Fish are the oldest vertebrates. They inhabit almost all aquatic environments on Earth—from freshwater lakes and rivers to saltwater seas and oceans.
The earliest fish were jawless, round-mouthed creatures that appeared on Earth during the Cambrian period, 450 million years ago. Fish are easily distinguishable by their appearance, and they make up the largest group of vertebrates: of the 50,000 species of vertebrates, more than 22,000 are fish.
School of fish in the sea
Not all animals that live in water are fish. For example, whales are mammals. However, all fish are well adapted to aquatic life. They move using fins. Fish have two pairs of fins—pectoral fins and pelvic fins—located on either side of their body; a caudal fin, located at the tail; and depending on the species, they have one or two dorsal fins on their back and one anal fin on their anus. They have a gas-filled sac called a swim bladder, which allows them to sink, float, and maintain position in the water. Only rays and sharks lack this organ. Fish also have gills for breathing; in most species, the gills are covered by an operculum. The gills are located on the sides of the head, behind the mouth, and are used to filter water swallowed from the mouth, obtain oxygen from the water, and then expel the water through openings called gill slits. Different species of fish vary greatly in size. Their bodies consist of three parts: the head, the trunk, and the tail. Their skin is covered with scales of varying sizes and numbers. There is a distinct line on each side called the lateral line, a sensory organ used to determine direction. The muscles of some bony fish are separated by tiny bones.
lamprey
The first fish to appear on Earth had a round, jawless mouth; now only about 70 species of such fish exist, forming jawless fish. Among these fish, lampreys are the most famous; they have no scales, and their slender, round bodies resemble eels. Lampreys attach themselves to other fish using their sucker-like mouths, feeding on the host's blood. Other fish are divided into two main categories: bony fish and cartilaginous fish.
Bony fish have a skeleton. Among these fish are primitive bony fish whose skeletons are only partially bony. Examples include lobe-finned fish (including coelacanths), lungfish, and sturgeon (such as sturgeon). These fish differ from more evolved bony fish in that their skeletons are entirely composed of bone. Moray eels, soles, and scorpionfish are representative of bony fish, varying in appearance but all possessing highly symmetrical caudal fins covered in tiny scales (with only a few exceptions, including eels and some carp). Bony fish are divided into several groups. Eels are fish whose juveniles look very different from their adults. Herring are fish that live in groups. Carp include almost all freshwater fish. Perch and tuna are fish with caudal fins supported by hard spokes. They are called "scorpion-finned fish" and constitute the largest group of bony fish.
Cartilaginous fish have a skeleton made entirely of cartilage, reinforced with calcium. These fish are primarily sharks and rays.
Eels are born as small, flat-bodied fish, called "baby eels." When mature, they have long, smooth bodies, generally scaleless, with a continuous fin running along their backs. Eels live in rivers and lakes in Europe and the Americas, migrating to the Sargasso Sea in the northeastern West Indies of the North Atlantic to reproduce. The fry die after birth. On their return journey across the Atlantic, the baby eels develop into adult eels, and during this return journey, they also begin to reproduce in the Sargasso Sea.
Herrings inhabit the North Sea, the English Channel, and the Baltic Sea. Adults have a pale belly and a dark blue or nearly black back. Like sardines and herring, they live in schools, sometimes in groups of thousands. This is an effective defense mechanism for each fish, as a predator can only choose one target from among many. When attacked, the school disappears almost immediately.
There are thousands of species of carp, found almost worldwide. These freshwater fish have large scales, and their teeth are fixed to their throats rather than their jaws. Their mouths can move forward to suck up food. Many species of the genus *Cyprinus* primarily live in calm rivers, small ponds, and lakes in Asia and Europe, and there is great variation in shape and color among the species. Some species have only a few large scales (mirror carp) or almost no scales (grass carp). These fish are easy to raise, and breeders have created many varieties. Carp mainly feed on plants and invertebrates. Spawning season depends on water temperature; it cannot be too cold (at least 20°C). Females lay hundreds or thousands of eggs, but most of the fry become food for other fish and even adult carp as soon as they hatch.
Koi
Spinyfins appeared approximately 60 million years ago. The perch is a typical example, with fins supported by hard, sharp spokes and a large, spiny tail fin. Perch live in lakes and rivers in Europe and North America, feeding on invertebrates and small fish, including their own juveniles. Other species of these fish live in the ocean, such as tuna and swordfish, both powerful swimmers capable of reaching speeds of up to 100 kilometers per hour. Tuna can weigh up to 500 kilograms and are carnivorous. Among fish, some possess the unique ability to maintain a body temperature higher than the water temperature; examples include the longfin tuna of the Pacific Ocean and the bluefin tuna of the Mediterranean and Atlantic Oceans.
Sharks and rays are the main representatives of modern cartilaginous fishes, which may have appeared as early as 410 million years ago. As their name suggests, they have skeletons made of cartilage. Cartilage is a flexible material that hardens when filled with calcium, and is a solid, bone-like substance. Cartilaginous fish thrive in temperate and warm oceans. They breathe in the water using gills. The gills communicate directly with the outside world through several gill slits behind the head. There are approximately 550 species of cartilaginous fish, of which 370 are sharks, and the rest are mainly composed of flattened rays and electric rays.
