of the great adaptations this shark has undergone to become one of the most abundant sharks in the world.


@ Fiorite- How is it that a bull shark can survive in both salt water and fresh water? I thought that the process of osmosis is what dictates whether a fish is able to survive in salt water or fresh water. I don't understand how a bull shark can be both a saltwater and fresh water fish species.


The bull shark is a unique fish species that gives birth to its young in the mouths of rivers, and have been seen swimming very far up stream. They have even been observed in freshwater lakes (Lake Nicaragua). It was once thought that the sharks in Lake Nicaragua were an endemic species, but tagging and witnesses have proven that these sharks are simply bull sharks that have jumped up the rapids to this lake much like a salmon. Bull sharks tagged in the lake have been found in the Caribbean sea and vice versa.

The fact that these sharks can swim in freshwater and saltwater, frequent tropical shallows, and like to attack unprovoked make them one of the deadliest sharks in the world. They have been found thousands of miles up the Mississippi, Amazon, and Ganges rivers as well as along every tropical coastline.

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    • An Atlantic salmon, a boney fish.
      By: Witold Krasowski
      An Atlantic salmon, a boney fish.
    • Catfish.
      By: boguslaw
    • Hoki.
      By: FOOD-pictures
    • Goldfish are members of the bony fishes group.
      By: fivespots
      Goldfish are members of the bony fishes group.
    • The origins of modern fish species can be detected in the fossil record.
      By: Bastos
      The origins of modern fish species can be detected in the fossil record.
    • Freshwater fish are caught in lakes, ponds, rivers and streams.
      By: Evgeniy Drogozhilov
      Freshwater fish are caught in lakes, ponds, rivers and streams.
    • A great white shark.
      By: FAUP
      A great white shark.