Fish

A Fish is an Aquatic Animal which is covered with Scales and equipped with two sets of Paired Fins and several Unpaired Fins. Fish live and breathe in water. All Fish are Vertebrates and most breathe through Gills. It is the presence of Gills, Fins and Water as their only Habitat, that makes them different from other Animals. Some Species of Fish are restricted to Fresh Water e.g., Largemouth Bass, while others are found only in the Open Ocean e.g. Halibut. Such species are called Stenohaline, meaning that they cannot tolerate much change in salinity. Many Fishes, on the other hand, are found in Estuaries where salinity is constantly changing. Such Species are called 'Euryhaline'. Examples are Summer Flounder, Red Drum and Striped Bass. Unlike Groupings such as Birds or Mammals, Fish are not a Single Clade but a Paraphyletic collection of Taxa, including Hagfishes, Lampreys, Sharks and Rays, Ray-Finned Fishes, Coelacanths and Lungfishes. Fish are the oldest Vertebrates of the world. Fish do not represent a Monophyletic Group, and therefore the evolution of Fish is not studied as a single event. Fish may have evolved from a creature similar to a coral-like Sea squirt, whose Larvae resemble primitive Fish in important ways. At 31,500 species, Fish exhibit greater Species diversity than any other Class of Vertebrates. Thereby making their population about half of all known Vertebrate Species. The 2006 IUCN Red List names 1,173 Fish Species that are threatened with extinction. Some of these include Atlantic Cod, Devil's Hole Pupfish, Coelacanths and Great White Sharks. Fish is an important human food source for Man. Commercial and subsistence Fishers hunt Fish in wild Fisheries or Farm them in Ponds or in cages in the Ocean. They are also caught by recreational Fishers and raised by Fish-keepers, and are exhibited in public Aquaria. Some animals are thought of as Fish simply because they live in the water, but they are not True Fishes. Fish-shaped Dolphins and Seals are mammals and must come to the surface to breathe. Shellfish, like Mussels and Cuttlefish are in fact Molluscs. Non-Fish Animals with Fishy names also include Crayfish and the Spiny-Skinned Starfish. The word 'Fish' is used either as singular noun or to describe a group of specimens from a Single Species. The word 'Fishes' describes a group of different Species.

Fish are classified into the following Major Groups:

1.  Jawless Fish - These Fish ave no jaws have almost become extinct - there are only about 45 surviving species, including the Lamprey and the Hagfish. These are also known as great survivors. Jawless Fish. They are also known as Agnathas. Fish with no Jaws have almost become extinct - there are only about 45 surviving Species, including the Lamprey and the Hagfish.

2.  Cartilaginous Fish - These Fishes developed about 100 million years later. The skeleton of these Fish is made of Cartilage, which is not as hard as bone. These Fish have jaws, as well as teeth which are usually hard and sharp. Their bodies are covered with hard Scales. They are also known as Chondrichthyes. Some of the Cartilaginous Fish are Sharks, Rays, Skates and Sawfishes.

3.  Bony Fish - These Fish with bony skeletons appeared about the same time as Cartilaginous Fish. They are by far the largest group, with about 20,000 Species alive today. These Fish have an Organ called a Swim Bladder to help them float. They are also known as 'Osteichthyes'. Some of the Bony Fishes are Lion Fish, Puffer Fish, Butterfly Fish etc.

Physical Features

Weight - Fish Weight varies from Species to Species. The Heaviest Fish averages about 20 tons and the Lightest Fish weigh 4-5 mg.

Size - Fish Size varies from Species to Species. The largest Fish is 16 meters (51 ft) long and the smallest measures about 8 mm (1/4 in.).

Scales - Most Fish have Scales, but there are some Species which lack them like, the Hagfishes and Lampreys. The Scales of Fish originate from the Mesoderm (skin); they may be similar in structure to teeth. Larger, heavier Scales supply more protection, but restrict movement, smaller, lighter Scales offer less protection but allow for greater freedom of movement. The Scales of Modern Fish are embedded in, and grow out of, the dermis and are covered entirely by the epidermis. They grow as the Fish grow, thus, in many cases, they reflect a history of the Fish's life. Experts in scale patterns can not only tell you how old a Fish is from its Scales, but also how many times it has spawned and if it has been seriously ill. Mostly a Fishes Scales lie shallowly in the dermis and are overlapping like the tiles on a roof, making them easily removed (Herring Pilchard and Sardines for instance) but in other species such as Plaice they are more deeply embedded and do not overlap.

There are Four main kinds of Scales and numerous variations of each kind:

1.  Placoid - Placoid Scales consist of a flattened rectangular base plate which is embedded in the Fish, and variously developed structures, such as spines, which project posteriorly on the surface. The spines give many species a rough texture.

2.  Cosmoid - Cosmoid Scales are similar to Placoid Scales  and probably evolved from the fusion of Placoid Scales. They consist of two basal layers of bone, a layer of dentine-like cosmine, and an outer layer of vitrodentine.

3.  Ganoid - Ganoid Scales are usually rhomboid in shape and have articulating peg and socket joints between them. They are modified Cosmoid Scales which consist of a bony basal layer, a layer of dentine, and an outer layer of ganoine (an inorganic bone salt).

4.  Cycloid and Ctenoid - Ctenoid Scales have a variously developed spiny posterior margin resembling a comb. Cycloid Scales have a smooth posterior margin lacking ctenii.

• Majority of Fishes are essentially torpedo-shaped. Most Fish have a well-developed swimming ability and are able to move very rapidly. Their tails are used to provide the thrust needed for high-speed swimming. Salmon, for example, are able to leap over low waterfalls on their upstream spawning migrations. Not all Fish are strong swimmers, however. The Ocean Sunfish, for example, is a large Fish weighing up to few hundred kilograms, can be found floating at the surface of the water in the Pacific Ocean. It has small Fins and limited mobility.

• In some Species the Males and Females have different shaped bodies or different colouring; in other Species there is no visible difference.

There are two types of muscle tissue in Fish:

1.  White Muscle - It is for speed over short distances. Ambush predators like Bass have mostly white muscle. Red muscle is for sustained swimming.

2.  Red Muscle - Almost all Salt-Water Fish have a line of red muscle down each side for swimming in ocean currents. Fish found in rivers also will have red muscle tissue. The Blue Catfish is a good example of this.

• Fish like Grass Carp that eat plant material (herbivores) have long intestines and a dark lining of the body cavity (peritoneum). Fish that eat other Fish have short intestines and a light peritoneum.

• Most Fish exchange gases using Gills on either side of the Pharynx. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their Gills. The Gills push the oxygen-poor water out through openings in the sides of the Pharynx. Some Fishes, like Sharks and Lampreys, possess multiple Gill openings. However, most Fishes have a single gill opening on each side.

Fish are known to have various techniques of Breathing according to which they can be divided into:

1.  Obligate Air Breathers - Such Fish need to reach the surface of water to breathe. The African Lungfish, must breathe air periodically or it will suffocate.

2.  Facultative Air Breathers - Such Fish rely on their Fills for oxygen. The 'Catfish - Hypostomus plecostomus', only breathe air only if it needs to, otherwise it relies on its Gills for oxygen. Most Air Breathing Fish are Facultative Air Breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators. A number of Fishes have evolved so-called accessory breathing organs  that extract oxygen from the air. Labyrinth Fish (such as gouramis and bettas) have a Labyrinth Organ above the Gills that performs this function. A few other Fish have structures resembling Labyrinth Organs in form and function, most notably Snakeheads, Pikeheads and the Clariidae Catfish family. Some Air-Breathing Fish are able to survive in damp burrows for weeks without water, entering a state of Aestivation (summertime hibernation) until water returns.

• Fish have a closed-loop circulatory system. The heart pumps the blood  in a single loop throughout the body. In most Fish, the heart consists of four parts, including two chambers and an entrance and exit.

• 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.

• Salt Water 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 fresh water to salt water.

• 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.

• Most Fish possess highly developed sense organs. Nearly all daylight Fish have colour vision that is at least as good as a humans. 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.

Geographical Range and Habitat

Fishes are spread in all the water bodies of the world. They are found in all Continents of the World. Colouration is another clue to where a Fish lives. The vertical stripes on bream help them hide in weeds. Horizontal stripes on Fish like Striped Bass help to break up their appearance in open water. Brightly Colorado Fish inhabit clear water; Drab-Coloured Fish are usually found in muddy water. A good example of this is Black Crappie, which is usually found in clear water, and White Crappie, which prefer dingy water. Bream found in a muddy pond will appear washed- out. Interestingly, Blind Catfish and Bass (and some others) will turn almost black.

Behaviour

To avoid being eaten, Some Species often swim in Schools that number many thousands of individuals. A predator seeing such a School may avoid it on the basis that the School could, in fact, be a Fish larger than itself. If the predator does attack, it will be able to take only a small fraction of the total number of Fish available.

Many Fish do not Swim in schools but depend on camouflage, swimming ability, or the tendency to seek refuge in crevices to avoid predation. Anemone Fish can live among the stinging tentacles of Sea Anemones. These Fish are immune to the stinging cells, so they are protected from predators.

Most Fish seek out their food by sight and actively search for it by swimming around. A few, on the other hand, wait for food to come to them. Anglerfish have a fleshy protrusion on their heads that acts like a Fishing lure. Some Anglerfish rest on the bottom, camouflaged to match their surroundings, and wave their lure until an unsuspecting smaller Fish investigates. Anglerfish that inhabit the deep ocean, where light does not penetrate, have lures that glow in the dark to attract prey.

Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behaviour in mazes reveals that they possess spatial memory and visual discrimination

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.

Diet - The majority of Fishes are Carnivores or Omnivores, although a few Species are Herbivores. Herbivores eat Aquatic Alants and Algae as food. Carnivorous Fish eat a variety of Fishes, Birds, Snakes, Mammals, Amphibians, Insects etc.

Predators - Fish Predators include other Fish, Mammals, Amphibians, Birds like Pelicans, Cormorants, Herons etc., Snakes, Bears, Turtles, Crustaceans, some Insect Larvae like Dragonfly Larvae), and Humans. There are some Fish Specialists like the Fishing Cat, Fishing Bat, Fish Owl, Fish Eagle, Kingfisher, Otters, Gharial etc. whose main diet is Fish.

Reproduction

Over 97% of all known Fishes are Oviparous, i.e. 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 ad hatch 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, meaning that they produce Live Young Ones. 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 the 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'. Intrauterine Cannibalism is an even more unusual mode of Vivipary, in which the largest embryos eat weaker and smaller siblings. This behaviour is also most commonly found among Sharks, such as the Grey Nurse Shark, but has also been reported for 'Nomorhamphus ebrardtii'.

In Folklore and Culture

• The Fish was sacred to the Greco-Roman mythology, where it held symbolic meaning of change and transformation. It is evident in the myth of Aphrodite and Heros when they turned themselves into Fish in order to escape from the ferocious Typhon.

• In Christianity, the Fish is a symbol of abundance and faith as observed in the Biblical story of Fishes and loaves.

• In Hinduism, Vishnu transformed himself into a Fish (Matsya) to save the world from a great flood. In this form, he guided King Manu’s boat (which contained the select few survivors & seeds of life to re-create the world after the flood subsided) to safety.

In the Dhamma of Buddha the Fish symbolize happiness as they have complete freedom of movement in the water. They also represent fertility and abundance. Often drawn in the form of Carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span.

The Fish also makes an appearance as one of the Eight sacred symbols of the Buddha:

1) Conch.

2) Lotus.

3) Parasol.

4) Wheel.

5) Knot.

6) Pair of Golden Fish.

7) Banner of Victory.

8) Vase.

• Legends of Half-Woman, Half-Fish like creatures known as Mermaids have featured in various stories like those of Hans Christian Andersen and movies like 'Splash'.

• The astrological symbol Pisces is based on a constellation of the same name, but there is also a second Fish constellation in the night sky, Piscis Austrinus.

• Fish feature prominently in art and literature, in movies such as 'Finding Nemo' and books such as 'The Old Man and the Sea'.

• Large Fish, particularly Sharks, have frequently been the subject of horror movies and thrillers, most notably the novel 'Jaws', which spawned a series of films of the same name that in turn inspired similar films or parodies such as 'Shark Tale', 'Snakehead Terror', and Piranha.

• The name of the Canadian city of Coquitlam, British Columbia is derived from 'Kwikwetlem', which is said to be derived from a Coast Salish term meaning 'Little Red Fish'.