Hypoxia in fish
Fish are exposed to large oxygen fluctuations in their aquatic environment since the inherent properties of water can result in marked spatial and temporal differences in the concentration of oxygen (see
Hypoxia tolerance
A fish's hypoxia tolerance can be represented in different ways. A commonly used representation is the critical O2 tension (Pcrit), which is the lowest water O2 tension (PO2) at which a fish can maintain a stable O2 consumption rate (MO2).[2] A fish with a lower Pcrit is therefore thought to be more hypoxia-tolerant than a fish with a higher Pcrit. But while Pcrit is often used to represent hypoxia tolerance, it more accurately represents the ability to take up environmental O2 at hypoxic PO2s and does not incorporate the significant contributions of anaerobic glycolysis and metabolic suppression to hypoxia tolerance (see below). Pcrit is nevertheless closely tied to a fish's hypoxia tolerance,[3] in part because some fish prioritize their use of aerobic metabolism over anaerobic metabolism and metabolic suppression.[4] It therefore remains a widely used hypoxia tolerance metric.[5]
A fish's hypoxia tolerance can also be represented as the amount of time it can spend at a particular hypoxic PO2 before it loses dorsal-ventral equilibrium (called time-to-LOE), or the PO2 at which it loses equilibrium when PO2 is decreased from normoxia to anoxia at some set rate (called PO2-of-LOE). A higher time-to-LOE value or a lower PO2-of-LOE value therefore imply enhanced hypoxia tolerances. In either case, LOE is a more holistic representation of overall hypoxia tolerance because it incorporates all contributors to hypoxia tolerance, including aerobic metabolism, anaerobic metabolism and metabolic suppression.
Oxygen sensing
Oxygen sensing structures
In mammals there are several structures that have been implicated as oxygen sensing structures; however, all of these structures are situated to detect aortic or internal hypoxia since mammals rarely run into environmental hypoxia. These structures include the type I cells of the carotid body,[6] the neuroepithelial bodies of the lungs[7] as well as some central and peripheral neurons and vascular smooth muscle cells. In fish, the neuroepithelial cells (NEC) have been implicated as the major oxygen sensing cells.[8] NEC have been found in all teleost fish studied to date, and are likely a highly conserved structure within many taxa of fish. NEC are also found in all four gill arches within several different structures, such as along the filaments, at the ends of the gill rakers and throughout the lamellae. Two separate neural pathways have been identified within the zebrafish gill arches both the motor and sensory nerve fibre pathways.[9] Since neuroepithelial cells are distributed throughout the gills, they are often ideally situated to detect both arterial as well as environmental oxygen.[10]
Mechanisms of neurotransmitter release in neuroepithelial cells
Neuroepithelial cells (NEC) are thought to be
Signal transduction up to higher brain centres
If the post-synaptic cell is a sensory neuron, then an increased firing rate in that neuron will transmit the signal to the central nervous system for integration. Whereas, if the post-synaptic cell is a connective pillar cell or a vascular smooth muscle cell, then the serotonin will cause vasoconstriction and previously unused lamellae will be recruited through recruitment of more capillary beds, and the total surface area for gas exchange per lamella will be increased.[12]
In fish, the hypoxic signal is carried up to the brain for processing by the
Locations of oxygen sensors
Through studies using mammalian model organisms, there are two main hypotheses for the location of oxygen sensing in chemoreceptor cells: the membrane hypothesis and the mitochondrial hypothesis. The membrane hypothesis was proposed for the carotid body in mice,[13] and it predicts that oxygen sensing is an ion balance initiated process. The mitochondrial hypothesis was also proposed for the carotid body of mice, but it relies on the levels of oxidative phosphorylation and/or reactive oxygen species (ROS) production as a cue for hypoxia. Specifically, the oxygen sensitive K+ currents are inhibited by H2O2 and NADPH oxidase activation.[14] There is evidence for both of these hypotheses depending on the species used for the study. For the neuroepithelial cells in the zebrafish gills, there is strong evidence supporting the "membrane hypothesis" due to their capacity to respond to hypoxia after removal of the contents of the cell. However, there is no evidence against multiple sites for oxygen sensing in organisms.
Acute responses to hypoxia
Many hypoxic environments never reach the level of
Typically, acute hypoxia causes hyperventilation, bradycardia and an elevation in gill vascular resistance in teleosts.[16] However, the benefit of these changes in blood pressure to oxygen uptake has not been supported in a recent study of the rainbow trout.[17] It is possible that the acute hypoxia response is simply a stress response, and the advantages found in early studies may only result after acclimatization to the environment.
Behavioral responses
Hypoxia can modify normal behavior.[18] Parental behaviour meant to provide oxygen to the eggs is often affected by hypoxia. 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,[19][20] gobies,[21][22] and clownfishes,[23] 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.[24][25] Rainbow cichlids often move their young fry closer to the water surface, where oxygen is more available, during hypoxic episodes.[26]
Behavioural adaptations meant to survive when oxygen is scarce include reduced activity levels, aquatic surface respiration, and air breathing.
Reduced activity levels
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.[27] Other examples of fishes that reduce their activity levels under hypoxia include the common sole,[28] the guppy,[29] the small-spotted catshark,[30] and the viviparous eelpout.[31] Some sharks that ram-ventilate their gills may understandably increase their swimming speeds under hypoxia, to bring more water to the gills.[32]
Aquatic surface respiration
In response to decreasing
But ASR is not limited to the intertidal environment. Most tropical and temperate fish species living in stagnant waters engage in ASR during hypoxia.[40] One study looked at 26 species representing eight families of non-air breathing fishes from the North American great plains, and found that all but four of them performed ASR during hypoxia.[41] Another study looked at 24 species of tropical fish common to the pet trade, from tetras to barbs to cichlids, and found that all of them performed ASR.[42] An unusual situation in which ASR is performed is during winter, in lakes covered by ice, at the interface between water and ice or near air bubbles trapped underneath the ice.[43][44][45]
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).[46] One example is the mummichog, whose upturned mouth suggests surface feeding, but whose feeding habits are not particularly restricted to the surface. 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.[47] Swimming upside down may also help fishes perform ASR, as in some upside-down catfish.[48]
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.[49] Another way to reduce buoyancy costs is to perform ASR on rocks or plants that provide support near the water surface.
ASR significantly affects survival of fish during severe hypoxia.[50] In the shortfin molly for example, survival was approximately four times higher in individuals able to perform ASR as compared to fish not allowed to perform ASR during their exposure to extreme hypoxia.[51]
ASR may be performed more often when the need for oxygen is higher. In the sailfin molly, gestating females (this species is a livebearer) spend about 50% of their time in ASR as compared to only 15% in non-gestating females under the same low levels of oxygen.[52]
Aerial respiration (air breathing)
Aerial respiration is the 'gulping' of air at the surface of water to directly extract oxygen from the atmosphere. 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.[53]
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
Predation risk associated with ASR and aerial respiration
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
Gill remodelling in hypoxia
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
The
The
Oxygen uptake
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.[64] Except for the Antarctic ice fish that does not, most fish use hemoglobin (Hb) within their red blood cells to bind chemically and deliver 95% of the oxygen extracted from the environment to the working tissues. 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: hematocrit and the binding properties of hemoglobin.
Hematocrit
In general, hematocrit is the number of red blood cells (RBC) in circulation and is highly variable among fish species. Active fish, like the
Changing the binding affinity of hemoglobin
An alternative mechanism to preserve O2 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 PaO2 and is illustrated using an oxygen equilibrium curve (OEC). Fish hemoglobins, with the exception of the agnathans, are tetramers that exhibit cooperativity of O2 binding and have sigmoidal OECs.
The binding affinity of hemoglobin to oxygen is estimated using a measurement called P50 (the partial pressure of oxygen at which hemoglobin is 50% bound with oxygen) and can be extremely variable.[70] If the hemoglobin has a weak affinity for oxygen, it is said to have a high P50 and therefore constrains the environment in which a fish can inhabit to those with relatively high environmental PO2. 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 PO2 environments. The use of high affinity (low P50) hemoglobins results in reduced ventillatory and therefore energetic requirements when facing hypoxic insult.[65] The oxygen binding affinity of hemoglobin (Hb-O2) is regulated through a suite of allosteric modulators; the principal modulators used for controlling Hb-O2 affinity under hypoxic insult are:
- Increasing RBC pH
- Reducing inorganic phosphate interactions
pH and inorganic phosphates (Pi)
In
Changing Hb- isoforms
Nearly all animals have more than one kind of Hb present in the RBC. Multiple Hb isoforms (see
Metabolic challenge
To deal with decreased ATP production through the electron transport chain, fish must activate anaerobic means of energy production (see
Switch from aerobic to anaerobic metabolism
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.[77] However, these pathways are much less efficient than aerobic metabolism. For example, when using the same substrate, the total yield of ATP in anaerobic metabolism is 15 times lower than in aerobic metabolism. 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
Metabolic suppression is the regulated and reversible reduction of metabolic rate below basal metabolic rate (called standard metabolic rate in ectothermic animals).[1] This reduces the fish's rate of ATP use, which prolongs its survival time at severely hypoxic sub-Pcrit PO2s by reducing the rate at which the fish's finite anaerobic fuel stores (glycogen) are used. 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.[78][79][80] Physiologically, metabolic rate can be lowered through reduced growth, digestion, gonad development, and ventilation efforts.[81][82] 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.[83][84][85] 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
Fish that are capable of hypoxia-induced metabolic suppression reduce their metabolic rates by 30% to 80% relative to standard metabolic rates.[93][94][95][90] Because this is not a complete cessation of metabolic rate, metabolic suppression can only prolong hypoxic survival, not sustain it indefinitely. If the hypoxic exposure lasts sufficiently long, the fish will succumb to a depletion of its glycogen stores and/or the over-accumulation of deleterious anaerobic end-products. 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.[90]
Energy conservation
In addition to a reduction in the rate of protein synthesis, it appears that some species of hypoxia-tolerant fish conserve energy by employing Hochachka's ion channel arrest hypothesis. This hypothesis makes two predictions:
- Hypoxia-tolerant animals naturally have low membrane permeabilities
- Membrane permeability decreases even more during hypoxic conditions (ion channel arrest)[96][97]
The first prediction holds true. When membrane permeability to Na+ and K+ ions was compared between reptiles and mammals, reptile membranes were discovered to be five times less leaky. Although evidence is limited, ion channel arrest enables organisms to maintain ion channel concentration gradients and membrane potentials without consuming large amounts of ATP.
Enhanced glycogen stores
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. It can be converted into glucose and subsequently used as the starting material in glycolysis. A key adaptation to long-term survival during hypoxia is the ability of an organism to store large amounts of glycogen. Many hypoxia-tolerant species, such as carp, goldfish, killifish, and oscar contain the largest glycogen content (300-2000 μmol glocosyl units/g) in their tissue compared to hypoxia-sensitive fish, such as rainbow trout, which contain only 100 μmol glocosyl units/g.[102] The more glycogen stored in a tissue indicates the capacity for that tissue to undergo glycolysis and produce ATP.
Tolerance of waste products
When anaerobic pathways are turned on, glycogen stores are depleted and accumulation of acidic waste products occurs. This is known as a Pasteur effect. A challenge hypoxia-tolerant fish face is how to produce ATP anaerobically without creating a significant Pasteur effect. Along with a reduction in metabolism, some fish have adapted traits to avoid accumulation of lactate. For example, the crucian carp, a highly hypoxia-tolerant fish, has evolved to survive months of anoxic waters. A key adaptation is the ability to convert lactate to ethanol in the muscle and excrete it out of their gills.[103] Although this process is energetically costly is it crucial to their survival in hypoxic waters.
Gene expression changes
Microarray studies done on fish species exposed to hypoxia typically show a metabolic switch, that is, a decrease in the expression of genes involved in aerobic metabolism and an increase in expression of genes involved in anaerobic metabolism. Zebrafish embryos exposed to hypoxia decreased expression of genes involved in the
Research in mammals has implicated
See also
References
- ^ ISBN 9780123746320.
- PMID 26768976.
- S2CID 9920660.
- PMID 27913601.
- PMID 27293760.
- PMID 7938227.
- PMID 11882682.
- PMID 15331683.
- S2CID 997986.
- PMID 18375847.
- S2CID 16605017.
- S2CID 24882272.
- PMID 2456613.
- PMID 10760304.
- PMID 21056112
- PMID 6033996.
- PMID 16574450
- ^ Reebs, S.G. (2009) Oxygen and fish behaviour
- ^ Iersel, J.J.A. van. 1953. An analysis of the parental behaviour of the male three-spine stickleback (Gasterosteus aculeatus L.). Behaviour Supplement 3: 1-159.
- ^ Sevenster, P. 1961. A causal analysis of a displacement activity (fanning in Gasterosteus aculeatus L.). Behaviour Supplement 9: 1-170.
- JSTOR 4534416.
- ^ Takegaki, T.; Nakazono, A. (1999). "Responses of the egg-tending gobiid fish Valenciennea longipinnis to the fluctuation of dissolved oxygen in the burrow". Bulletin of Marine Science. 65: 815–823.
- .
- .
- S2CID 22455103.
- .
- .
- doi:10.1139/z98-141.
- S2CID 25168337.
- ^ Metcalfe, J.D.; Butler, P.J. (1984). "Changes in activity and ventilation in response to hypoxia in unrestrained, unoperated dogfish (Scyliorhinus canicula L.)". Journal of Experimental Biology. 180: 153–162.
- .
- S2CID 30061199.
- S2CID 41805602.
- .
- PMID 18276177.
- S2CID 12981509.
- doi:10.1139/z84-051.
- PMID 10841932.
- .
- S2CID 36058867
- doi:10.1139/z78-263.
- S2CID 36058867.
- S2CID 40460613.
- S2CID 26567277.
- JSTOR 1442367.
- JSTOR 1441653.
- PMID 10708642.
- JSTOR 1446679.
- JSTOR 1446244.
- S2CID 23174540
- ^ Plath M, Tobler M, Riesch R, García de León FJ, Giere O, Schlupp I. 2007. Survival in an extreme habitat: the roles of behaviour and energy limitation. Die Naturwissenschaften 94: 991-6. PMID
- S2CID 20707941.
- PMID 21177940.
- ^ S2CID 624488.[permanent dead link]
- ^ ISBN 9780123746320.
- PMID 17472921.
- S2CID 7674442.
- PMID 28476894.
- PMID 15618479.
- ^ PMID 12966058.
- PMID 15767311.
- ^ PMID 18344480.
- S2CID 7459192.
- S2CID 22391627
- ^ a b c d Perry, SF, Esbaugh, A, Braun, M, and Gilmour, KM. 2009. Gas Transport and Gill Function in Water Breathing Fish. In Cardio-Respiratory Control in Vertebrates, (ed. Glass ML, Wood SC), pp. 5-35. Berlin: Springer-Verlag.
- .
- ^ Yamamoto, K.; Itazawa, Y.; Kobayashi, H. (1985). "Direct observations of fish spleen by an abdominal window method and its application to exercised and hypoxic yellowtail". Jpn J Ichthyol. 31: 427–433.
- PMID 16809464.
- PMID 14766767.
- .
- PMID 3040965.
- S2CID 46536552
- ^ Nikinmaa, M; Boutilier, RG (1995). "Adrenergic control of red cell pH, organic phosphate concentrations and haemoglobin function in teleost fish". In Heisler, N (ed.). Advances in Comparative and Environmental Physiology. Vol. 21. Berlin: Springer-Verlag. pp. 107–133.
- PMID 30713504.
- PMID 11454005.
- PMID 17626121.
- S2CID 18361836.
- .
- ISBN 9780123746320.
- ISBN 9780123746320.
- PMID 17210965.
- ISBN 9780123746320.
- PMID 18805810.
- PMID 17515419.
- PMID 8897979.
- PMID 24072793.
- PMID 26768970.
- .
- .
- ^ PMID 27913601.
- .
- PMID 29093174.
- S2CID 23164136.
- .
- .
- PMID 2417316
- PMID 8238602.
- PMID 3605374.
- S2CID 25835417.
- PMID 15879061.
- S2CID 28016845.
- ISBN 9780123746320.
- PMID 7384807
- ^ PMID 11172064.
- ^ PMID 15994372.
- PMID 16469844.
- S2CID 15538913.
- ^ S2CID 27670309.
- PMID 18651837.
- PMID 16623959.
- PMID 17467194