There are over thirty thousand different species of fish, and they are the most numerous vertebrates. Vertebrates are animals that have a bony spine that contains a nerve (spinal) chord. Vertebrates usually have an internal skeleton that provides support and protection for internal organs. This spine and skeleton allow vertebrates to move quickly and to have great strength.
Fish usually live surrounded entirely by water. They all have gills for breathing and fins for swimming. Most fish are ectotherms, which means that their bodies are nearly the same temperature as the water in which they live. About 60% of all fish live entirely in saltwater, while the rest live in freshwater or both freshwater and saltwater.
One of the most remarkable things about fish is their diversity. Fish can be very large, like the whale sharks that can reach 90,000 pounds (41,000 kilograms) or very small, like gobies that can weigh as little as 0.0004 ounce (0.1 gram). They have a variety of diets, including plants, other fish, invertebrates (animals without a backbone) and microscopic plankton (free-floating plants and animals).
Fish find their food in many different ways, including hunting with their eyes, grazing, scraping the sea floor, and digging. Some fish have special organs that bioluminesce (create light) and lure prey towards their mouths, while others use this glow to make themselves blend in with coral or other ocean features, disguising themselves from predators.
Fish come in a variety of colors that can serve to scare predators away or blend into their environment. Some fish live their entire lives within one bay or cove, while others may migrate thousands of miles (kilometers) across the ocean. Fish can live alone or they may swim with many other fish, called schools, to protect against predators.
Fish are classified into three groups. The jawless fish belong to the class Agnatha. They include hagfish and lampreys. The rays and sharks belong to the class Chondrichthyes. They have skeletons that are made of a tough material called cartilage. The bony fish belong to the class Osteichthyes. This largest group includes many familiar fish like tuna, halibut, anchovy, and cod.
Hagfish live in colonies (groups) on the sea floor. They dig in sediments (sand, gravel, and silt) for worms to eat. If approached by predators, hagfish emit large quantities of foul-smelling slime from glands along the sides of their bodies. This usually discourages or confuses predators. After the danger has passed, the hagfish will tie itself in a knot, which it slides along the length of its body to scrape off the slime.
The mouth of the lamprey is called an oral disc. It is cone-shaped and contains sharp teeth that it uses to bore a hole into the side of another animal. The lamprey then attaches its oral disk to the live animal’s wound and feeds off the blood and tissue of its host. Lampreys usually detach after some period of time without killing their host. Lampreys are usually most often found attached to bony fish, but they also have been seen on whales and dolphins.
It is the same material that is found in human ears and noses. Sharks and rays do not have gas bladders (internal sacs that fill with gas to help the fish rise or fall in the water so that it does not waste energy by continually swimming). Sharks and rays must continually swim in order to prevent themselves from sinking.
However, the liver of sharks is large and it contains a lot of oily materials. As oil is less dense than water, this special liver helps the shark stay afloat. Cartilaginous fish have several rows of teeth that fall out as they age. They are then replaced with new teeth that grow in from behind.
Sharks do not have scales; instead they have rough plates called dentricles embedded in their skin. These dentricles make the skin feel abrasive, like sandpaper. Many sharks have electroreceptors on their heads. These specialized organs allow the shark to sense the electrical currents generated by fish as they swim through the water. The shark’s well-developed nervous system, including a large brain, also helps it locate its prey (animals that are food).
Rays have a more flattened shape than sharks. Their fins are attached to their bodies so that they look like triangular or semicircular wings. Large rays, like the manta ray, can measure 22 feet (7 meters) from fin to fin. These huge animals feed on plankton. Other rays, like the stingray, have sharp barbs attached to the base of their tail. These are used as defense against predators. Another family of rays can actually produce an electric current, which they can use to stun prey.
The class Osteichthyes, or the bony fish, make up the majority of fish, with almost 28,000 different species. These fish all have a strong, but lightweight, skeleton that supports their organs. They have gas bladders that help them maintain buoyancy. The teeth of bony fish are fused to their jawbone and do not fall out as do the teeth of the cartilinagous fish.
Osteichthyes are found in every type of marine environment from near-shore tidepools and coral reefs to the very bottom of the deep ocean. Nearly 90% of all the bony fish are categorized into one order, Teleostei. These fish include many common fish, like the cod, tuna, seabass, and perch.
Teleostei also includes unusual fish like the mola, which floats near the surface of the ocean in warm currents; the angler fish, which lives on the seafloor and lures its prey using a worm-shaped appendage; and the football fish, which permanently fuses with its mate.
Movement Many teleost fish have bodies that are shaped to allow them to move easily through water. In particular, fast or constantly swimming fish have body shapes that minimize drag, or resistance to movement. The less surface area comes into contact with water in the forward direction, the less drag the fish will have.
A torpedo shape, with a body that tapers towards the rear, is one of the most effective shapes for minimizing drag, and many fast-swimming fish have this sort of shape. An example of a fish with a shape that minimizes drag is the swordfish, which can reach speeds up to 75 miles per hour (120 kilometers per hour) in short bursts.
Some fish, like eels, wave their entire bodies back and forth in an “S” shape to move through the water. This is not a very efficient way of swimming as it requires a lot of energy and it increases the surface that confronts the water as the fish swims forward.
More advanced swimmers have a stiff body, with a tail that bends back and forth behind the fish. This allows the fish to conserve energy because it only moves its tail, not its entire body. It also minimizes the area that confronts the water to the head of the fish.
Water and gas balance Just like humans, metabolism (the process of cells burning food to produce energy) in fish requires oxygen and produces carbon dioxide as a waste product. Fish breathe through gills, which are found underneath flaps on both sides of the head.
Water containing dissolved oxygen is brought in through the mouth and pumped over the gills. The gills are packed with blood vessels that absorb the oxygen from the water and produce the carbon dioxide waste that is generated within the fish.
The body fluids of saltwater teleost fish are about one-third as salty as ocean water. Another way of thinking about this is that the concentration of water inside these fish is greater than the concentration of water in the ocean. Because of osmosis (the passage of a liquid from a weak solution to a more concentrate solution), the water from the inside of the fish is constantly diffusing (moving outward) from the fish.
As a result, saltwater fish must regularly drink seawater to replenish the water that is lost by osmosis. The fish have special salt glands in their gills that remove the excess salt that comes inside the fish with the seawater.
Lateral line Teleost fish have developed an interesting sensory organ that helps to sense vibrations in the water. Along both sides of most teleost fish is a long row of canals (tubes) that are packed with nerve cells. These nerves detect movements of currents, changes in water pressure, and even noises.