Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the
esophagus. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called
pyloric caeca, which secrete digestive
enzymes and absorb nutrients. Organs such as the
liver and
pancreas add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.
Excretion
As with many aquatic animals, most fish release their nitrogenous wastes as
ammonia. Some of the wastes
diffuse through the gills. Blood wastes are
filtered by the
kidneys.
Saltwater fish tend to lose water because of
osmosis. Their kidneys return water to the body. The reverse happens in
freshwater fish: they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.
Scales
The scales of fish originate from the mesoderm (skin); they may be similar in structure to teeth.
Sensory and nervous system
Central nervous system
Fish typically have quite small
brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal.
However, some fish have relatively large brains, most notably
mormyrids and
sharks, which have brains about as massive relative to body weight as
birds and
marsupials.
Fish brains are divided into several regions. At the front are the
olfactory lobes, a pair of structures that receive and process signals from the
nostrils via the two
olfactory nerves.
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 structural equivalent to the
cerebrum in higher vertebrates. In fishes the telencephalon is concerned mostly with
olfaction.
Together these structures form the
forebrain.
Connecting the forebrain to the
midbrain is the
diencephalon (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with
hormones and
homeostasis.
The
pineal body lies just above the diencephalon. This structure detects light, maintains
circadian rhythms, and controls color changes.
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.
The
hindbrain or
metencephalon is particularly involved in swimming and balance.
The cerebellum is a single-lobed structure that is typically the biggest part of the brain.
Hagfish and
lampreys have relatively small cerebellae, while the
mormyrid cerebellum is massive and apparently involved in their
electrical sense.
The
brain stem or
myelencephalon is the brain's posterior.
As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs
respiration and
osmoregulation.
Sense organs
Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's. Many fish also have chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears, many fish may not hear very well. Most fish have sensitive receptors that form the
lateral line system, which detects gentle currents and vibrations, and senses the motion of nearby fish and prey.
Some fish, such as catfish and sharks, have organs that detect low-level electric current.
Other fish, like the electric eel, can produce electric current.
Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.
Capacity for pain
Experiments done by William Tavolga provide evidence that fish have
pain and fear responses. For instance, in Tavolga’s experiments,
toadfish grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode.
In 2003, Scottish
scientists at the
University of Edinburgh and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with
pain in other animals.
Bee venom and
acetic acid injected into the lips resulted in fish rocking their bodies and rubbing their lips along the sides and floors of their tanks, which the researchers concluded were attempts to relieve pain, similar to what mammals would do.
Neurons fired in a pattern resembling human neuronal patterns.
Professor James D. Rose of the
University of Wyoming claimed the study was flawed since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours".
Rose argues that since fish brains are so different from human brains, fish are probably not conscious in the manner humans are, so that reactions similar to human reactions to pain instead have other causes. Rose had published a study a year earlier arguing that fish cannot feel pain because their brains lack a
neocortex.
However, animal behaviorist
Temple Grandin argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions."
Animal welfare advocates raise concerns about the possible
suffering of fish caused by angling. Some countries, such as Germany have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish.
Muscular system
Most fish move by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the back fin, backward force is applied to the water, and in conjunction with the fins, moves the fish forward. The fish's fins function like an airplane's flaps. Fins also increase the tail's surface area, increasing speed. The streamlined body of the fish decreases the amount of friction from the water. Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a
swim bladder that adjusts their buoyancy through manipulation of gases.
Homeothermy
Although most fish are exclusively aquatic and
ectothermic, there are exceptions to both cases.
Fish from multiple groups can live out of the water for extended time periods.
Amphibious fish such as the
mudskipper can live and move about on land for up to several days.
Certain species of fish maintain elevated body temperatures.
Endothermic teleosts (bony fishes) are all in the suborder
Scombroidei and include the
billfishes, tunas, and one species of "primitive"
mackerel (
Gasterochisma melampus). All sharks in the family
Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are endothermic, and evidence suggests the trait exists in family
Alopiidae (
thresher sharks). The degree of endothermy varies from the
billfish, which warm only their eyes and brain, to
bluefin tuna and
porbeagle sharks who maintain body temperatures elevated in excess of 20
°C above ambient water temperatures.
See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central
nervous system processing, and higher rates of
digestion.
Reproductive system
Organs
Fish reproductive organs include
testes and
ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused.
There may also be a range of secondary organs that increase reproductive 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 release phase of germ cells in cysts to the seminiferous tubules
lumen.
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 from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the
oviduct.
Gymnovaries are the primitive condition found in
lungfish,
sturgeon, and
bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.
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.
Postovulatory
follicles are structures formed after oocyte release; they do not have
endocrine function, present a wide irregular lumen, and are rapidly reabsorbed 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.
Some fish are
hermaphrodites, having both testes and ovaries either at different phases in their life cycle or, as in hamlets, have them simultaneously.
Reproductive method
Over 97% of all known fishes are
oviparous,
that is, the eggs develop outside the mother's body. Examples of oviparous fishes include
salmon,
goldfish,
cichlids,
tuna, and
eels. In the majority of these species, fertilization takes place outside the mother's body, with the male and female fish shedding their
gametes into the surrounding water. However, a few oviparous fishes practice internal fertilization, with the male using some sort of
intromittent organ to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the
horn shark, and oviparous rays, such as
skates. In these cases, the male is equipped with a pair of modified
pelvic fins known as
claspers.
Marine fish can produce high numbers of eggs which are often released into the open water column. The eggs have an average diameter of 1 millimetre (0.039 in).
The newly hatched young of oviparous fish are called
larvae. They are usually poorly formed, carry a large
yolk sac (for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed
metamorphosis) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on
zooplankton prey, a process which depends on typically inadequate zooplankton density, starving many larvae.
In
ovoviviparous fish the eggs develop inside the mother's body after internal fertilization but receive little or no nourishment directly from the mother, depending instead on the
yolk. Each embryo develops in its own egg. Familiar examples of ovoviviparous fishes include
guppies,
angel sharks, and
coelacanths.
Some species of fish are
viviparous. In such species the mother retains the eggs and nourishes the embryos. Typically, viviparous fishes have a structure analogous to the
placenta seen in
mammals connecting the mother's blood supply with that of the embryo. Examples of viviparous fishes include the
surf-perches,
splitfins, and
lemon shark. Some viviparous fishes exhibit
oophagy, in which the developing embryos eat other eggs produced by the mother. This has been observed primarily among sharks, such as the
shortfin mako and
porbeagle, but is known for a few bony fish as well, such as the
halfbeak Nomorhamphus ebrardtii.
[32] Intrauterine cannibalism is an even more unusual mode of vivipary, in which the largest embryos eat weaker and smaller siblings. This behavior is also most commonly found among sharks, such as the
grey nurse shark, but has also been reported for
Nomorhamphus ebrardtii.
Aquarists commonly refer to ovoviviparous and viviparous fishes as
livebearers.
Immune system
Immune organs vary by type of fish.
In the
jawless fish (lampreys and hagfishes), true
lymphoid organs are absent. These fish rely on regions of
lymphoid tissue within other organs to produce immune cells. For example,
erythrocytes,
macrophages and
plasma cells are produced in the anterior kidney (or
pronephros) and some areas of the gut (where
granulocytes mature.) They resemble primitive
bone marrow in hagfish.
Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to
chondrichthyes; the epigonal organs (lymphoid tissue similar to mammalian bone) that surround the gonads, the
Leydig's organ within the walls of their esophagus, and a
spiral valve in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable
thymus and a well-developed
spleen (their most important immune organ) where various
lymphocytes, plasma cells and macrophages develop and are stored.
Chondrostean fish (sturgeons, paddlefish and
bichirs) possess a major site for the production of granulocytes within a mass that is associated with the
meninges (membranes surrounding the central nervous system.) Their heart is frequently covered with tissue that contains lymphocytes,
reticular cells and a small number of
macrophages. The chondrostean kidney is an important
hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.
Like chondrostean fish, the major immune tissues of bony fish (or
teleostei) include the kidney (especially the anterior kidney), which houses many different immune cells.
In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.
In 2006, a lymphatic system similar to that in mammals was described in one species of teleost fish, the
zebrafish. Although not confirmed as yet, this system presumably will be where naive (unstimulated)
T cells accumulate while waiting to encounter an
antigen.
http://en.wikipedia.org/wiki/Fish
![Drawing of animal with large mouth, long tail, very small dorsal fins, and pectoral fins that attach towards the bottom of the body, resembling lizard legs in scale and development.[43]](http://en.wikipedia.org/wiki/File:Dunkleosteus_BW.jpg)
