A fish (: fish or fishes) is an aquatic animal, Anamniotes, gill-bearing vertebrate animal with swimming fish fin and craniate, but lacking limbs with digits. Fish can be grouped into the more basal jawless fish and the more common jawed fish, the latter including all extant taxon cartilaginous and bony fish, as well as the extinct and . In a break to the long tradition of grouping all fish into a single class (Pisces), modern phylogenetics views fish as a paraphyletic group.
Most fish are ectotherm, their body temperature varying with the surrounding water, though some large nekton like white shark and tuna can hold a higher core temperature. Many fish can communicate acoustically with each other, such as during courtship displays. The study of fish is known as ichthyology.
The earliest fish appeared during the Cambrian as small ; they continued to evolution through the Paleozoic, diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins, the , had heavy bony plates that served as protective against invertebrate . The first fish with , the placoderms, appeared in the Silurian and greatly diversified during the Devonian, the "Age of Fishes".
Bony fish, distinguished by the presence of and later ossified , emerged as the dominant group of fish after the end-Devonian extinction wiped out the , the placoderms. Bony fish are further divided into the lobe-finned fish and ray-finned fish. About 96% of all living fish species today are , a crown group of ray-finned fish that can cranial kinesis. The Tetrapoda, a mostly terrestrial clade of vertebrates that have dominated the top in both aquatic and terrestrial ecosystems since the Late Paleozoic, evolved from lobe-finned fish during the Carboniferous, developing air-breathing homologous to swim bladders. Despite the cladistic lineage, tetrapods are usually not considered to be fish.
Fish have been an important natural resource for since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or fish farm them in fish pond or in breeding cages in the ocean. Fish are caught for recreation, or raised by fishkeeping as ornamental fish for private and public exhibition in aquarium and . Fish have had a role in human culture through the ages, serving as Deity, religious symbols, and as the subjects of art, books and movies.
Etymology
The word
fish is inherited from Proto-Germanic, and is related to
German language Fisch, the
Latin language piscis and Old Irish īasc, though the exact root is unknown; some authorities reconstruct a Proto-Indo-European root *peysk-, attested only in
Italic languages,
Celtic languages, and Germanic.
[Winfred Philipp Lehmann, Helen-Jo J. Hewitt, Sigmund Feist, A Gothic etymological dictionary, 1986, s.v. fisks p. 118]
Evolution
Fossil history
About 530 million years ago during the Cambrian explosion, fishlike animals with a
notochord and eyes at the front of the body, such as
Haikouichthys, appear in the
fossil record.
During the late
Cambrian, other jawless forms such as
appear.
Gnathostomata appear in the Silurian, with giant armoured Placodermi such as Dunkleosteus.
During the Devonian, fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes".
Phylogeny
Fishes are a
paraphyly group, since any
clade containing all fish, such as the
Gnathostomata or (for bony fish)
Osteichthyes, also contains the clade of
(four-limbed vertebrates, mostly terrestrial), which are usually not considered fish.
Some tetrapods, such as
and
Ichthyopterygia, have secondarily acquired a fish-like body shape through convergent evolution.
On the other hand,
Fishes of the World comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using the taxon Osteichthyes as a clade, which now includes all tetrapods".
The
biodiversity of extant fish is unevenly distributed among the various groups;
teleosts, bony fishes
Cranial kinesis, make up 96% of fish species.
The
cladogram shows the
phylogeny of all groups of living fishes (with their respective diversity
) and the tetrapods.
[ Table 1a: Number of species evaluated in relation to the overall number of described species, and numbers of threatened species by major groups of organisms] Extinct groups are marked with a dagger (†); groups of uncertain placement
are labelled with a question mark (?) and dashed lines (- - - - -).
Taxonomy
Fishes (without tetrapods) are a
paraphyletic group and for this reason, the class
Pisces seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three
extant taxon classes ("
Agnatha",
Chondrichthyes, and "
Osteichthyes"), and with extinct forms sometimes classified within those groups, sometimes as their own classes.
Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within the nine largest families; from largest to smallest, these are Cyprinidae, Gobiidae, Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families are monotypic, containing only one species.
Diversity
Fish range in size from the huge
whale shark to some tiny teleosts only long, such as the cyprinid
Paedocypris progenetica and the
stout infantfish.
File:Rhincodon typus fgbnms (cropped).jpg|Largest: whale shark
File:Paedocypris progenetica 001.jpg|Smallest: e.g. Paedocypris progenetica
Swimming performance varies from fish such as tuna, salmon, and Carangidae that can cover 10–20 body-lengths per second to species such as and Batoidea that swim no more than 0.5 body-lengths per second.
File:Salmo salar.jpg|Fastest: e.g. salmon, 10–20 body lengths/second
File:Anguilla japonica 1856.jpg|Slowest: e.g. eel, 0.5 body lengths/second
A typical fish is ectothermic, 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.
File:Humpback anglerfish.png|Ambush predator:
anglerfish
File:Atl mackerel photo3 exp.jpg|Streamlined, somewhat warm-blooded:
mackerel
File:Hippocampus hippocampus (cropped).jpg|Tail not Fish locomotion:
seahorse
File:Phycodurus eques P2023146 (cropped).JPG|:
leafy seadragon
File:Eastern Cleaner Clingfish (cropped).jpg|No fish scale:
clingfish
File:Cyphotilapia frontosa mouthbrooding.jpg|Mouthbrooder: front cichlid with juvenile fish in mouth
Ecology
Habitats
Fish species are roughly divided equally between
freshwater and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species.
in the
Indo-Pacific constitute the center of diversity for marine fishes,
whereas continental freshwater fishes are most diverse in large
of tropical rainforests, especially the
Amazon Basin,
Congo Basin, and
Mekong basins.
More than 5,600 fish species inhabit
freshwaters alone, such that
represent about 10% of all
vertebrate species on the Earth.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high (e.g., Salvelinus and gudgeon) to the abyssal zone and even hadal zone depths of the deepest oceans (e.g., cusk-eels and snailfish), although none have been found in the deepest 25% of the ocean.
In terms of temperature, Jonah's icefish live in cold waters of the Southern Ocean, including under the Filchner–Ronne Ice Shelf at a latitude of 79°S,
A few fish live mostly on land or lay their eggs on land near water.
Parasites and predators
Like other animals, fish suffer from
parasitism. Some species use
cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of
in the
Indian Ocean and
Pacific Ocean oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract the attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus,
Orange chromide, the cleaner, and the much larger
E. suratensis.
Fish occupy many in freshwater and marine . Fish at the higher levels Predatory fish, and a substantial part of their prey consists of other fish.
File:Initial phase parrotfish feeding at Shaab Marsa Alam, Red Sea, Egypt -SCUBA (6336981391).jpg|A parrotfish feeding on algae on a coral reef
File:Arothron hispidus is being cleaned by Hawaiian cleaner wrasses, Labroides phthirophagus 1.jpg|A cleaner fish removing Parasitism from its client, a Tetraodontidae
File:Barracuda with prey.jpg|A barracuda preying on a smaller fish
File:1031 california sealion wright odfw (35281910502).jpg|Sea lion, a predatory mammal, eating a large Salmonidae
File:Cormorant with fish (cropped).jpg|Cormorant with fish prey
Anatomy and physiology
Locomotion
The body of a typical fish is adapted for efficient swimming by alternately contracting paired sets of
on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as control surfaces like an aircraft's flaps, enabling the fish to steer in any direction.
File:Lampanyctodes hectoris (Hector's lanternfish).svg|Anatomy of a typical fish (lanternfish shown):
1) gill cover 2) lateral line 3) dorsal fin 4) fat fin
5) caudal peduncle 6) caudal fin 7) anal fin 8) 9) pelvic fins 10) pectoral fins
Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that allows them to adjust their buoyancy by increasing or decreasing the amount of gas it contains.
The fish scale provide protection from Predation at the cost of adding stiffness and weight.
File:Swim bladder.jpg|Gas-filled swim bladder of a rudd helps maintain neutral buoyancy.
File:Fish scales.jpg|Silvered fish scale of a rohu provide protection and camouflage.
Circulation
Fish have a closed-loop circulatory system. The
heart pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through the gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart.
Respiration
Gills
Fish exchange gases using
on either side of the
pharynx. Gills consist of comblike structures called filaments. Each filament contains a
capillary network that provides a large
surface area for exchanging
oxygen and
carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in the gills flows in the opposite direction to the water, resulting in efficient countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath a protective bony cover or operculum. They are able to oxygenate their gills using muscles in the head.
(1977). 003910284X, Holt-Saunders International. 003910284X
Air breathing
Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land.
The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to the heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of
and the
intertidal zone, are facultative air breathers, able to breathe air when out of water, as may occur daily at
low tide, and to use their gills when in water. Some coastal fish like rockskippers and
choose to leave the water to feed in habitats temporarily exposed to the air.
Some catfish absorb air through their digestive tracts.
Digestion
The digestive system consists of a tube, the gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An
esophagus carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches,
pyloric caeca, around the pylorus, of doubtful function. The
pancreas secretes enzymes into the intestine to digest the food; other enzymes are secreted directly by the intestine itself. The
liver produces
bile which helps to break up fat into an emulsion which can be absorbed in the intestine.
Excretion
Most fish release their nitrogenous wastes as
ammonia. This may be excreted through the gills or filtered by the
. Salt is excreted by the rectal gland.
Saltwater fish tend to lose water by
osmosis; their kidneys return water to the body, and produce a concentrated urine. The reverse happens in
freshwater fish: they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater.
Brain
Fish have 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, notably
Mormyridae and
, which have brains about as large for their body weight as birds and
. At the front of the brain are the
olfactory bulb, a pair of structures that receive and process signals from the
via the two
. Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the
telencephalon, which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to the midbrain is the
diencephalon; it works with
and
homeostasis. The
pineal body is just above the diencephalon; it detects light, maintains
circadian rhythms, and controls color changes. The
midbrain contains the two optic lobes. These are very large in species that hunt by sight, such as
rainbow trout and
. The
metencephalon controls swimming and balance.The single-lobed cerebellum is the biggest part of the brain; it is small in hagfish and
, but very large in
, processing their
electric fish. The brain stem or
myelencephalon controls some muscles and body organs, and governs respiration and
osmoregulation.
Sensory systems
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.
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,
that detect weak electric currents on the order of millivolt.
Vision is an important sensory system in fish.[ See also Lamb et al.'s "The origin of the Vertebrate Eye", 2008.]
Hearing too is an important sensory system in fish. Fish sense sound using their lateral lines and 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.
Cognition
The cognitive capacities of fish include
self-awareness, as seen in
.
Manta rays and
placed in front of a mirror repeatedly check whether their reflection's behavior mimics their body movement.
wrasse,
archerfish, and
Atlantic cod can solve problems and invent tools.
The monogamous cichlid
Amatitlania siquia exhibits pessimistic behavior when prevented from being with its partner.
Fish orient themselves using landmarks; they may use mental maps based on multiple landmarks. Fish are able to learn to traverse mazes, showing that they possess spatial memory and visual discrimination.
Behavioral research suggests that fish are
Sentience, capable of experiencing pain.
Electrogenesis
Electric fish such as
Mormyridae, the
Gymnarchus, and electric eels have some of their muscles adapted to generate electric fields. They use the field to locate and identify objects such as prey in the waters around them, which may be turbid or dark.
Strongly electric fish like the electric eel can in addition use their electric organs to generate shocks powerful enough to stun their prey.
Endothermy
Most fish are exclusively cold-blooded or
ectothermic. However, the
Scombroidei are
warm-blooded (endothermic), including the
and tunas.
The
Lampris guttatus, a
lampriform, uses whole-body endothermy, generating heat with its swimming muscles to warm its body while countercurrent exchange minimizes heat loss.
Among the cartilaginous fishes, sharks of the families
Lamnidae (such as the great white shark) and
Alopiidae (thresher sharks) are endothermic. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the
bluefin tuna and the
porbeagle shark, which maintain body temperatures more than above the ambient water.
Reproduction and life-cycle
The primary reproductive organs are paired
and
ovary.
Eggs are released from the ovary to the
.
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
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
Ichthyoplankton. 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
metamorphosis to become juveniles. During this transition, larvae must switch from their yolk sac to feeding on
zooplankton prey.
Some fish such as
Embiotocidae,
Goodeidae, and
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.
DNA repair
Embryos of externally fertilized fish species are directly exposed during their development to environmental conditions that may damage their DNA, such as pollutants,
ultraviolet and reactive oxygen species.
[Dey A, Flajšhans M, Pšenička M, Gazo I. DNA repair genes play a variety of roles in the development of fish embryos. Front Cell Dev Biol. 2023 Mar 1;11:1119229. doi: 10.3389/fcell.2023.1119229. PMID 36936683; PMCID: PMC10014602] To deal with such DNA damages, a variety of different
DNA repair pathways are employed by fish embryos during their development.
[ In recent years zebrafish have become a useful model for assessing environmental pollutants that might be genotoxic, i.e. cause DNA damage.][Canedo A, Rocha TL. Zebrafish (Danio rerio) using as model for genotoxicity and DNA repair assessments: Historical review, current status and trends. Sci Total Environ. 2021 Mar 25;762:144084. doi: 10.1016/j.scitotenv.2020.144084. Epub 2020 Dec 14. PMID 33383303]
Defenses against disease
Fish have both non-specific and immune defenses against disease. Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the epidermis that traps and inhibits the growth of . If breach these defenses, the innate immune system can mount an inflammation that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens, non-specifically. Specific defenses respond to particular antigens, such as on the surfaces of pathogenic bacteria, recognised by the adaptive immune system. Immune systems evolved in as shown in the cladogram.
Immune organs vary by type of fish. The jawless fish have lymphoid tissue within the pronephros, and in the gut. They have their own type of adaptive immune system; it makes use of variable lymphocyte receptors (VLR) to generate immunity to a wide range of antigens, The result is much like that of jawed fishes and tetrapods, but it may have evolved separately.
Behavior
Shoaling and schooling
A shoal is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. A school is a much more tightly organised group, synchronising its swimming so that all fish move at the same speed and in the same direction. Schooling is sometimes an antipredator adaptation, offering improved vigilance against predators. It is often more efficient to gather food by working as a group, and individual fish optimise their strategies by choosing to join or leave a shoal. When a predator has been noticed, prey fish respond defensively, resulting in collective shoal behaviours such as synchronised movements. Responses do not consist only of attempting to hide or flee; antipredator tactics include for example scattering and reassembling. Fish also aggregate in shoals to spawn.
Communication
Fish communicate by transmitting acoustic signals (sounds) to each other. This is most often in the context of feeding, aggression or courtship.
Some fish produce sounds by rubbing or grinding their bones together. These sounds are stridulatory. In Haemulon flavolineatum, the French grunt fish, as it produces a grunting noise by grinding its teeth together, especially when in distress. The grunts are at a frequency of around 700 Hz, and last approximately 47 milliseconds.
Some fish species create noise by engaging specialized muscles that contract and cause swimbladder vibrations. Oyster toadfish produce loud grunts by contracting sonic muscles along the sides of the swim bladder.
The red drum, Sciaenops ocellatus, produces drumming sounds by vibrating its swimbladder. Vibrations are caused by the rapid contraction of sonic muscles that surround the dorsal aspect of the swimbladder. These vibrations result in repeated sounds with frequencies from 100 to >200 Hz. S. ocellatus produces different calls depending on the stimuli involved, such as courtship or a predator's attack. Females do not produce sounds, and lack sound-producing (sonic) muscles.
Conservation
The 2024 IUCN Red List names 2,168 fish species that are endangered or critically endangered.
Overfishing
The Food and Agriculture Organization reports that "in 2017, 34 percent of the fish stocks of the world's marine fisheries were classified as overfished".
Other threats
A key stress on both freshwater and marine ecosystems is habitat degradation including water pollution, the building of dams, removal of water for use by humans, and the introduction of invasive species species including predators. Freshwater fish, especially if Endemism to a region (occurring nowhere else), may be threatened with extinction for all these reasons, as is the case for three of Spain's ten endemic freshwater fishes.[Atlas of Exotic Fishes in the Mediterranean Sea. 2nd Edition. 2021. (F. Briand Ed.) CIESM Publishers, Paris, Monaco 366 p.[3]] and ecology.
Importance to humans
Economic
Throughout history, humans have used fish as a food source for dietary protein. Historically and today, most fish harvested for human consumption has come by means of catching wild fish. However, fish farming, which has been practiced since about 3,500 BCE in ancient China,
Recreation
Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in in homes, offices, or public settings. Recreational fishing is fishing primarily for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit, or artisanal fishing, which is fishing primarily for food. The most common form of recreational fishing employs a fishing rod, fishing reel, fishing line, fish hook, and a wide range of baits. Recreational fishing is particularly popular in North America and Europe; government agencies often actively manage target fish species.
Culture
Fish themes have symbolic significance in many religions. In ancient Mesopotamia, fish offerings were made to the gods from the very earliest times.[Ovid Fasti 2.457ff]
Fish feature prominently in art,
File:Matsya painting.jpg|Avatar of Vishnu as a Matsya, India
File:Bartolomeo Passerotti - The Fishmonger's Shop - WGA17072.jpg| The Fishmonger's Shop, Bartolomeo Passerotti, 1580s
File:Goldfish Matisse.jpg| Goldfish by Henri Matisse, 1912
See also
Notes
Sources
Further reading
External links
-
ANGFA – Illustrated database of freshwater fishes of Australia and New Guinea
-
FishBase online – Comprehensive database with information on over 29,000 fish species
-
United Nation – Fisheries and Aquaculture Department: Fish and seafood utilization
