What are Fish?

All about fish

If you have ever wondered what fish are, the world’s largest fish, and whether fish have ears, you will find plenty of fun fish facts for kids within this teaching wiki.

What are fish?

Fish are aquatic vertebrates that live in water. There are over 34,000 different species of fish with a wide range of features.

Are all fish the same? – Characteristics

Fish are extremely diverse creatures, with individual species carrying their unique characteristics. In addition, each fish species has adapted to its specific environment over time, which means that, depending on where they live, fish tend to look different. As a result, fish are the most diverse

So, are all fish the same? No, it would be almost impossible for all 32,000 types of fish to be the same. However, there are still a few evolutionary adaptations and characteristics that all fish share.

All fish have the same characteristics:

• All fish are cold-blooded

Regardless of their species, all fish are ectothermic, meaning they are cold-blooded. As such, fish are not able to stabilize their body temperature. Fish, therefore, rely completely on their surrounding environment to keep them at a steady temperature. This means a fish’s body temperature will change and adapt depending on its surroundings.

Fish are super sensitive to temperature, so they must be very careful about what environments they enter. Most fish must stay in warm waters to maintain a healthy body temperature, while others can travel into more diverse temperature ranges.

• All fish live in water

This is perhaps one of the most obvious similarities between all fish species. All fish must live in water. If they are taken out of the water, they will die. However, some fish species can spend considerable periods outside the water without suffocating. For instance, a fish called the ‘walking catfish’ has a respiratory system that allows it to live out of water for several days.

• All fish use gills to breathe

Fish do not have the same respiratory system as humans, but they still need to breathe to survive. So instead, fish live through things called gills. Gills are respiratory organs that all fish possess, along with many other water-dwelling animals. They operate by extracting dissolved oxygen from the water and excreting carbon dioxide. In some instances, creatures with gills have adapted to respire on land as long as they are kept moist. Hermit crabs are an example of this.

While all fish have gills, some species of fish that live in environments where oxygen is sparse have developed lungs as well.

• All fish have a swim bladder

One of the uniting features among all fish is that they all have a swim bladder. What is this? A swim bladder is a specialized organ filled with air designed to help fish stay buoyant in the water. Without a swim bladder, fish constantly risk sinking or floating too much in the water.

Swim bladders also allow fish to sleep in the water without sinking to the bottom. In addition, some fish swallow air, which is then sent to their swim bladder. It is this adaptation that helps fish survive in environments where oxygen is very sparse.

• All fish have fins for movement

All modern-day fish have fins that allow them to move through the water. There are lots of different types of fins that fish have. Typically, fish will have the following fins:

• a tail fin,
• a matching pair of side fins,
• dorsal fins,
• an anal fin.

Overall, fins propel fish through the water, helping them move around and maintain stability as they do so. The tail fin is what drives fish through the water. Almost all fish also have pelvic and pectoral fins, which allow them to move around and maintain stability. Most fish also have dorsal and ventral fins, which reduce the rolling motion that happens while fish are swimming. These fins also help fish when making turns.

Are all fish the same? Unfortunately, no, but there are many similarities across the different species. Here are some other characteristics that many fish share:

• Over time, fish have adapted and developed special senses that allow them to thrive in their aquatic environment. For example, water can transmit sounds, disperse chemicals, and conduct electricity better than the air. Therefore, fish have evolved to rely much less on their vision and, instead, become more dependent on their hearing, taste, and smell.
• Certain species of fish possess something called a ‘lateral line.’ This organ can detect movements and vibrations that fish use to spot predators and prey.
• Sharks and many other fish species have a specialized structure in their eye, the tapetum lucidum, which amplifies incoming light.
• You have probably never seen a fish ear, but they have them! Fish ears are internal organs and, like in humans, help with balance and hearing.

Characteristics of modern fish:

Modern fish are divided into three categories:

• Agnatha: the jawless fish.
• Chondrichthyes: jawed fish that have skeletons made of cartilage, as well as sharks, rays, skates, and ratfish.
• Osteichthyes: jawed fish that have skeletons made of bone.

The structure of a fish

Fish are fascinating creatures with complex and intricate structures. This section will take a look into the inner workings of a fish.

The muscle system

The majority of a fish’s body is made up of muscles. These muscles are connected and work together to help the fish move smoothly and quickly through the water. The muscles are typically arranged in rows of V-shaped segments, each attached to vertebrae. These segments contract, allowing the fish’s body to bend and move through the water, propelled by strong tail strokes.

The digestive system

Teeth

In fish, the digestive process begins with the mouth, where they use their teeth to capture their prey or collect plant foods. The shape and structure of a fish’s mouth and teeth vary depending on its food type. Most fish are known as predacious, meaning that they feed on small invertebrates or other fish. Predacious fish have simple, cone-shaped teeth along their jaws, on some of the bones in the roof of their mouths, and special gill arch structures in front of the esophagus. Despite having so many teeth, most predacious fish do not use them to chew their prey; they swallow their prey whole. Instead, they use their teeth to hold

Some fish have sharp teeth, which they use for chewing and tearing up their prey. These fish include sharks and piranhas, with extremely sensitive teeth for biting chunks off of their food. Parrotfish are another species of fish that have very interesting teeth. Parrotfish have beak-like mouths with short, sharp teeth that they use for breaking off the coral and pavement-like teeth in their throats to crush coral. On the other hand, catfish have tiny, brush-like teeth arranged in rows on their jaws. These teeth are designed to allow catfish to scrape plant and animal growth off of rocks. There are also many species of fish that have no jaw teeth but instead have strong throat teeth.

Intestines

Another aspect of a fish’s digestive system that varies greatly depending on its diet is the intestine. For example, in predacious fish, the intestine can be no longer than the body cavity. In herbivorous fish, the intestine can be coiled around and have a length several times longer than the entire length of the fish. The use of intestine is primarily for absorbing nutrients into the bloodstream.

The respiratory system

One of the big questions that people ask about fish is, “How do they breathe?”

Fish live underwater, so their breathing is completely different from humans, but they still go through a process of exchanging oxygen and carbon dioxide. Most fish breathe by exchanging dissolved oxygen and carbon dioxide in water via their gills. The gills are located just behind and to the side of their mouths. Breathing can be broken down into a few steps:

1. Fish take in water continuously through their mouths.
2. This water then passes backward between the gill bars and over the gill filaments.
3. The gill filaments are where the exchange of oxygen and carbon dioxide takes place.

The circulatory system

Similar to humans, a fish’s circulatory system consists of the heart, the arteries, the capillaries, and the veins. However, fish have a single circuit for blood flow and a two-chambered heart with only one atrium and one ventricle. The atrium collects blood that has come back from the body, and the ventricle pumps it to the gills, where the exchange of gases occurs. This is where the blood is re-oxygenated.

Fish senses

Smell

Though humans cannot smell anything underwater, the sense of smell is extremely important to most fish. A fish’s nasal organ is lined with special sensory cells that pick up on chemicals dissolved in the water, such as substances from food. These cells send sensory information to the brain through the first cranial nerve, which is how fish smell.

Many fish also rely on their sense of smell as an alarm system. Other fish species, like eels, rely on their sense of smell rather than vision to locate food.

Taste

Many species of fish also have a very well-developed sense of taste. Catfish are a great example of how flavor works for fish, known as “swimming tongues.” Their taste receptors – small, pit-like taste buds – cover their bodies. Catfish also have whiskers to supplement their poor vision and boost their taste organs, as they are also covered in taste buds.

Sight

Sight is another important sense for the majority of fish. A fish’s eye is not overly different from other vertebrates but varies in structure and adaptation. Fish living in dark and dim conditions usually have large eyes unless they have a special adaptation that ensures their sight is not their dominant sense. In this case, the eyes will be smaller. Fish that live in brighter, often shallow waters tend to have fairly small eyes that are efficient at seeing.

Cyclostomes – strange-looking fish with no jaws – have basic eyes compared to other fish species. The skin of the Cyclostomes is stretched over their eyeballs, making their vision much less effective. Most fish have spherical lenses designed to see objects near and far away by moving with eyeballs.

Archaeologists have found that fish developed color vision over 300 million years ago by studying fish fossils. However, not all living fish have retained this ability.

Keep reading to learn what fish eat!

Hearing

Unlike humans, fish do not have ears that protrude from their heads and help them to hear. Instead, all of the organs for hearing are internal for fish, located within their skulls on either side of the brain and behind the eyes. The process of hearing for fish is as follows:

1. Sound waves, particularly those at low frequencies, travel through water.
2. These sound waves directly impact a fish’s bones and the fluids of the head and body.
3. These sound waves are then transmitted to the hearing organs.

Fish are extremely sensitive and responsive to sound, as it is often warning them of danger. For example, a trout has been conditioned to escape the clutches of fishermen by fleeing the instant that they hear footsteps on a stream bank. However, the range of sound frequencies that fish can listen to is very restricted compared to humans. Most fish communicate with one another by producing sounds in their swim bladders, rasping their teeth in their throats, and many other ways.

Other senses

In addition to the main senses listed above, fish have a range of other reasons.

• Touch – Fish seldom rely on one type of sensory information to interact with and interpret the world around them. Communication is one of the senses fish use to engage with the environment around them and determine whether it is safe. Like most other animals, fish have touch receptors all over the surface of their bodies, so their sense of touch is very effective. Touch, pain, and temperature receptors are also very important for fish. Fish’s reactions to pain and danger are very similar to those of humans. This suggests that fish experience a sensation of pain in the same way that humans do.

• Lateral line system – The lateral line system is a sensory system that is vital for fish but cannot be found in other vertebrates except some amphibians. The lateral line system consists of small, heavily innervated (supplied with nerves) canals. These canals are located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently placed along these canals are small sensory organs designed to pick up on changes in pressure. This lateral line system enables fish to sense changes in water currents and stress, which, in turn, helps them to align themselves to the various changes that occur in the physical environment.

Where do fish live?

There is no one clear answer to the question: Where do fish live? This is because there are loads of different species that require other things from their environments to survive and thrive. As such, fish live in a variety of habitats. However, regardless of where fish live, there are a few things that fish need to live comfortably in their environment:

• Oxygen

Like almost all living things, fish need oxygen to survive. If an environment does not have an adequate supply of oxygen, fish will not be able to stay there. In addition, depending on the fish species, some will require more oxygen than others. Carp, for example, can live with much less oxygen than other types of fish.

What can affect the oxygen supply in an aquatic environment? Like plants and trees provide oxygen for us to breathe on land, living plants in and around water habitats supply the water with oxygen through photosynthesis. Moreover, oxygen can also be added to the water from the air around it.

Several factors can also take oxygen away from an environment. For instance, decaying plants use oxygen from the water to decompose. Pollution also contributes to reducing the amount of oxygen in the water.

The temperature of the water also affects its capacity to hold oxygen. Cold water can have much more oxygen than warm water.

• Food

Another feature that must be present in all places where fish live is food. Just like we, as humans, need a sufficient amount of food to survive, fish also must eat to live. Not only does there have to be an adequate amount of food available for fish to live somewhere, but there also has to be the right type of food and not too much competition. The kind of food that fish look for will change from species to species.

• Water Quality

Different species of fish require different levels of water quality. Some species of fish, for instance, can live in poor quality waters, such as carp, while others need much higher quality water to survive.

• Shelter

For a lot of fish, places to seek shelter are a necessity for any living environment. So, where fish live depends on what kinds of plants, rocks, logs, reefs, etc., are available to shelter under. There are a couple of reasons why fish seek cover. On the one hand, being able to hide away under different objects allows fish to avoid detection from predators. But, on the other hand, it gives them the element of surprise when capturing their prey.

Fish have adapted to live in a range of habitats, such as:

• Coastal waters
• The deep sea
• Coral reefs
• Rivers
• Streams
• Inland lakes
• Great lakes
• Coastal wetlands

How do fish adapt to their environments?

While fish are abundant on the planet, some species have shared characteristics depending on their environment. Here are a few examples of how fish adapt to different habitats:

Freshwater fish:

Freshwater habitats have low dissolved salt concentrations, so fish in these environments require different physiological adaptations.

Freshwater fish have less saline body fluids than their surroundings. This is because their gills diffuse dissolved gasses while keeping the salts in the body fluid inside, while their scales limit the amount of water diffusion through their skin. In addition, their kidneys are well-developed enough to reclaim salts from body fluids before excretion.

Reef fish:

Reef fish tend to be bright and conspicuous, matching their colorful environment. Their appearance can camouflage against the colorful background, protecting them from predators. Plus, it helps fish identify their species for mating.

Open-water fish have bodies adapted to swim faster. However, speed is less important in a coral reef. Coral reef fish often have flat and maneuverable bodies, allowing them to better navigate the different nooks and crannies.

Deep-sea fish:

Deep-sea fish are often drab in color or even transparent. The absence of red light at these depths means they are concealed from predators and prey, helping them to eat and avoid being eaten.

It can be difficult to mate with no light, but deep-sea anglerfish have adapted to find a way around this. The tiny male anglerfish attach themselves to their much larger mates with their hooked teeth and establish a parasitic relationship for life. The male receives nourishment from the female, while the female never has to find another mate.

Fish habitats and humans

Several factors have impacted water quality and fish habitats over the last century, leading to negative consequences for fish populations. These include:

• Coastal development: Developing coastal areas threatens the function and diversity of coastal habitats, which are fundamental for many fish species. For example, removing vegetation and trees from riverbanks limits shade and increases the water temperature. Plus, less vegetation leads to an increase in erosion, affecting spawning areas. In addition, dams and other infrastructure prohibit fish from migrating to reach spawning habitats and harm aquatic life.

• Invasive species: Introducing new species to ecosystems creates more competition for food and habitat, meaning fish species can be displaced.

• Pollution: Pollution is a huge problem for fish habitats. Industrial pollutants, waste, sewage, and plastic continue to impair water quality and destroy fish habitats.

What do fish eat?

Fish are such diverse creatures that their diet varies greatly from species to species. Fish can be separated into three categories based on what they eat:

• Carnivores
• Herbivores
• Omnivores

What fish eat depends on their species and environment, but they aren’t picky overall. Instead, fish roam around, devouring whatever they can catch or forage. Most fish are omnivorous, meaning they feed on meat and plant materials. Regarding their macronutrient breakdown, fish diets consist of around 40% to 50% protein, with the rest being made up of fats and carbohydrates.

Across the board, fish tend to feed on whatever is closest to them and in the most abundant supply in their surrounding environment. There are some fish, however, who have adapted to eat very specific types of food. For instance, some scale-eating fish feed primarily off other fish’s scales.

As we have discussed, what fish eat depends on their environment and genetic makeup, but there are a variety of key foods and food groups that tend to crop up in most fish diets:

• Insects: flies, crickets, grasshoppers, beetles, etc.
• Worms: mealworms, bloom worms, leeches, etc.

Larger fish also tend to prey on the following creatures:

• Rats
• Mice
• Frogs
• Turtles
• Smaller fish

Ten fish facts for kids:

Impress your class with your aquatic knowledge using these ten fish facts for kids!

1. Fish have been around for 500 million years – longer than the dinosaurs!
2. Most fish reproduce by laying eggs.
3. However, great white sharks give birth to live babies called pups.
4. Most fish have taste buds all over their body.
5. Fish can drown if there is not enough oxygen in the water.
6. Seahorses can swim upright.
7. The largest fish in the world is the whale shark, which can reach lengths of 41.5 feet! Despite its size, the whale shark eats plankton and is relatively harmless to humans.
8. By contrast, the recently-discovered Paedocypris progenetica is the smallest fish in the world at less than a third of an inch long!
9. The sailfish is the fastest fish in the world and can reach speeds of over 68 mph (ca. 109 km/h).
10. A fish species traveling in synchrony is called a school or shoal.