What Is a Fish?
Fishes come in an amazing variety of shapes and colors, but they all have three important things in common: All fishes live in water, have fins, and use gills to get oxygen from the water. We have also included a few sea creatures -- some jellyfish and octopods -- in this category.

Nesting and Reproduction
Most fishes reproduce by external fertilization—a female fish releases eggs into the water, where they are fertilized by sperm from a male. This process is called spawning. Some fishes, such as sharks, reproduce by internal fertilization— the male deposits sperm into the female fish, where the eggs are fertilized. These fishes are called “livebearers,” which means that they give birth to live young.

Fish Eggs
Fish eggs range in size from a barely visible dot to the size of a pea. Some eggs float in the open water, while others are heavier than water and sink to the bottom. Eggs on the bottom may clump together in masses, or they may stick to sand grains or to hard objects. Some fishes carefully place eggs on the undersides of rocks or on sticks in the water. Some eggs are not sticky at all and simply fall into the spaces between pebbles. Fish eggs may hatch within several days of spawning or many weeks later, depending on the species and the water temperature.


Fish Attraction
Many fishes develop bright colors during their breeding seasons. Usually it is the male that is more colorful. He uses his bright colors to attract females. Color also makes it easier for fishes to recognize members of their own species.

Spawning Run
Many fishes move to special areas where conditions are just right for spawning and protecting their eggs. For some minnows and sunfishes, this is a matter of moving a few yards in a stream or into shallow water in a lake. But for salmons and shads, it can mean a journey of thousands of miles.

Salmon Life Cycle
Coho Salmon spend part of their lives in the salt water of the Pacific Ocean, but they travel many miles into freshwater streams and rivers to spawn. Along the journey, their bodies change to a brilliant red color. Females release eggs along the gravel bottom, where they are fertilized by males. Both males and females die after spawning. The young fish hatch and spend a year in the fresh water before traveling out to sea. Two to three years later, the salmon return to the river as breeding adults. They spawn, and the cycle continues.

Fish Nests
Many fishes simply release their eggs and sperm into the water and pay no attention at all to the fertilized eggs, but other fishes build nests and guard the eggs and young. Fishes usually spawn during a particular time of year. In freshwater ponds and lakes, late spring and early summer are excellent times to observe fishes making nests and laying eggs. It’s also a good time to observe hatchlings (baby fishes) schooling together.

Fish Parenting
Most fish parents spend little time caring for their offspring. But some, such as certain catfishes, guard the nests and young for a few days after the eggs hatch. In a few species the adults carry eggs on their bodies. Some fishes actually keep the eggs in their mouths until they hatch.

Baby Fishes
When young fishes hatch they may look very different from adult fish of their species. Newly hatched fishes may be nearly transparent, but in a few days they develop color patterns. Juvenile freshwater fishes tend to resemble their parents, but most saltwater fishes have larval stages. These larval fish drift freely in the open sea and many are bizarre-looking, with odd shapes or long trailing fins.

Defense
Like most animals, fishes have to find food to eat and at the same time avoid being eaten by other animals. Fishes defend themselves in many ways, including swimming away from predators or blending into the background. Some puff up their bodies so they can’t be swallowed. Many fishes are also protected by pointy spines and hard, scaly skin.

Escape
When approached by a predator or some other threat, the first reaction of most fishes is to swim away. Some try to escape into open water, others swim to shelter among weeds, or around submerged rocks or logs.

Camouflage Color
One way fishes hide from their enemies is by camouflage (blending into the background of their habitat). Sometimes a fish can remain prefectly still, and if it is the right color and shape, its predator may not see it, even when they are looking right at it. Camouflage also works the other way: It makes a predator hard to see as it stalks its prey.

Bottom Feeder
Winter Flounder live on the bottom in saltwater bays and oceans, blending into the soft mud and sand. Bigger fishes swim by and don’t notice them at all.

Seaweed Fish
Sargassumfish are colored to match the seaweed that floats where they live. It protects them from hungry predators, but it also makes them nearly invisible to prey.

Silent but Deadly
Muskellunge are big fish with enormous appetites. They hide quietly in grassy areas of fresh water, ready to attack and eat fishes and frogs that swim by.

Body Armor
Scales are a kind of armor. Some fishes have extremely hard, bony scales that provide excellent protection. The Longnose Gar has tough diamond-shaped scales all over its body that protect it from predators.

Pointy Spines
Many fishes have spines to protect them from their enemies. Spines are sharp bones usually located near the fins or on the head. Some species have spines that are flat against the body and can be pointed outward to discourage predators. When threatened by a predator, the Porcupinefish fills itself with water and puffs up like a spiky ball. The predator usually swims away, discouraged and hungry.

Senses
Fishes have the same five senses that humans have: sight, hearing, touch, smell, and taste. They use their senses to look for food, avoid predators, find a partner to spawn with, and to find their way around their habitat. Many fishes have highly adapted senses that allow them to smell food from very far away or detect vibrations in the water.
Fish Eyes
Fishes’ eyes are like humans’ eyes but they focus in a different way. Humans have muscles that change the shape of the lens in the eye, but fishes have muscles that move their lenses back and forth. They focus on objects much as a camera focuses. Fishes see in color, just as people do.

Do Fishes Smell?
The smell detectors in fishes are in sacs in front of the eyes. Fishes smell by pumping water in through two front nostrils and use these smell detectors to sample chemicals in the water. The water then passes out through the rear two nostrils. Sharks have an amazing sense of smell. Even under the sea they can detect very small amounts of blood and oil from an injured fish miles away.

Fish Ears
Although fishes have no external ears (their ears are inside their head), they still have a keen sense of hearing. Some fishes communicate by making sounds and listening for a response from a member of their own species.

Fishy Taste
Human tastebuds are on the tongue but in some fishes the tastebuds are scattered over the outside of the body! The tastebuds of catfishes are concentrated on barbels, the whiskerlike projections near the mouth. The Brown Bullhead uses its barbels (or whiskers) to find food in murky waters and to find other catfishes.

Lateral Line
Fishes have a special sense organ called the lateral line that detects vibrations in the water. It runs along both sides of the fish just below the skin and sometimes appears as a thin black stripe. Fishes use the lateral line to sense movement in the water, the temperature of the water, and to maintain their balance.

Electric Fishes
Some fishes can detect weak electrical currents in the water using special organs called electroreceptors. They use this ability mostly to “sense” when other fishes are nearby. Other fishes, like electric rays and catfishes, produce a powerful electrical charge that stuns their prey.

Locomotion
Locomotion means how something moves from place to place through a medium like air or water. Water is about 50 times as thick as air, so most fishes are streamlined to slip easily through their watery habitats: Their heads are connected directly to their bodies without a neck, and their fins are close in to their bodies.

Speedsters
Great Barracudas swim with amazing bursts of speed, slicing through the water to catch and eat their favorite foods: fast-moving smaller fishes and speedy squids.

Wing Flappers
Skates and rays are flattened and disk-shaped. They move through the water by flapping their wing fins. Remora fish often attach themselves to Mantas (the largest of the skates and rays) and ride along.

Swim Like a Fish
Most fishes propel themselves through the water by moving their bodies in S-shaped waves. Below are examples of typical fish shapes (shown from above) and the way fishes with these shapes swim.

Over the Speed Limit
Billfishes like the Striped Marlin are some of the fastest fishes in the sea. They can swim for short periods at over 70 miles per hour!

Take a Walk
The Walking Catfish actually walks across land to move from one body of water to another. It uses its pectoral fins like legs and has a modified gill chamber to get oxygen from the air.

Swim Bladders
All fishes have an organ called a swim bladder that is filled with gas. By adding or subtracting the amount of gas (mostly oxygen) in the swim bladder, fishes control how much they float in the water. Without a swim bladder, fishes would have to swim constantly to hold a position at any depth beneath the water.

Slowpokes
Some fishes, like porcupine fishes, trunkfishes, and puffers, are slow swimmers. They mostly use their tail fins to move their wide bodies through the water.

Fish School
Some fish of the same species live in large groups called schools. They band together to protect themselves from predators, to migrate, and to search for food. Fish schools can contain thousands of individuals.

What Do Fishes Eat?
Fishes eat many kinds of food, including insects, worms, snails, clams, and other fishes. Only a few kinds of fishes eat plants. Fishes will usually eat whatever is plentiful and easy to catch, but most are specialized in some way to feed on a particular food. You can get an idea of what a fish likes to eat by studying its teeth and the shape of its body.

Throat Teeth
Besides the teeth that grow out of their jaws, some fishes also have teeth located behind the gills near the throat. They use them to eat prey in much the same way as they would use their regular teeth. Minnows and suckers have no teeth in their mouths. They shred, crush, and chew up all their food using their throat teeth.

Chew on This
Fishes usually swallow their food whole, but some have special teeth for shredding food or for crushing shells. Fishes like gars and pikes use their sharp, pointed teeth to grab and hold their prey before swallowing. Damselfishes and other species with patches of short, blunt teeth eat mostly small, soft-shelled animals. The Freshwater Drum crushes clams, snails, and other hard-shelled animals with its large throat teeth before it swallows them.

Chasers and Pouncers
How a fish catches its prey depends on the shape of its body. Slender, streamlined species, like billfishes, are fast swimmers that chase down speedy prey over a distance. Fishes with stout bodies, like basses, lie in wait for prey and ambush it when it swims by. Sunfishes have short, compressed bodies that enable them to make quick sharp turns and chase prey through vegetation.

Vegetarian Menu
Only a few fishes feed on plants, but those that do have specially designed teeth and intestines to help them digest vegetation. Teeth with flat surfaces are used for grinding plants. Very long intestines allow digestive fluids more time to work on the food.

Raking It In
Some fishes feed mainly on plankton (small plants and animals that drift freely in the open water). They use bony parts in the mouth and throat called gill rakers to strain prey out of water passing through the gills.