Changing perceptions: The ‘cold’ fish with a warm heart…


Opah held up by Nicholas Wegner (opah researcher)

I have nothing against Shakespeare personally, (best comeback yet invented “peace, ye fat guts, thou cream-faced loon”), but sometimes even the Bard himself is allowed a slip up… His expression, “as cold as a fish” (The Winter’s Tale, 4:4), has become a common cliché, implying that nothing else on earth provides quite as strong a simile as the frigid temperatures of these cold-blooded creatures of the deep. However, recent research suggests that we should question such cool reception of fishes and perhaps it is time to revert their proverbial cold shoulder.

A study by Nicholas Wegner, published in Science earlier this year, ( has demonstrated that we should reconsider the classification of all fishes as ectotherms. By studying the opah (Lampris guttatus), it appears that this fish species actually displays whole body endothermy, or internal heat. Of course, scientists have known for many years that some fishes can warm specific areas or organs to improve vision or brain functioning (demonstrated by some tuna and sharks), however to maintain the entire body temperature consistently warmer than the outside environment had yet to be discovered in any fish species.


Opah in fish market, although not yet evaluated by the IUCN for conservation status, this is a popular fish for human consumption

Perhaps most interestingly, the opah is a deep water fish, which are usually believed to be fairly slow swimmers, and this discovery suggests they are active predators which can pursue agile prey such as squid. Recent studies suggest that even fish with limited warming abilities are able to swim further and faster than their cold bodied relatives. Of course there is a trade-off; to achieve such high internal temperatures, a greater energy intake is required, which requires more time and effort spent foraging/eating. However the benefits of such a strategy include average swimming speeds of up to 2.7 times faster than similar sized, cold bodied fish!

The opah traps internal heat using some pretty cool (pun intended) thermal engineering. Heat is generated as the fish flaps its pectoral fins (the opah’s primary form of locomotion), and this heat is retained by thick, insulating fatty tissues. As blood is pumped into the gill surfaces from the body, it becomes cooled by seawater, but a dense network of blood vessels (the rete mirabile), enables this cooled, oxygen rich blood coming from the gills to pass next to warm, oxygen depleted blood from the body creating a counter-current heat exchange. Like a car radiator, this enables heat transfer between key blood vessels, which the fish can then use to boost performance of vital organs and enable key ambushes of fast prey items. This enables the opah to remain 5oC warmer than the surrounding ocean water, even at depths of 300m where water temperature can reach 8oC!

So as science marches on, perhaps we can allow Shakespeare another chance, after all: “knowledge is the wing wherewith we fly.”


Opah are also popular when caught for sport (catch and release)

Any fishy questions? Seen an ocean sunfish, stranded or otherwise, please get in touch! I am very keen for little sunfish tissue samples (and of course I can explain how to take little fin snips from dead fish and pay for postage etc.) so do contact me on twitter @SunfishResearch or by email:   🙂

Further reading/reference websites on the Opah and it’s amazing physiology:


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