In order to survive, any living thing often increases its chances of survival and reproduction through changes in its behavior, physiology or structure during the process of coordination with its living environment. The word adaptation has many meanings in biology, such as evolutionary adaptation, ecological adaptation, physiological adaptation, behavioral adaptation, sensory adaptation, etc.
Evolutionary adaptation is the result of nature's selection of widespread variations in organisms. Natural selection enables organisms to have an appearance and structure that is suitable for the environment, and passes the genes it carries to its offspring. These genes further reproduce, expanding and consolidating the population that adapts to the environment. The adaptation of fish morphological structure is first reflected in body shape. In order to adapt to life in the water, the body shape of fish is generally spindle-shaped, with pointed ends and a convex middle, so that the head and tail in the water will not be too heavy and sink to the bottom. Although the heads of silver carp and bighead carp are large, the brain cavity only accounts for 1/10 of the head cavity. The head is rich in fat, which makes its specific gravity light. Although the head is large, it is light. Eels and eels living in caves have rounded bodies in order to move in and out of caves quickly and freely. The body of the ray becomes flat when it lives at the bottom of the water, so these are the results of the body shape adapting to the environment. In order for fish to stay freely in any still water body, and to maintain balance and swim, the changes in their fins play an extremely important role.
The formation of dorsal and anal fins prevents fish from swaying in the water; the pectoral and pelvic fins balance the body and serve as oars for paddling; and the tail fin serves as a rudder to control direction. Because different fish live in different living environments, in order to adapt to the environment, fish fins have undergone various metamorphoses. For example, the pectoral fins of flying fish are particularly extended, allowing flying fish to glide when flying out of the water; ventral suction in rapids in mountain streams In the loach, the pectoral fins and pelvic fins become flat suction cup shapes to allow the fish to cling to the rocks and not be washed away by the water; in order to protect itself, the anchovy fish living in the ocean moves its first dorsal fin forward to the top of its head and It turns into a big sucker. It usually relies on this sucker to stick to the belly of big fish or boats to avoid enemies and travel far away. In order to adapt to lying on its side and crawling on the bottom of the water, flatfish move the eyes on both sides of their heads to the upward side.
The eyes of fish living in the upper layer of the water are on both sides of the head, while the eyes of bottom fish are mostly lower. However, fish in caves have been unable to see light for a long time, and their eyes have degenerated into blind fish. The changes in the mouth and jaws are also obvious. The mouths of carnivorous fish, such as snakeheads, catfish, catfish, etc., are wide. The mouths of algae-eating fishes, such as white turtles and white turtles, are located below the head and have hard horny lower jaws. layer. For some carnivorous fish that are not good at swimming at the bottom, such as anglers, their mouths are particularly large, and the first few rays of their dorsal fins move forward to the top of their heads, turning them into fishing lines and bait to lure small animals. For fish that live in rapids, such as cuttlefish, the lower lip of the mouth turns into a sucker. The lamprey's mouth is shaped into a round sucker, which is used to attach to the surface of other fish to suck its blood and flesh. In addition, in order to adapt to the environment, fish have made many changes in the body surface and inner cavity to adapt to the environment. All these changes are changes formed in the long-term evolutionary process.
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