Organs

Organs

Fish reproductive organs include testes and ovaries. In most fish species, gonads are paired

organs of similar size, which can be partially or totally fused.[23] There may also be a range of secondary reproductive organs that help in increasing a fish's fitness.
In terms of spermatogonia distribution, the structure of teleosts testes has two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorph fishes they are confined to the distal portion of these structures. Fishes can present cystic or semi-cystic spermatogenesis in relation to the phase of release of germ cells in cysts to the seminiferous tubules lumen.[23]
Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom and then they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.[24] Gymnovaries are the primitive condition found in lungfishes, sturgeons, and bowfins. Cystovaries are the condition that characterizes most of the teleosts, where the ovary lumen has continuity with the oviduct.[23] Secondary gymnovaries are found in salmonids and a few other teleosts.
Oogonia development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the nucleus, ooplasm, and the surrounding layers characterize the oocyte maturation process.[23]
Postovulatory follicles are structures formed after oocyte release; they do not have endocrine function, present a wide irregular lumen, and are rapidly reabosrbed in a process involving the apoptosis of follicular cells. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.[23]
Some fish are hermaphrodites, having testes and ovaries either at different phases in their life cycle or, like hamlets, can be simultaneously male and female.

Sensory and nervous system

Sensory and nervous system

Dorsal view of the brain of the rainbow trout.

Central nervous system
Fish typically have quite small brains relative to body size when compared with other vertebrates, typically one-fifteenth the mass of the brain from a similarly sized bird or mammal.[16] However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains of about as massive relative to body weight as birds and marsupials.[17]
The brain is divided into several regions. At the front are the olfactory lobes, a pair of structure the receive and process signals from the nostrils via the two olfactory nerves.[16] The olfactory lobes are very large in fishes that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the equivalent structure to the cerebrum in higher vertebrates. In fishes the telencephalon is concerned mostly with olfaction.[16] Together these structures form the forebrain.
Connecting the forebrain to the midbrain is the diencephalon (in the adjacent diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs a number of functions associated with hormones and homeostasis.[16] The pineal body lies just above the diencephalon. This structure performs many different functions including detecting light, maintaining circadian rhythms, and controlling colour changes.[16]
The midbrain or mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids.[16]
The hindbrain or metencephalon is particularly involved in swimming and balance.[16] The cerebellum is a single-lobed structure that is usually very large, typically the biggest part of the brain.[16] Hagfish and lampreys have relatively small cerebellums, but at the other extreme the cerebellums of mormyrids are massively developed and apparently involved in their electrical sense.[16]
The brain stem or myelencephalon is the most posterior part of the brain.[16] As well as controlling the functions of some of the muscles and body organs, in bony fish at least the brain stem is also concerned with respiration and osmoregulation.[16]

Sense organs
Most fish possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least as good as a human's. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the lateral line system. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey.[18] Some fishes such as catfishes and sharks, have organs that detect low levels electric current.[19] Other fish, like the electric eel, can produce their own electricity.

Pain reception in fish
In 2003, Scottish scientists at the University of Edinburgh performing research on rainbow trout concluded that fish exhibit behaviors often associated with pain.[20] Professor James D. Rose of the University of Wyoming critiqued the study, claiming it was flawed.[21] Rose had published his own study a year earlier arguing that fish cannot feel pain as they lack the appropriate neocortex in the brain.[