Fish: General Characteristics

Fish were the first vertebrate with bony skeletons to appear on the earth. The doubtless form the most numerous group of vertebrates. Unlinke today's fish, the earliest fish had no scales, fins or jawbone, but they did have type of dorsal fin. Overtime they have been changing in form and size to adapt to different environments, in both fresh water and salt water. Their bodies are generally streamlined, being covered with smooth scales and having fins that enable them to move with energy, direction and stability. In place of lungs, these complex creatures normally breathe through gills that capture oxygen dissolved in the water, and they are cold-blooded.

Distinguishing Features

Similar characteristics nearly define all fish, with a few rare exceptions. These aquatic animals are designed to live underwater, and they have jawbone and lidless eyes and are cold-blooded. They breathe through gills and are vertebrates - that is, they have a spinal column. They live in the oceans, from the poles to the equator, as well as in bodies of fresh water and in streams. Some fish migrate, but very few can pass from salt water to fresh water or vice versa. Their fish enable them to swim and move in different directions. Animals such as dolphins, seals and whales are at times mistaken for fish, but they are actually mammals.

Bony Fish

The group of fish that have evolved and diversified most in the last few million years are the osteichthyes, fish with spines and jawbones. In general, their skeletons are relatively small but firm, being made mostly of bones. Flexible fins enable them to control their movements with precision. The various species of osteichtyes have adapted to wide variety of environments and even to extreme conditions.

Cartilaginous Fish

As indicated by the name, the skeleton of cartilaginous fish is made of cartilage, a flexible, durable substance that is softer than bone. They have jaws and teeth, which are usually hard and sharp. Their body is covered with hard scales. However, they lack characteristic shared by most bony fish - the swim bladder, an organ that helps fish to float. Their pectoral fins, tail and flat head give this group a streamlined profile.

Anatomy

Most fish have the same internal organs as amphibians, reptiles, birds and mammals. The skeleton acts as a support, and the brain receives information through the eyes and the lateral line to coordinate the motions of the muscle in propelling the fish through the water. Fish breathe with gills, they have a digestive system designed to transform food into nutrients and they have a heart that pumps blood through a network or blood vessels.

Protective layer

Most fish are covered with scales, and external layer of transparent plates. All fish of a given species have the same number of scales. Depending on the family and genus of a fish, its scales can have a variety of characteristics. Scales on the lateral line of the body have small orifices that link the surface with a series of sensory cells and nerve endings. It is also possible to determine a fish's age by studying its scales.

The Art of Swimming

To swim, fish move in three dimensions: forwards and back, left and right and up and down. The main control surfaces that fish use for manoeuvring are the fins, including the tail, or caudial fin. To change direction, the fish tilts the control surfaces at an angle to the water current. The fish must also keep its balance in the water; it accomplishes this by moving its paired and unpaired fins.

Life Cycle

In an underwater environment, animals can simply secrete their sex cells into the water. But for the fertilization to be effective, the male and the female must synchronize their activities. Many species, such as the salmon, travel great distances to meet with potential mates. Upon meeting a mate they release their sex cells. The time and place are important because of the survival of the eggs depends on the water temperature. Parent-child relations are extremely varied, from the complete neglect of the eggs once laid to constant watchfulness and protection of the young. Matters of Life and Death To survive, most fish need adaptations to enable them to flee from their predators or to find food. The European place can lie on the ocean floor with its flat body. Its ivory colours makes it almost invisible. The flying fish, on the other hand, developed pectoral fins to lift itself up over the surface of the water and flee its enemies.

Long and Flexible

The sea horse is a small ocean fish that belongs to the same family as pipefish and sea dragons (syngnathidae). Its name comes from its horselike head. In fact, no other fish genus has its head at a right angle to the rest of its body. Because it cannot use speed to escape from its predators, the sea horse has the ability to change colour to blend in with its environment. The reproduction process of these is also very unique. The male has an incubatinng pouch in which the female deposits the fertilized eggs.

Deadly Weapon

One of the greatest predators in the ocean is the great white shark, easily identified by its distinctive white colouring, black eyes and fierce teeth and jaws. Many biologists believe that attacks on humans result from the shark's exploratory behaviour, because these fish often lift their heads above the water and explore things by biting them. This activity is often dangerous because of the sharpness of the sharks' teeth and the strength of their jaws. Great white sharks are implicated in most fatal shark attacks on humans, especially on surfers and divers.

The Journey to Home

After living in the oceans for five to six years, the Pacific red Salmon (Oncorhynchusnerka) returns to the river where it was born to reproduce. The journey last from two to three months, and it demands a great deal of energy. The salmon must swim against the current, climb waterfalls and evade predators, including bears and eagles. Once the salmon reach the river, the female lays her eggs, and the male fertilizes them. Typically, the same locations in specific rivers are sought year after year. This species of salmon dies after completing the reproductive cycle, unlike the Atlantic salmon, which repeats the cycle three or four times. Once the eggs hatch, the cycle begins anew.

Habitat, Tastes and Preferences

The oceans cover 70 per cent of the earth's surface. That is where life began on this planet and where the most primitive species live side by side with the most highly evolved ones. This abundance of species is due in part to the wide variety of environments found in the ocean. As one descends in depth, the water's temperature decreases, as does the amount of light. These factors determine deffirent echosystems, feeding patterns and adaptation strategies among a wide variety of fish species.

Kings of Darkness

In depths below 2,500 m (8,200 ft), where barely any light penetrates, live rare species known as abyssal fish. In this environment, life is possible near hydrothermal vents in the sea floor that warm the nearby waters. In spite of this natural warmth, in many areas the temperature never rises above 2 degree celsius (36 fahrenheit). At this depth fish have peculiar shapes, with large heads and strong teeth for eating other fish, because no vegetation can grow there. To attract their prey, many have 'lure' organs made of photophores that shine in the darkness. They also are usually black or dark brown for purposes of camouflage.

Sea Snakes

Eels (Anguilliformes), an order of ray-finned fish (Actinopterygii), are distictive for their elongated, snakelike shape. In the past they were an important food source. There are about 600 species of true eels, including morays, congers and snake eels. Eels come in a wide variety of colours and patterns, ranging from solid grey to mottled yellow. Their bodies lack scals and are covered with a protective mocous membrane. One of the most striking eels is the green moray, which lives in the Caribbean Sea and hides in the coral reefs awaiting its prey. Although it is not poisonous, it is feared by divers because its bite can inflict grave wounds.