The Fish

Features

A fish (pl.: fish or fishes) is an aquatic, gill-bearing animal with a hard skull that lacks limbs with digits. This includes hagfish, lampreys, and both cartilaginous and bony fishes. Some 96% of living fish species are teleosts, bony fishes able to protrude their jaws. The bony fishes or Osteichthyes include the tetrapods, which evolved from lobe-finned fishes; major features such as having jaws and an adaptive immune system evolved in the Osteichthyes.

Senses

The lateral line system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey.[58] This can be considered both a sense of touch and of hearing. Blind cave fish navigate almost entirely through the sensations from their lateral line system. Some fish, such as catfish and sharks, have the ampullae of Lorenzini, electroreceptors that detect weak electric currents on the order of millivolt. Vision in fishes is an important sensory system. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rods and cones (for scotopic and photopic vision); most species have colour vision. Some fish can see ultraviolet, while others can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Hearing too is an important sensory system in fish. Fish sense sound using their lateral lines and otoliths in their ears, inside their heads. Some can detect sound through the swim bladder. Some fish, including salmon, are capable of magnetoreception; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field. The mechanism of fish magnetoreception remains unknown; experiments in birds imply a quantum radical pair mechanism.

Eggs

Eggs are released from the ovary to the oviducts. Over 97% of fish, including salmon and goldfish, are oviparous, meaning that the eggs are shed into the water and develop outside the mother's body. The eggs are usually fertilized outside the mother's body, with the male and female fish shedding their gametes into the surrounding water. In a few oviparous fish, such as the skates, fertilization is internal: the male uses an intromittent organ to deliver sperm into the female's genital opening of the female. Marine fish release large numbers of small eggs into the open water column. Newly hatched young of oviparous fish are planktonic larvae. They have a large yolk sac and do not resemble juvenile or adult fish. The larval period in oviparous fish is usually only some weeks, and larvae rapidly grow and change in structure to become juveniles. During this transition, larvae must switch from their yolk sac to feeding on zooplankton prey. Some fish such as surf-perches, splitfins, and lemon sharks are viviparous or live-bearing, meaning that the mother retains the eggs and nourishes the embryos via a structure analogous to the placenta to connect the mother's blood supply with the embryo's.

Species

Fish range in size from the huge 16-metre (52 ft) whale shark to some tiny teleosts only 8-millimetre (0.3 in), such as the cyprinid Paedocypris progenetica and the stout infantfish. Swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10-20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second. A typical fish is cold-blooded, has a streamlined body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and a tail fin, jaws, skin covered with scales, and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes.

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