Fish that use air breathing organs ABO tend to live in environments with highly variable oxygen content and rely on aerial respiration during times when there is not enough oxygen to support water-breathing. However, many species of teleost fish are obligate water breathers and do not display either of these surface respiratory behaviours.
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Typically, acute hypoxia causes hyperventilation , bradycardia and an elevation in gill vascular resistance in teleosts. Hypoxia can modify normal behavior.
Fish Physiology: Hypoxia Vol. 27 by Colin J. Brauner (2009, Hardcover)
For example, fanning behavior swimming on the spot near the eggs to create a flow of water over them, and thus a constant supply of oxygen is often increased when oxygen is less available. This has been documented in sticklebacks,   gobies,   and clownfishes,  among others. Gobies may also increase the size of the openings in the nest they build, even though this may increase the risk of predation on the eggs.
Behavioural adaptations meant to survive when oxygen is scarce include reduced activity levels, aquatic surface respiration, and air breathing. As oxygen levels decrease, fish may at first increase movements in an attempt to escape the hypoxic zone, but eventually they greatly reduce their activity levels, thus reducing their energetic and therefore oxygen demands.
Atlantic herring show this exact pattern. In response to decreasing dissolved oxygen level in the environment, fish swim up to the surface of the water column and ventilate at the top layer of the water where it contains relatively higher level of dissolved oxygen, a behavior called aquatic surface respiration ASR.
This is true only in stagnant water; in running water all layers are mixed together and oxygen levels are the same throughout the water column. One environment where ASR often takes place is tidepools, particularly at night. Examples of tidepool species that perform ASR include the tidepool sculpin , Oligocottus maculosus ,   the three-spined stickleback ,  and the mummichog.
But ASR is not limited to the intertidal environment. Most tropical and temperate fish species living in stagnant waters engage in ASR during hypoxia. Some species may show morphological adaptations, such as a flat head and an upturned mouth, that allow them to perform ASR without breaking the water surface which would make them more visible to aerial predators.
Hypoxia in fish - Wikipedia
In the tambaqui , a South American species, exposure to hypoxia induces within hours the development of additional blood vessels inside the lower lip, enhancing its ability to take up oxygen during ASR. Some species may hold an air bubble within the mouth during ASR. This may assist buoyancy as well as increase the oxygen content of the water passing over the bubble on its way to the gills. ASR significantly affects survival of fish during severe hypoxia. ASR may be performed more often when the need for oxygen is higher. Aerial respiration evolved in fish that were exposed to more frequent hypoxia; also, species that engage in aerial respiration tend to be more hypoxia tolerant than those which do not air-breath during the hypoxia.
There are two main types of air breathing fish—facultative and non-facultative.
Under normoxic conditions facultative fish can survive without having to breathe air from the surface of the water. However, non-facultative fish must respire at the surface even in normal dissolved oxygen levels because their gills cannot extract enough oxygen from the water. Many air breathing freshwater teleosts use ABOs to effectively extract oxygen from air while maintaining functions of the gills. ABOs are modified gastrointestinal tracts , gas bladders , and labyrinth organs ;  they are highly vascularized and provide additional method of extracting oxygen from the air.
Both ASR and aerial respiration require fish to travel to the top of water column and this behaviour increases the predation risks by aerial predators or other piscivores inhabiting near the surface of the water. When fish can visually detect the presence of their aerial predators, they simply refrain from surfacing, or prefer to surface in areas where they can be detected less easily i. Gill remodelling happens in only a few species of fish, and it involves the buildup or removal of an inter-lamellar cell mass ILCM.
As a response to hypoxia, some fish are able to remodel their gills to increase respiratory surface area, with some species such as goldfish doubling their lamellar surface areas in as little as 8 hours. The crucian carp is one species able to remodel its gill filaments in response to hypoxia. Their inter-lamellar cells have high rates of mitotic activity which are influenced by both hypoxia and temperature. This same transition in gill morphology occurs in the goldfish when the temperature was raised from 7. Temperature also affects the speed at which the gills can be remodelled: Covering the gill lamellae may protect species like the crucian carp from parasites and environmental toxins during normoxia by limiting their surface area for inward diffusion while still maintaining oxygen transport due to an extremely high hemoglobin oxygen binding affinity.
The naked carp , a closely related species native to the high-altitude Lake Qinghai , is also able to remodel their gills in response to hypoxic conditions. Fish exhibit a wide range of tactics to counteract aquatic hypoxia, but when escape from the hypoxic stress is not possible, maintaining oxygen extraction and delivery becomes an essential component to survival.
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Maintaining oxygen extraction and delivery to the tissues allows continued activity under hypoxic stress and is in part determined by modifications in two different blood parameters: In general, hematocrit is the number of red blood cells RBC in circulation and is highly variable among fish species. Active fish, like the blue marlin , tend to have higher hematocrits,  whereas less active fish, such as the starry flounder exhibit lower hematocrits. Increasing hematocrit in response to erythropoietin is observed after approximately one week and is therefore likely under genetic control of hypoxia inducible factor hypoxia inducible factor HIF.
First, A higher hematocrit results in more viscous blood especially in cold water increasing the amount of energy the cardiac system requires to pump the blood through the system and secondly depending on the transit time of the blood across the branchial arch and the diffusion rate of oxygen, an increased hematocrit may result in less efficient transfer of oxygen from the environment to the blood. An alternative mechanism to preserve O 2 delivery in the face of low ambient oxygen is to increase the affinity of the blood.
The oxygen content of the blood is related to PaO 2 and is illustrated using an oxygen equilibrium curve OEC. Fish hemoglobins, with the exception of the agnathans , are tetramers that exhibit cooperativity of O 2 binding and have sigmoidal OECs. Conversely, fish hemoglobins with a low P50 bind strongly to oxygen and are then of obvious advantage when attempting to extract oxygen from hypoxic or variable PO 2 environments.
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The use of high affinity low P50 hemoglobins results in reduced ventillatory and therefore energetic requirements when facing hypoxic insult. The dilution of the cell contents causes further spatial separation of hemoglobin from the inorganic phosphates and again serves to increase Hb-O 2 affinity. Nearly all animals have more than one kind of Hb present in the RBC. Multiple Hb isoforms see isoforms are particularly common in ectotherms , but especially in fish that are required to cope with both fluctutating temperature and oxygen availability.
Hbs isolated from the European eel can be separated into anodic and cathodic isoforms. The anodic isoforms have low oxygen affinities high P50 and marked Bohr effects, while the cathodic lack significant pH effects and are therefore thought to confer hypoxia tolerance. They demonstrated there were Hb isoforms specific to the hypoxia-raised individuals. To deal with decreased ATP production through the electron transport chain, fish must activate anaerobic means of energy production see anaerobic metabolism while suppressing metabolic demands.
The ability to decrease energy demand by metabolic suppression is essential to ensure hypoxic survival due to the limited efficiency of anaerobic ATP production. Aerobic respiration, in which oxygen is used as the terminal electron acceptor, is crucial to all water-breathing fish. When fish are deprived of oxygen, they require other ways to produce ATP. Thus, a switch from aerobic metabolism to anaerobic metabolism occurs at the onset of hypoxia. Glycolysis and substrate-level phosphorylation are used as alternative pathways for ATP production.
For example, when using the same substrate, the total yield of ATP in anaerobic metabolism is 15 times lower than in aerobic metabolism.
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This level of ATP production is not sufficient to maintain a high metabolic rate, therefore, the only survival strategy for fish is to alter their metabolic demands. Metabolic suppression is the regulated and reversible reduction of metabolic rate below basal metabolic rate called standard metabolic rate in ectothermic animals . Metabolic suppression also reduces the accumulation rate of deleterious anaerobic end-products lactate and protons , which delays their negative impact on the fish. The mechanisms that fish use to suppress metabolic rate occur at behavioral, physiological and biochemical levels.
Behaviorally, metabolic rate can be lowered through reduced locomotion, feeding, courtship, and mating   . Physiologically, metabolic rate can be lowered through reduced growth, digestion, gonad development, and ventilation efforts  . And biochemically, metabolic rate can be further lowered below standard metabolic rate through reduced gluconeogenesis, protein synthesis and degradation rates, and ion pumping across cellular membranes   .
Reductions in these processes lower ATP use rates, but it remains unclear whether metabolic suppression is induced through an initial reduction in ATP use or ATP supply. The prevalence of metabolic suppression use among fish species has not been thoroughly explored.
This is partly because the metabolic rates of hypoxia-exposed fish, including suppressed metabolic rates, can only be accurately measured using direct calorimetry , and this technique is seldom used for fish  . The species that employ metabolic suppression are more hypoxia-tolerant than the species that do not, which suggests that metabolic suppression enhances hypoxia tolerance. Consistent with this, differences in hypoxia tolerance among isolated threespine stickleback populations appear to result from differences in the use of metabolic suppression, with the more tolerant stickleback using metabolic suppression .
Because this is not a complete cessation of metabolic rate, metabolic suppression can only prolong hypoxic survival, not sustain it indefinitely. Furthermore, the severely limited energetic scope that comes with a metabolically suppressed state means that the fish is unable to complete critical tasks such a predator avoidance and reproduction. Perhaps for these reasons, goldfish prioritize their use of aerobic metabolism in most hypoxic environments, reserving metabolic suppression for the extreme case of anoxia .
This hypothesis makes two predictions:. The first prediction holds true. The limiting factor for fish undergoing hypoxia is the availability of fermentable substrate for anaerobic metabolism; once substrate runs out, ATP production ceases. Endogenous glycogen is present in tissue as a long term energy storage molecule. Internationally respected contributors review and synthesize the morphological, behavioral, physiological, biochemical and molecular strategies used by fish to survive hypoxia exposure, while placing these adaptations within an environmental and ecological context.
Through the development of a synthesis chapter, this book serves as the cornerstone for directing future research into the effects of hypoxia exposures on fish physiology and biochemistry. Periods of environmental hypoxia Low Oxygen Availability are extremely common in aquatic systems due to both natural causes such as diurnal oscillations in algal respiration, seasonal flooding, stratification, under ice cover in lakes, and isolation of densely vegetated water bodies, as well as more recent anthropogenic causes e.
Hypoxia in fish
In view of this, it is perhaps not surprising that among all vertebrates, fish boast the largest number of hypoxia tolerant species; hypoxia has clearly played an important role in shaping the evolution of many unique adaptive strategies. These unique adaptive strategies either allow fish to maintain function at low oxygen levels, thus extending hypoxia tolerance limits, or permit them to defend against the metabolic consequences of oxygen levels that fall below a threshold where metabolic functions cannot be maintained.
The aim of this volume is two-fold.