FishPosted on May 18, 2018 - Last modified: September 8, 2018
The fishes are aquatic animals and there are more than 30.000 species of vertebrate animals (phylum Chordata) found in the fresh and salty waters of the world. Living species range from primitive jawless lampreys and shellfish, through cartilaginous sharks and rays, to abundant and diverse bony fish. Most species of fish are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.
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Fish have been around for more than 450 million years, during which time they have repeatedly evolved to fit into almost every imaginable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fish: when fish colonized the land habitat, they became tetrapod (four-legged) land vertebrates.
The popular conception of a fish as a slick, streamlined aquatic animal that possesses fins and breathes through its gills applies to many fish, but many more fish deviate from that conception to fit it. For example, the body lengthens in many ways and shortens a lot in others; the body is flattened in some (mainly bottom-dwelling fish) and laterally compressed in many others; the fins can be elaborately extended, forming intricate shapes, or they can be shortened or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air respirators have appeared in various evolutionary lines.
Many fish are cryptic in color and shape, closely matching their respective environments; others are among the brightest of all organisms, with a wide range of hues, often of striking intensity, in a single individual. The brightness of pigments can be enhanced by the structure of the fish's surface, so that it almost appears to shine. A number of unrelated fish have true light-producing organs. Many fish are capable of altering their coloration, some for the purpose of camouflage, others to enhance behavioral signals.
The length of the adult fish ranges from less than 10 mm to more than 20 meters, and their weight ranges from 1,5 grams to many thousands of kg. Some live in shallow hot springs at temperatures slightly above 42 ° C, others in cold Arctic seas a few degrees below 0 ° C or in cold deep waters more than 4.000 meters below the ocean's surface. The structural and especially physiological adaptations to life at such extremes are relatively poorly understood and provide curious scientists with great incentive to study.
Almost all natural water bodies have fish life, with the exception of very hot thermal ponds and extremely saline alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America.
The current distribution of fish is the result of the Earth's geological history and development, as well as the ability of fish to undergo evolutionary changes and adapt to available habitats. It can be seen that the fish are distributed according to the habitat and the geographical area. The main habitat differences are marine and freshwater.
For the most part, fish in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, like salmon, migrate from one to another. Freshwater habitats can be considered to be of many types. The fish found in mountain streams, arctic lakes, tropical lakes, temperate streams, and tropical rivers differ from each other, both in their obvious structure and in their physiological attributes. Even in adjacent habitats where, for example, a tropical mountain stream enters a lowland stream, the fish fauna will be different.
Marine habitats can be divided into deep ocean bottoms (benthic), midwater ocean bottoms (bathypelagic), surface ocean bottoms (pelagic), rocky shoreline, sandy shoreline, muddy shoreline, bays, estuaries, and others. Also, for example, rocky coasts in tropical and temperate regions will have different fish fauna, even when such habitats are found along the same coastline.
All living things need to stay close to a food source and fish are no exception. Fish tend to concentrate where there is a lot of food. Schools of fish or other prey will attract larger fish to feed on them. Fish take advantage of many foraging opportunities, including hatching insects and migratory frogs.
The signs of small fish activity can lead you to hot fishing spots. Fish that jump above the surface often try to escape larger fish. Numerous small fish in the shallow waters could mean that larger fish will arrive later to feed.
All aspects of fish life are closely correlated with adaptation to the total environment: physical, chemical and biological.
Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fish display. The vast majority hatch from relatively small eggs a few days to several weeks or more after the eggs disperse in the water.
Newly hatched hatchlings are still partially underdeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but can be very long, and some lampreys continue as larvae for at least five years. Young and larval fish must grow considerably before reaching sexual maturity, and their small size and other factors often dictate that they live in a different habitat than adults. For example, most tropical marine fish have pelagic larvae. The food of the larvae is also different, and larval fish often live in shallow water, where they may be less exposed to predators.
After a fish reaches adulthood, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fish live for only one to three years at most. In some species, however, individuals can live up to 10 or 20 or even 100 years.
Fish perceive the world around them by the usual senses of sight, smell, hearing, touch and taste and by special side-line water current detectors. In the few fish that do generate electric fields, a process that might best be called electrolocation aids perception. One or other of these senses is often emphasized at the expense of others, depending on the fish's other adaptations. In fish with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed mainly on smell (like some eels).
Specialized behavior mainly refers to the three most important activities in fish life: feeding, breeding, and escaping from enemies.
Predatory fish are often solitary, lurking to swoop in search of prey, a kind of locomotion impossible for beaked, coral-feeding parrotfish, swimming in small groups from one coral head to the next. Additionally, some predatory fish that inhabit pelagic environments, such as tunas, often breed.
Sleep in fish, all of which lack true eyelids, consists of an apparently listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can run away. Some types of fish lie on the bottom to sleep.
Communication between members of one species or between members of two or more species is often extremely important, especially in reproductive behavior. The mode of communication can be visual, as between the so-called cleaner small fish and a large fish of a very different species. Larger fish often allow the cleaner to enter their mouth to remove parasites from the gills. The cleaner is recognized for its distinctive color and actions and is therefore not eaten, even if the largest fish is normally a predator. Communication is often chemical, the signals are sent by specific chemicals called pheromones.
Many fish have a streamlined body and swim freely in open water. Locomotion is closely correlated with habitat and ecological niche (the general position of the animal with respect to its environment).
Many others, both marine and freshwater, swim on the surface and have their mouths adapted to feed better on the surface. These are often long and slender, capable of darting at surface insects or other surface fish and in turn keeping away from predators.
The so-called calm water swimmers, the most common type of fish, are of many types and live in many habitats. Fish that live in relatively calm waters, such as bays, lake shores, or slow rivers, are generally not strong, fast swimmers, but are capable of reaching short, fast speeds to escape a predator. Many of these have flattened sides, such as aquarist sunfish and freshwater angelfish. Fish associated with the bottom or substrate are often slow swimmers. Fish that feed on plankton in open water almost always remain spindle-shaped and are able to move quickly and strongly.
Bottom-dwelling fish are of many types and have undergone many types of modification to their body shape and swimming habits. Stingrays, which evolved from deep-sea sharks that swam hard, generally stay close to the bottom and move by waving their large pectoral fins. Flounders live in a similar habitat and move across the bottom by waving their entire body. Many move from place to place, resting on the bottom between movements.
Some other fish venture out onto wet ground to find more promising waters than they left behind. They move by moving their bodies, sometimes using strong pectoral fins; most have accessory organs that breathe air. Many that live at the bottom live in mud holes or rock crevices. Sea eels and gobies are commonly found in such habitats and for the most part they venture far beyond their cavernous homes. Some bottom dwellers have developed powerful adhesive discs that allow them to remain on the substrate in areas such as rocky shores, where wave action is great.
Some fish are hermaphrodites: an individual that produces sperm and eggs, usually at different stages of their life. However, self-fertilization is probably rare.
Reproduction methods are varied, but most lay large numbers of small, fertilized, scattered eggs outside the body. The eggs of pelagic fish often remain suspended in open water. Many coastal and freshwater fish lay eggs on the bottom or between plants. Some have sticky eggs. The mortality of the hatchlings and especially of the eggs is very high, and often only a few individuals reach the maturity of hundreds, thousands, and in some cases millions of eggs laid.
Males produce sperm, usually as a milky white substance called milk, in two (sometimes one) testicles within the body cavity. In bony fish, a sperm duct leads from each testicle to a urogenital opening behind the ventilation duct or anus. In sharks and rays and cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milk to the eggs in the female's ventilation shaft or in the substrate where the female has placed them. Accessory organs are sometimes used to fertilize females internally, for example, the classifiers of many sharks and rays.
In females the ovules are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and outwards. In some fish, the eggs are fertilized internally, but are removed before development occurs.
In some bony fish the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are fed by the ovarian tissues after hatching (viviparous). There are also other methods used to feed the young inside the female. In all life carriers, the young are born relatively large in size and few in number.
Successful reproduction and, in many cases, defense of the eggs and young are ensured by a rather stereotypical but often elaborate courtship and paternal behavior, whether on the part of the male, female, or both. Some fish prepare their nests by emptying depressions in the sandy bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (pimples), or blow a group of mucus-covered bubbles on the surface of the water. (gouramis). The eggs are deposited in these structures. Some varieties of cichlids and catfish hatch the eggs in their mouths.
The basic structure and function of the fish's body are similar to those of all other vertebrates. The four usual types of tissues are present: superficial or epithelial, connective (bone, cartilage and fibrous tissues, as well as their derivatives, blood), nervous and muscular. In addition, the organs and organ systems of fish are parallel to those of other vertebrates.
The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter located in the midline just below the gill chamber. The body cavity, which contains the vital organs, is located behind the head in the lower front part of the body. The anus usually marks the posterior termination of the body cavity and occurs most often just in front of the base of the anal fin. The spinal cord and vertebral column continue from the back of the head to the base of the caudal fin, passing dorsally into the body cavity and through the caudal region (tail) behind the body cavity. Most of the body is made of muscle tissue, a large part of which is necessary for swimming.
The skeleton is an integral part of the fish's locomotion system, in addition to serving to protect vital parts. The internal skeleton is made up of the bones of the skull (except the bones of the roof of the head, which are actually part of the external skeleton), the spine, and the fin supports (fin rays). The fin supports are derived from the external skeleton. The internal skeleton of cyclostomes, sharks and rays is made of cartilage; that of many fossil groups and some primitive live fish is mostly cartilage but may include some bone. Instead of the spinal column, early vertebrates had a fully developed noto chord, a rigid and flexible rod of viscous cells surrounded by a strong fibrous sheath.
The skull, including the arches of the gills and the jaws of bony fish, is completely ossified, or at least partially ossified. That of sharks and rays remains cartilaginous, sometimes partially replaced by calcium deposits, but never by true bone.
The supporting elements of the fins (basal or radial spines or both) have changed a lot during the evolution of fish. Some of these changes are described in the next section (Evolution and paleontology). Most fish have a single dorsal fin in the midline of their back. Many have two and a few have three dorsal fins. The other fins are the single tail and the anal fins and the paired pelvic and pectoral fins. A small fin, the adipose fin, with hairy fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the tail fin.
The skin of a fish must fulfill many functions. It helps maintain osmotic balance, provides physical protection to the body, is the site of staining, contains sensory receptors, and in some fish, works on respiration. Mucous glands, which help maintain water balance and offer protection against bacteria, are extremely numerous on fish skin, especially cyclostomes and teleosts. Since mucous glands are present in modern lampreys, it is reasonable to assume that they were present in primitive fish, such as ancient Silurian and Devonian agnathans.
Protection against abrasion and predation is another function of fish skin, and the dermal (skin) spine emerged early in fish evolution in response to this need. It is believed that the bone first evolved into the skin and only later invaded the cartilaginous areas of the fish's body, to provide additional support and protection. Dermal spines have played an important role in fish evolution and have different characteristics in different groups of fish. Several groups are characterized at least in part by the type of bony scales they possess.
The scales have played an important role in the evolution of fish. Primitive fish generally had thick bony plates or thick scales in various layers of bone, enamel, and related substances. Modern teleost fish have bone scales that, despite being protective, allow much more freedom of movement in the body. A few modern teleosts (some catfish, spinach, and others) have secondarily acquired bony plates on the skin. Modern and primitive sharks possessed placoid scales, a relatively primitive type of scale with a structure similar to that of a tooth, consisting of an outer layer of enamel-like substance (vitrodentine), an inner layer of dentin, and a pulp cavity that it contained nerves and blood vessels.
Early bony fish had thick scales of the ganoid or cosmoid type. Cosmoid scales have a hard enamel-like outer layer, an inner layer of cosmina (a form of dentin), and then a layer of vascular bone (isopedin). In ganoid scales the hard outer layer is chemically different and is called ganoin. Beneath this is a cosmin-like layer and then a vascular bone layer. The fine translucent bony scales of modern fish, called cycloids and cenoids (the latter distinguished by striations at the edges), lack enamel layers and dentines.
The skin has several other functions in fish. It is well supplied with nerve endings and presumably receives tactile, thermal, and painful stimuli. The skin is also well supplied with blood vessels. Some fish breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the surface of the skin.
The skin serves as protection through the control of coloration. Fish exhibit an almost unlimited range of colors. The colors often blend closely with the surroundings, effectively hiding the animal. Many fish use bright colors for territorial advertising or as marks of recognition for other members of their own species, or sometimes for members of other species.
Many fish can change their color to a greater or lesser degree, due to the movement of the pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), almost universally occurring in fish, are often juxtaposed with other pigment cells. When placed under iridocytes or leucophores (which contain the silver or white pigment of guanine), the melanophores produce structural colors of blue and green. These colors are often extremely intense, because they are formed by the refraction of light through the sharp crystals of guanine.
Refracted blue and green colors are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. The yellow, orange and red colors are produced by erythrophores, cells that contain the appropriate carotenoid pigments. Other colors are produced by combinations of melanophores, erythrophores, and iridocytes.