Most fish are ectothermic, so their body temperature largely follows the surrounding water, though many use behavior to find warmer or cooler conditions. Tuna are an important exception: they retain metabolic heat in selected tissues through countercurrent blood vessels, but their bodies are not uniformly warm like a mammal’s. Opah, by contrast, has been identified by NOAA as a fully warm-blooded fish.
Key takeaways
- Most fish depend substantially on external conditions for body heat and can use habitat choice to encounter different water temperatures.
- Tuna retain metabolic heat in selected tissues using vascular countercurrent heat exchangers; the extent and location of warming vary among species.
- Some tuna, including bigeye tuna, combine behavioral and physiological thermoregulation.
- NOAA distinguishes tuna’s regional warming from opah, which it describes as the first fully warm-blooded fish identified.
How do fish regulate body temperature?
Most fish are ectothermic: their body temperature depends substantially on external conditions, especially the temperature of the surrounding water. Many can also influence their temperature by moving among habitats or depths with different water temperatures. Ectothermy does not mean a fish is always cold or unable to affect its thermal conditions.
In ordinary ectothermic fish, body temperature broadly tracks the water. Behavioral thermoregulation lets a fish encounter a more suitable temperature by selecting where it swims or stays. In some fish, including tuna, internal heat production and heat retention also warm selected tissues.
Are all fish cold-blooded?
“Cold-blooded” is common everyday language, but it can obscure the range of fish temperature strategies. Most fish are ectothermic, while some warm particular tissues through metabolic heat and heat-retaining blood vessels. NOAA also identifies opah as fully warm-blooded, a different condition from the regional endothermy of tuna.
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| Strategy | How temperature is influenced | Example or qualification |
|---|---|---|
| Ectothermy | Body temperature depends substantially on external conditions. | Describes most fish. |
| Behavioral thermoregulation | Movement or habitat choice changes the water temperatures a fish encounters. | Fish may move between depths or water masses. |
| Regional endothermy | Metabolic heat is retained in selected tissues by countercurrent blood-vessel networks. | Tuna; extent and warmed tissues vary by species. |
| Fully warm-bodied condition | Warming is not limited to the regional pattern described for tuna. | NOAA describes opah as the first fully warm-blooded fish identified. |
NOAA’s overview of whether all fish are cold-blooded discusses ectothermy, tuna, sharks, and opah. The review by Dizon and Brill distinguishes behavioral, passive, and physiological thermoregulation in tuna.
How do tuna stay warmer than the water?
Tuna generate metabolic heat, including through muscle activity, and retain some of it in selected tissues with vascular countercurrent heat exchangers. Warm venous blood transfers heat to cooler arterial blood returning from the gills. This helps conserve heat that would otherwise be lost as blood circulates through the gills.
This is called regional endothermy: some tissues can be warmer than the surrounding water, but the fish is not necessarily warm throughout its body. The tissues affected differ among tuna species. A comparative review reports that some tuna warm axial muscle and brain tissue; bluefin and albacore also warm regions of the abdominal viscera.
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Reported temperature differences depend on the study and fish. Dizon and Brill’s 1979 review reports tuna muscle temperatures 1° to 21°C above the surrounding water. A NOAA-hosted overview reports 2° to 21°C above ambient. These are reported ranges, not a fixed difference shared by every tuna.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsBigeye tuna combine behavioral and physiological thermoregulation, helping them forage in cold water below the thermocline, according to Holland and colleagues’ study. Do not assume this finding applies equally to every tuna species. A NOAA repository overview also cautions that physiological thermoregulation was not demonstrated in every tuna species covered by its review.
Are tuna the only fish that warm their bodies?
No. Some sharks also show regional warming, and opah is a distinct case: NOAA describes it as the first fully warm-blooded fish identified. These examples should not be treated as equivalent to one another or to the broad ectothermy seen in most fish.
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| Group | How much of the body is warmed? | Evidence or mechanism |
|---|---|---|
| Most fish | Body temperature largely follows surrounding conditions. | Ectothermy; some use behavioral thermoregulation. |
| Tuna | Selected tissues; extent varies by species. | Metabolic heat retained through vascular countercurrent exchange; some species also select thermal habitat. |
| Some sharks | Regional warming. | NOAA’s overview describes regional warming in tuna and some sharks. |
| Opah | NOAA describes it as fully warm-blooded. | Distinguished from tuna’s regional endothermy. |
For more on tuna, see the comparative review by Wainwright and Longo and the NOAA-hosted overview.
What is the difference between behavioral and physiological thermoregulation?
Behavioral thermoregulation changes the temperature a fish encounters: the fish moves to a different depth or water mass. Physiological thermoregulation involves internal processes that alter heat production or retention. Tuna can use both strategies, but evidence and the degree of each vary by species.
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Frequently asked questions
Are all fish cold-blooded?
No. Most fish are ectothermic, but some fish warm selected tissues through metabolic heat and countercurrent exchange. NOAA describes opah as fully warm-blooded.
Are tuna warm-blooded?
Tuna are regionally endothermic: they can retain metabolic heat in selected tissues, rather than maintaining a uniformly warm body like a mammal. The extent of warming varies among tuna species.
How do tuna keep heat from escaping through their gills?
Countercurrent blood vessels allow warm venous blood to transfer heat to cooler arterial blood returning from the gills, helping retain heat around selected tissues.
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How much warmer can tuna muscle be than the water?
Dizon and Brill’s 1979 review reports tuna muscle temperatures 1° to 21°C above surrounding water. A NOAA-hosted overview gives a range of 2° to 21°C above ambient. These are source-specific reported ranges, not a universal value for all tuna.
Is opah warm in the same way as tuna?
No. Tuna are described as regionally endothermic, with selected tissues warmed. NOAA describes opah as the first fully warm-blooded fish identified.
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