Aquatic respiration

Aquatic respiration is the process whereby an aquatic animal obtains oxygen from water.

Pleurobranchaea meckelii
Sea slugs breathe through a gill (or ctenidium)

Respiratory systems


Most fish exchange gases using gills on either side of the pharynx (throat), forming the Splanchnocranium; the Splanchnocranium being the portion of the skeleton where the cartilage of the cranium converges into the cartilage of the pharynx and its associated parts[1]. Gills are tissues which consist of threadlike structures called filaments. These filaments have many functions and are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange[2]. Each filament contains a capillary network that provides a large surface area for the exchange of gases and ions. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In species like the Spiny dogfish and other sharks and rays, a spiracle exists near the top of the head that pumps water into the gills when the animal is not in motion[3]. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The muscles on the sides of the pharynx push the oxygen-depleted water out the gill openings. In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the Operculum (fish)[4].


Molluscs generally possess gills that allow exchange of oxygen from an aqueous environment into the circulatory system. These animals also possess a heart that pumps blood which contains hemocyaninine as its oxygen-capturing molecule. Therefore, this respiratory system is similar to that of vertebrate fish. The respiratory system of gastropods can include either gills or a lung.


Aquatic arthropods generally possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton. Others may breathe atmospheric air while remaining submerged, via breathing tubes or trapped air bubbles, though some aquatic insects may remain submerged indefinitely and respire using a plastron. A very few Arachnids have adopted an aquatic life style including the Diving bell spider. In all cases, oxygen is provided from air trapped by hairs around the animals body.

Aquatic reptiles

All aquatic reptiles breathe air into lungs. The anatomical structure of the lungs is less complex in reptiles than in mammals, with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli however, reptiles do not possess a diaphragm. Thus, breathing occurs via a change in the volume of the body cavity which is controlled by contraction of intercostal muscles in all reptiles except turtles. In turtles, contraction of specific pairs of flank muscles governs inspiration or expiration.[5]

See also reptiles for more detailed descriptions of the respiratory system in these animals.


Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. While the lungs are of primary importance to breathing control, the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water.[6]

Aquatic birds

The respiratory system of birds differs significantly from that found in mammals, containing unique anatomical features such as air sacs. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli. See Avian respiratory system for a detailed description of these and other features.


Tuna Gills in Situ 01
Posterior view of the gills of a tuna

Many aquatic animals have developed gills for respiration which are specifically adapted to their function. In fish, for example, they have:

  • A large surface area to allow as much oxygen to enter the gills as possible because more of the gas comes into contact with the membrane
  • Good blood supply to maintain the concentration gradient needed
  • Thin membrane to allow for a short diffusion pathway
  • each gill arch has two rows (hemibranchs) of gill filaments
  • each gill filament has many lamellae

In osteichthyes, the gills contain 4 gill arches on each side of the head, two on each side for chondrichthyes or 7 gill baskets on each side of the fish's head in Lampreys. In fish, the long bony cover for the gill (the operculum) can be used for pushing water. Some fish pump water using the operculum. Without an operculum, other methods, such as ram ventilation, are required. Some species of sharks use this system. When they swim, water flows into the mouth and across the gills. Because these sharks rely on this technique, they must keep swimming in order to respire.

Bony fish use countercurrent flow to maximize the intake of oxygen that can diffuse through the gill. Countercurrent flow occurs when deoxygenated blood moves through the gill in one direction while oxygenated water moves through the gill in the opposite direction. This mechanism maintains the concentration gradient thus increasing the efficiency of the respiration process as well and prevents the oxygen levels from reaching an equilibrium. Cartilaginous fish do not have a countercurrent flow system as they lack bones which are needed to have the opened out gill that bony fish have.

Control of respiration

Scientists have investigated what part of the body is responsible for maintaining the respiratory rhythm. They found that neurons located in the brainstem of fish are responsible for the genesis of the respiratory rhythm.[7] The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the homology of respiratory centers between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see Involuntary control of respiration).

Another important feature of the respiratory rhythm is that it is modulated to adapt to the oxygen consumption of the body. As observed in mammals, fish “breathe” faster and heavier when they do physical exercise. The mechanisms by which these changes occur have been strongly debated over more than 100 years between scientists.[8] The authors can be classified in 2 schools:

1. Those who think that the major part of the respiratory changes are pre-programmed in the brain, which would imply that neurons from locomotion centers of the brain connect to respiratory centers in anticipation of movements.

2. Those who think that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. This would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.

Many now agree that both mechanisms are probably present and complementary, or working alongside a mechanism that can detect changes in oxygen and/or carbon dioxide blood saturation.

See also


  1. ^ "Introduction to the skeletal system". Retrieved 2019-06-07.
  2. ^ Evans, David H. (2010-06-18). "A Brief History of the Study of Fish Osmoregulation: The Central Role of the Mt. Desert Island Biological Laboratory". Frontiers in Physiology. 1: 13. doi:10.3389/fphys.2010.00013. ISSN 1664-042X. PMC 3059943. PMID 21423356.
  3. ^ Wischnitzer, Saul (1967). Atlas and Dissection Guide for Comparative Anatomy. United States of America. p. 22. ISBN 0-7167-0691-1.
  4. ^ Kimmel, Charles B.; Aguirre, Windsor E.; Ullmann, Bonnie; Currey, Mark; Cresko, William A. (2008). "Allometric Change Accompanies Opercular Shape Evolution in Alaskan Threespine Sticklebacks". Behaviour. 145 (4/5): 669–691. doi:10.1163/156853908792451395. ISSN 0005-7959. JSTOR 40295944.
  5. ^ "reptile - animal". Retrieved 8 September 2016.
  6. ^ Gottlieb, G; Jackson DC (1976). "Importance of pulmonary ventilation in respiratory control in the bullfrog". Am J Physiol. 230 (3): 608–13. doi:10.1152/ajplegacy.1976.230.3.608. PMID 4976.
  7. ^ Russell, David F. (1986). "Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting". Journal of Comparative Physiology A. 158 (1): 91–102. doi:10.1007/BF00614523.
  8. ^ Waldrop, Tony G.; Gary A. Iwamoto; Philippe Haouzi (10 November 2005). "Point:Counterpoint: Supraspinal locomotor centers do/do not contribute significantly to the hyperpnea of dynamic exercise". Journal of Applied Physiology. 100 (3): 1077–1083. doi:10.1152/japplphysiol.01528.2005. PMID 16467394.

In animal anatomy, a cloaca kloh-AY-kə (plural cloacae kloh-AY-see or kloh-AY-kee) is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals, opening at the vent. All amphibians, birds, reptiles, and a few mammals (monotremes, tenrecs, golden moles, and marsupial moles) have this orifice, from which they excrete both urine and feces; this is in contrast to most placental mammals, which have two or three separate orifices for evacuation. Excretory openings with analogous purpose in some invertebrates are also sometimes referred to as cloacae. Mating by cloaca is known as cloacal copulation, mostly referred to as cloacal kiss.

The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent-marking behavior of some reptiles, marsupials, amphibians, and monotremes.

Coarse fishing

In the United Kingdom and Ireland coarse fishing (Irish: garbhiascaireacht, Welsh: pysgota bras) refers to angling for freshwater fish which are traditionally considered undesirable as a food or game fish. Freshwater game fish are all salmonids—most particularly salmon, trout and char—so generally coarse fish, also known as rough fish, are freshwater fish that are not salmonids. There is disagreement over whether grayling should be classified as a game fish or a coarse fish.Fly fishing is the technique usually used for freshwater game fishing, while other angling techniques are usually used for coarse fishing. The sport of coarse fishing and the techniques it uses are particularly popular in the United Kingdom and mainland Europe, and as well as in some former British Commonwealth countries and among British expatriates.

The distinction between coarse fish and game fish has no taxonomic basis. It originated in the United Kingdom in the early 19th century. Prior to that time, recreational fishing was a sport of the gentry, who angled for salmon and trout and called them game fish. There was a view that other fish did not make as good eating, and they were disdained as coarse fish. Coarse fish have scales that are generally larger than the scales of game fish, and they tend to inhabit warmer and stiller waters.

Cutaneous respiration

Cutaneous respiration, or cutaneous gas exchange, is a form of respiration in which gas exchange occurs across the skin or outer integument of an organism rather than gills or lungs. Cutaneous respiration may be the sole method of gas exchange, or may accompany other forms, such as ventilation. Cutaneous respiration occurs in a wide variety of organisms, including insects, amphibians, fish, sea snakes, turtles, and to a lesser extent in mammals, including humans.

Dolphin (comics)

Dolphin is a fictional character, a superheroine in the DC Comics universe. Created by writer-artist Jay Scott Pike, she debuted in Showcase #79 (Dec. 1968).


Euryhaline organisms are able to adapt to a wide range of salinities. An example of a euryhaline fish is the molly (Poecilia sphenops) which can live in fresh water, brackish water, or salt water. The green crab (Carcinus maenas) is an example of a euryhaline invertebrate that can live in salt and brackish water. Euryhaline organisms are commonly found in habitats such as estuaries and tide pools where the salinity changes regularly. However, some organisms are euryhaline because their life cycle involves migration between freshwater and marine environments, as is the case with salmon and eels.

The opposite of euryhaline organisms are stenohaline ones, which can only survive within a narrow range of salinities. Most freshwater organisms are stenohaline, and will die in seawater, and similarly most marine organisms are stenohaline, and cannot live in fresh water.

F. E. J. Fry

Frederick Ernest Joseph Fry MBE, FRSC (April 17, 1908 – May 22, 1989) was a Canadian ichthyologist and aquatic ecologist. He is known for his early research in physiological ecology and population dynamics in fishes. In the late 1940s, he became the first scientist to model how environmental factors affect the activity of fish. He was a 1959 Guggenheim Fellow, and served as president of several organizations including the American Society of Limnology and Oceanography (1951) American Fisheries Society (1966) and American Institute of Fishery Research Biologists (1972).

Fry was born in the English town of Woking, Surrey, April 17, 1908, to parents Ernest and Mabel Fry. His family immigrated to Canada in 1912, and after the first World War settled in Toronto. Fry attended the University of Toronto, earning a B.A. (1933), M.A. (1935), and PhD (1936). He joined the University of Toronto faculty as lecturer in 1938.

During World War II Fry served in the Royal Canadian Air Force from 1941 to 1945, working in aviation medicine, where he helped develop equipment aiding in respiration at high altitudes.After the War, Fry returned to the University of Toronto as assistant professor in 1945, and became full professor of zoology in 1956, a position he held until his retirement in 1973.

One of his early studies of note was a long term field study of fishes of Lake Opeongo in Algonquin Provincial Park. In an influential 1949 paper, Fry developed "virtual population" analysis to understand effects of fishing on fish populations, a method which 50 years later was still in a chief way of determining total allowable catches in fisheries management.His physiology papers "Effects of the Environment on Animal Physiology" in 1947, and "The Aquatic Respiration of Fish" in 1957 have become classic works in fisheries science.

Fear of fish

Fear of fish or ichthyophobia ranges from cultural phenomena such as fear of eating fish, fear of touching raw fish, or fear of dead fish, up to irrational fear (specific phobia). Galeophobia is the fear specifically of sharks.

Fish gill

Fish gills are organs that allow fish to breathe underwater. Most fish exchange gases like oxygen and carbon dioxide using gills that are protected under gill covers on both sides of the pharynx (throat). Gills are tissues that are like short threads, protein structures called filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia. 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. In some fish, capillary blood flows in the opposite direction to the water, causing counter-current exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective bony cover called the operculum.

Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.

Previously, the evolution of gills was thought to have occurred through two diverging lines: gills formed from the endoderm, as seen in jawless fish species, or those form by the ectoderm, as seen in jawed fish. However, recent studies on gill formation of the little skate (Leucoraja erinacea) has shown potential evidence supporting the claim that gills from all current fish species have in fact evolved from a common ancestor.

Freshwater fish

Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations.

41.24% of all known species of fish are found in fresh water. This is primarily due to the rapid speciation that the scattered habitats make possible. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying island biogeography.


A gill ( (listen)) is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist. The microscopic structure of a gill presents a large surface area to the external environment. Branchia (pl. branchiae) is the zoologists' name for gills (from Ancient Greek).

With the exception of some aquatic insects, the filaments and lamellae (folds) contain blood or coelomic fluid, from which gases are exchanged through the thin walls. The blood carries oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, crustaceans, insects, fish, and amphibians. Semiterrestrial marine animals such as crabs and mudskippers have gill chambers in which they store water, enabling them to use the dissolved oxygen when they are on land.

Hallucinogenic fish

Several species of fish are claimed to produce hallucinogenic effects when consumed. For example, Sarpa salpa, a species of sea bream, is commonly claimed to be hallucinogenic. These widely distributed coastal fish are normally found in the Mediterranean and around Spain, and along the west and south coasts of Africa. Occasionally they are found in British waters. They may induce hallucinogenic effects that are purportedly LSD-like if eaten. In 2006, two men who apparently ate the fish experienced hallucinations lasting for several days. The likelihood of hallucinations depends on the season. Sarpa salpa is known as "the fish that makes dreams" in Arabic.Other species claimed to be capable of producing hallucinations include several species of sea chub from the genus Kyphosus. It is unclear whether the toxins are produced by the fish themselves or by marine algae in their diet. Other hallucinogenic fish are Siganus spinus, called "the fish that inebriates" in Reunion Island, and Mulloidichthys flavolineatus (formerly Mulloidichthys samoensis), called "the chief of ghosts" in Hawaii.


The Hippocampinae are a subfamily of small marine fishes in the family Syngnathidae. Depending on the classification system used, it comprises either seahorses and pygmy pipehorses, or only seahorses.

Marbled goby

Pomatoschistus marmoratus, the marbled goby, is a species of goby native to the eastern Atlantic from the Bay of Biscay down around the Iberian Peninsula through the Mediterranean Sea, the Black Sea and the Sea of Azov. It is also found in the Suez Canal in Egypt. It occurs in marine and brackish waters on sandy substrates in shallow waters, typically down to 20 m (66 ft), but occasionally to 70 m (230 ft) in the winter. It can reach a length of 8 cm (3.1 in) TL though most do not exceed 5 centimetres (2.0 in) TL.

Marbled lungfish

The marbled lungfish (Protopterus aethiopicus) is a lungfish of the family Protopteridae. Also known as the leopard lungfish, it is found in Africa. At 133 billion base pairs it has the largest known genome of any vertebrate and one of the largest of any organism on Earth, along with Polychaos dubium and Paris japonica at 670 billion and 150 billion, respectively.


An otolith (Greek: ὠτο-, ōto- ear + λῐ́θος, líthos, a stone), also called statoconium or otoconium or statolith, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule and utricle, in turn, together make the otolith organs. These organs are what allows an organism, including humans, to perceive linear acceleration, both horizontally and vertically (gravity). They have been identified in both extinct and extant vertebrates.Counting the annual growth rings on the otoliths is a common technique in estimating the age of fish.

Respiration (physiology)

In physiology, respiration is the movement of oxygen from the outside environment to the cells within tissues, and the transport of carbon dioxide in the opposite direction.

The physiological definition of respiration differs from the biochemical definition, which refers to a metabolic process by which an organism obtains energy (in the form of ATP) by oxidizing nutrients and releasing waste products. Although physiologic respiration is necessary to sustain cellular respiration and thus life in animals, the processes are distinct: cellular respiration takes place in individual cells of the organism, while physiologic respiration concerns the diffusion and transport of metabolites between the organism and the external environment.

In animals with lungs, physiological respiration involves respiratory cycles of inhaled and exhaled breaths. Inhalation (breathing in) is usually an active movement. The contraction of the diaphragm muscle cause a pressure variation, which is equal to the pressures caused by elastic, resistive and inertial components of the respiratory system. In contrast, exhalation (breathing out) is usually a passive process. Breathing in, brings air into the lungs where the process of gas exchange takes place between the air in the alveoli and the blood in the pulmonary capillaries

The process of breathing does not fill the alveoli with atmospheric air during each inhalation (about 350 ml per breath), but the inhaled air is carefully diluted and thoroughly mixed with a large volume of gas (about 2.5 liters in adult humans) known as the functional residual capacity which remains in the lungs after each exhalation, and whose gaseous composition differs markedly from that of the ambient air. Physiological respiration involves the mechanisms that ensure that the composition of the functional residual capacity is kept constant, and equilibrates with the gases dissolved in the pulmonary capillary blood, and thus throughout the body. Thus, in precise usage, the words breathing and ventilation are hyponyms, not synonyms, of respiration; but this prescription is not consistently followed, even by most health care providers, because the term respiratory rate (RR) is a well-established term in health care, even though it would need to be consistently replaced with ventilation rate if the precise usage were to be followed.


Teleostomi is an obsolete clade of jawed vertebrates that supposedly includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the extinct class Placodermi.

Recent studies indicate that Osteichthyes evolved from placoderms like Entelognathus, while acanthodians are more closely related to modern chondrichthyes. Teleostomi, therefore, is not a valid, natural clade, but a polyphyletic group of species.The clade Teleostomi should not be confused with the similar-sounding fish clade Teleostei.


The Trionychidae are a taxonomic family of a number of turtle genera. Softshells include some of the world's largest freshwater turtles, though many can adapt to living in highly brackish areas. Members of this family occur in Africa, Asia, and North America. Most species have traditionally been included in the genus Trionyx, but the vast majority have since been moved to other genera. Among these are the North American Apalone softshells that were placed in Trionyx until 1987.

Aquatic ecosystems

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