All fish are ectotherms or cold-blooded animals that live in the water. This means that they can’t control their body temperature. Because of that, their body temperature rises and falls along with the temperature of their outside environment. Fish also have fins and gills.
Fish are also vertebrates, meaning they have backbones. About half of all animals in the world with backbones are fish. As a matter of fact, fish were the first vertebrates on Earth and the first to evolve. They have been on the Earth’s waters for more than 500 million years. Yes, they are actually more ancient than dinosaurs.
According to scientists, there are more than 20,000 different species of fish in the world. They divided the fish based on the structure of their mouths and the types of skeletons they have. The three main types are jawless fishes, cartilaginous fishes, and bony fishes.
Jawless Fish (Agnatha)
There are only around 100 species of jawless fish in the whole world left. They were the earliest vertebrates; thus, they are very primitive in nature. They do not only lack jaws but also lack stomach. Unlike other fish, they also do not have fins and scales. In addition, their skeletons are made of cartilage. The mouths of these fishes have structures for sucking, stabbing, and scraping.
As to how they mate and reproduce, there is really not much information about it. However, like most fish, fertilization is believed to be external. The only existing jawless fishes are hagfishes and lampreys.
There are only about 76 species of hagfishes living in cold waters around the world, from shallow to as deep as 5,500 feet. The hagfish known today also pretty much looks like the ones from 300 million years ago, indicating that it had changed very little since then.
Hagfishes are pinkish in color and shaped like eels and are sometimes called “slime eels”. It may be because hagfishes produce slime to ward off attackers as well as other fish trying to steal their food.
Around their mouths, there are four pairs of thin sensory tentacles which they use to find food. They also have two rows of tooth-like structures made of keratin. They feed mainly on dead animals and they use these tooth-like structures to bore a tunnel deep into their carcasses. They are also capable of biting off chunks of food. Though their primary source of food is the dead animals, they also eat live prey. They twist their tails into knots to generate torque and increase the force of their bites.
Their eating habits may seem repulsive, but hagfishes actually help clean and recycle dead animals from the seabed. Interestingly, hagfishes can actually go without food for months.
There are only about 43 species of lampreys in the world. Lampreys are also eel-shaped jawless fish. Their long narrow body lack scales. They have a sucker-like mouth filled with a vicious-looking set of teeth that they attach to their prey. Like the hagfishes, the lampreys still look pretty much the same to how they were about 360 millions years ago. It has been a very long time, but their form has been virtually unchanged.
Lampreys can be both found in freshwater rivers and seas. Their size can range from 5 to 40 inches. They filter feed on microorganisms during the larval stage. They are commonly found on lake bottoms and muddy rivers during this period. This stage can last for seven years. Towards the end of the larval stage, they change into an eel-like creature that swims. Then, they start to attach themselves to fish in order to survive.
Because lampreys use their sucker-like mouth to clamp onto the sides of other fish, they are sometimes called as the vampires of the sea. They suck the blood, tissue, and fluids out of the fish they are attached to.
Cartilaginous Fish (Chondrichthyes)
The skeletons of the cartilaginous fish are made of cartilage, thus the name. However, unlike jawless fish which skeleton are also made of cartilage, they have jaws pair of fins. They are flexible but still tough. The cartilage gives enough structural support to allow these fish to grow to enormous sizes.
Cartilaginous fish have a skeleton, but they lack a ribcage. This means they are going to die if taken out of water, because their body will crush their internal organs. In addition, they do not have bone marrow.
The bodies of cartilaginous fish are covered in placoid scales or dermal denticles. These are pointed, toothlike scales that provide their bodies a texture rougher than sandpaper. These scales are totally unlike that of other fish. Cartilaginous fish have gills that open to the water directly through slits. Aside from gills, they may also breathe through spiracles. Spiracles are openings on top of the heads of all rays and skates. They are also present in some sharks. Spiracles allow them to breathe without taking in the sand.
All cartilaginous fish are carnivores. They feed on small fish and plankton. Some cartilaginous fish include sharks, skates, rays, and chimeras. There are about 500 different species of sharks and 600 rays and skates species. However, there are only about 47 species of chimeras.
Bony Fish (Osteichthyes)
The bony fishes are the most familiar type of fish, making up about 95 percent of all species. They have a skeleton made of bones. This characteristic is in contrast to the cartilaginous fish. They also have gills and a covering for them (operculum) which enable them to breathe when they are resting. Many species have swim bladders, a gas-filled organ which helps the fish control buoyancy. The primary organ for the respiration of bony fish is gills; however, a lot of the species can also breathe through either swim-bladders or air bladders, stomach, lungs, skin, or intestines. In addition, their bodies are covered with scales.
Bony fish can either live in freshwater or saltwater, and they can be found all over the world. With approximately 29, 000 living species, bony fish are the most diverse of all types of fish. They are also the most diverse group of vertebrates alive today.
Some examples of bony fish are tuna, salmon, tilapia, goldfish, piranha, trout, mackerel, barracuda, anchovy, and sardines.
Bony fish are classified into 4 groups based on their feeding pattern: herbivores, carnivores, omnivores, and detritivores.