Internal anatomy of a bony fish
The liver in fish produces bile which is stored in the gall bladder until a bolus passes the stomach, at which time the bile is expelled into the intestine. The circulatory systems of all vertebrates are closed , just as in humans. Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. 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. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure.
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Bony fish and sharks are known to be a single circulation species. The Heart What is single circulation? The Countercurrent System First, the blood in the gills flows in the opposite direction to the water passing over them. When the water has passed over the gills, the blood is "hungry" for the oxygen that is there. The blood then becomes rich in oxygen. Single circulation is when the blood passes through the heart once every time it circulates.
Gas Exchange Gas exchange is when diffusion takes place, in the gills, only if oxygen is more concentrated in the water than in the blood. However, once the oxygen is diffused into the gills, the water surrounding the fish is depleted, and gas exchange loses its efficiency.
This is then solved by a countercurrent system. The Circulatory System When the fish is in a stable state, deoxygenated blood flows into the fisrt chamber of the heart from the body. The blood is then pumped into the second chamber of the heart, and is brought to the gills where gas exchange takes place.
Once the oxygenated blood is carried back to the body by its arteries, it is then branched out into thin-walled capillaries that allow nutrients flow to a cell. Veins are the largest vessel that brings the deoxygenated blood to the heart which repeats the cycle. Modern fish of this class lack a swim bladder , and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes.
The bony fishes are by far the largest class. Examples range from the tiny sea horse to the kg 1,pound blue marlin , from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin, overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits. The study of fishes, the science of ichthyology , is of broad importance.
Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean.
For a detailed discussion of the technology and economics of fisheries, see commercial fishing. Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases. Fishes are valuable laboratory animals in many aspects of medical and biological research.
For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions.
Fishes have been especially important in the study of animal behaviour , where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression. There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them.
Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.
Of the approximately species of freshwater fish of the United States, Canada, and Mexico, about 40 have become extinct in the past…. Fishes have been in existence for more than million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat.
In a sense, land vertebrates are simply highly modified fishes: The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it.
For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some principally in bottom-dwelling fishes and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth , eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines. Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual.
The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration —some for the purpose of camouflage, others for the enhancement of behavioral signals. Fishes range in adult length from less than 10 mm 0. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.
Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of the Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area.
Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon , migrate from one to the other.
The freshwater habitats may be seen to be of many kinds. There is no small intestine as such in non-teleost fish, such as sharks , sturgeons , and lungfish. Instead, the digestive part of the gut forms a spiral intestine , connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns.
This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.
Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions. The large intestine is the last part of the digestive system normally found in vertebrate animals.
Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body. In sharks , this includes a rectal gland that secretes salt to help the animal maintain osmotic balance with the seawater. The gland somewhat resembles a caecum in structure, but is not a homologous structure.
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: 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. In sharks, digestion can take a long time. The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs. One of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines.
This short length is achieved by the spiral valve with multiple turns within a single short section instead of a long tube-like intestine.
The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca. Oxytocin is a group of neuropeptides found in most vertebrate. One form of oxytocin functions as a hormone which is associated with human love.
In , researchers injected cichlids from the social species Neolamprologus pulcher , either with this form of isotocin or with a control saline solution. They found isotocin increased "responsiveness to social information", which suggests "it is a key regulator of social behavior that has evolved and endured since ancient times".
Fish can bioaccumulate pollutants that are discharged into waterways. Estrogenic compounds found in pesticides, birth control, plastics, plants, fungi, bacteria, and synthetic drugs leeched into rivers are affecting the endocrine systems of native species.
The fish have been exposed to higher levels of estrogen, and leading to feminized fish. These endocrine-disrupting compounds are similar in structure to naturally occurring hormones in fish. They can modulate hormonal interactions in fish by: Two major types of osmoregulation are osmoconformers and osmoregulators. Osmoconformers match their body osmolarity to their environment actively or passively. Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater.
Osmoregulators tightly regulate their body osmolarity, which always stays constant, and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment.
An example is freshwater fish. The gills actively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very hypotonic dilute urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes salt out from the gills. Most fish are stenohaline , which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to.
However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e. Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.
In contrast to bony fish, with the exception of the coelacanth ,  the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea and trimethylamine N-oxide TMAO , allowing them to be in osmotic balance with the seawater. This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to marine environments. A few exceptions exist, such as the bull shark , which has developed a way to change its kidney function to excrete large amounts of urea.
Sharks have adopted a different, efficient mechanism to conserve water, i. They retain urea in their blood in relatively higher concentration. Urea is damaging to living tissue so, to cope with this problem, some fish retain trimethylamine oxide.
This provides a better solution to urea's toxicity. Sharks, having slightly higher solute concentration i. Homeothermy and poikilothermy refer to how stable an organism's temperature is. Most endothermic organisms are homeothermic, like mammals. However, animals with facultative endothermy are often poikilothermic, meaning their temperature can vary considerably. Similarly, most fish are ectotherms , as all of their heat comes from the surrounding water.
However, most are homeotherms because their temperature is very stable. Most organisms have a preferred temperature range, however some can be acclimated to temperatures colder or warmer than what they are typically used to.
An organism's preferred temperature is typically the temperature at which the organism's physiological processes can act at optimal rates. When fish become acclimated to other temperatures, the efficiency of their physiological processes may decrease but will continue to function. This is called the thermal neutral zone at which an organism can survive indefinitely.
Vernon has done work on the death temperature and paralysis temperature temperature of heat rigor of various animals. He found that species of the same class showed very similar temperature values, those from the Amphibia examined being To cope with low temperatures, some fish have developed the ability to remain functional even when the water temperature is below freezing; some use natural antifreeze or antifreeze proteins to resist ice crystal formation in their tissues.
Most sharks are "cold-blooded" or, more precisely, poikilothermic , meaning that their internal body temperature matches that of their ambient environment.
Members of the family Lamnidae such as the shortfin mako shark and the great white shark are homeothermic and maintain a higher body temperature than the surrounding water. In these sharks, a strip of aerobic red muscle located near the center of the body generates the heat, which the body retains via a countercurrent exchange mechanism by a system of blood vessels called the rete mirabile "miraculous net".
The common thresher shark has a similar mechanism for maintaining an elevated body temperature, which is thought to have evolved independently [ not in citation given ]. Tuna can maintain the temperature of certain parts of their body above the temperature of ambient seawater. However, unlike typical endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range. The rete mirabile "wonderful net" , the intertwining of veins and arteries in the body's periphery, transfers heat from venous blood to arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling.
This allows the tuna to elevate the temperatures of the highly aerobic tissues of the skeletal muscles, eyes and brain,   which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish. In some fish, a rete mirabile allows for an increase in muscle temperature in regions where this network of vein and arteries is found. The fish is able to thermoregulate certain areas of their body.
Additionally, this increase in temperature leads to an increase in basal metabolic temperature. The fish is now able to split ATP at a higher rate and ultimately can swim faster.
The eye of a swordfish can generate heat to better cope with detecting their prey at depths of feet.
Fish swim by contracting longitudinal red muscle and obliquely oriented white muscles. The red muscle is aerobic and needs oxygen which is supplied by myoglobin. The white muscle is anaerobic and it does not need oxygen. Red muscles are used for sustained activity such as cruising at slow speeds on ocean migrations. White muscles are used for bursts of activity, such as jumping or sudden bursts of speed for catching prey.
Mostly fish have white muscles, but the muscles of some fishes, such as scombroids and salmonids , range from pink to dark red. The red myotomal muscles derive their colour from myoglobin , an oxygen-binding molecule, which tuna express in quantities far higher than most other fish.
The oxygen-rich blood further enables energy delivery to their muscles. 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. A typical characteristic of many animals that utilize undulatory locomotion is that they have segmented muscles, or blocks of myomeres , running from their head to tails which are separated by connective tissue called myosepta.
In addition, some segmented muscle groups, such the lateral hypaxial musculature in the salamander are oriented at an angle to the longitudinal direction. For these obliquely oriented fibers the strain in the longitudinal direction is greater than the strain in the muscle fiber direction leading to an architectural gear ratio greater than 1.
A higher initial angle of orientation and more dorsoventral bulging produces a faster muscle contraction but results in a lower amount of force production. Most fishes bend as a simple, homogenous beam during swimming via contractions of longitudinal red muscle fibers and obliquely oriented white muscle fibers within the segmented axial musculature.
The deeper white muscle fibers fishes show diversity in arrangement. The body of a fish is denser than water, so fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a swim bladder , or gas bladder, that adjusts their buoyancy through manipulation of gases.
In this way, fish can stay at the current water depth, or ascend or descend without having to waste energy in swimming. The bladder is only found in bony fishes. In the more primitive groups like some minnows , bichirs and lungfish , the bladder is open to the esophagus and double as a lung. It is often absent in fast swimming fishes such as the tuna and mackerel families. The condition of a bladder open to the esophagus is called physostome , the closed condition physoclist.
In the latter, the gas content of the bladder is controlled through the rete mirabilis , a network of blood vessels effecting gas exchange between the bladder and the blood. In some fish, a rete mirabile fills the swim bladder with oxygen. A countercurrent exchange system is utilized between the venous and arterial capillaries.
By lowering the pH levels in the venous capillaries, oxygen unbinds from blood hemoglobin. This causes an increase in venous blood oxygen concentration, allowing the oxygen to diffuse through the capillary membrane and into the arterial capillaries, where oxygen is still sequestered to hemoglobin.
The cycle of diffusion continues until the concentration of oxygen in the arterial capillaries is supersaturated larger than the concentration of oxygen in the swim bladder. At this point, the free oxygen in the arterial capillaries diffuses into the swim bladder via the gas gland. Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains squalene , and their cartilage, which is about half the normal density of bone.
Sand tiger sharks store air in their stomachs, using it as a form of swim bladder. Most sharks need to constantly swim in order to breathe and cannot sleep very long without sinking if at all.
However, certain species, like the nurse shark , are capable of pumping water across their gills, allowing them to rest on the ocean bottom. Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's see vision in fishes. 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. 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.
Vision is an important sensory system for most species of fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have a more spherical lens.