How do warm-blooded fishes maintain a constant temperature?

Last week, we saw the mechanisms of poikilotherm fishes (or cold-blooded fishes) to fight against high and low temperatures. This week I will talk about endotherm fishes (or warm-blooded fishes). 


99% of fishes are cold-blooded, it is that the body temperature is similar to water temperature. Since some weeks ago, it was known that tuna, the group of basking sharks and swordfishes are regionally warm-blooded animals. Now, it is known that, in addition to this regionally warm-blooded specie, opah is totally endotherm.

This warm-blooded fishes, either regionally or totally, have, in general, something in common: they are big predators of fast preys and their bodies are hydrodynamic.


Tuna and basking sharks have myoglobin-rich muscles (a blood pigment useful for the diffusion and storage of oxygen inside muscles), called red muscles, which are the responsible of swimming, which increase a lot temperature during this activity. Thanks to these muscles, this animals can constantly swim because they give the necessary energy. But, as they breathe through gills, it is necessary something more to maintain a constant temperature.

Tonyina (Thunnus) (Foto de Greenpeace).
Tuna (Thunnus) (Picture from Greenpeace).
Tauró peregrí () (Foto de Ocio en Galicia).
Basking shark (Cetorhinus maximus) (Picture from Ocio en Galicia).

This “something” is counter-current circulatory system of the blood. The red muscles are placed close to vertebral column. Longitudinal arteries and veins carry the blood for all the body and are placed en each side of the body under the skin. Longitudinal arteries branch in small arteries that go to red muscles. Blood move away of red muscles through veins that flow into longitudinal veins, and go to heart. It is called a counter-current circulation because arteries carry the blood to red muscles and veins move away blood from there. In fact, main arteries and veins divide into small crossed vessels, what is called a rete mirabile.

Sistema a contracorrent de la sang (Foto extreta d'aquí).
Counter-current circulation: (a) bluefin tuna and (b) basking shark (Picture from here).

This counter-current system allows that heat received in veins from red muscles is transferred into arteries that get in in them, instead of going to the periphery of the body and gills, where this heat would be lost to water. So, it allows to maintain the heat produced in red muscles.

Some species of tuna, like bluefin tuna, and of basking sharks, moreover, can maintain a high temperature in other parts of the body, like stomach and guts, brain and eyes. This organs are irrigated by a rete mirabile.


Swordfishes have two particularities that differ from the prior examples:

  1. They just heat the brain and ocular retina.
  2. They have heater tissues.

Heater tissues consist on an extraocular muscles, that in the past were the responsible of moving the eyes in all directions. Nowadays, they are not contractile, but maintain a lot of mitochondria, which are the responsible to produce heat. This heat is maintained in the head of the animal due to a counter-current circulation, allowing the warming of brain and retina.

Peix espasa (Xiphis gladius) (Foto de Bajo el Agua)
Swordfish (Xiphis gladius) (Picture from Bajo el Agua)


A study published in Science this May has revealed that opah (Lampris guttatus) is a totally warm-blooded animal. According to this study, body temperature of opah is 5ºC higher than sea temperature.

Peix lluna real (Lampris guttatus) (Foto de IdentidadGeek)
Opah (Lampris guttatus) (Picture from IdentidadGeek)

Most of this heat is produced in pectoral fin muscles, which are surrounded by a fat layer of 1 cm that acts as a thermal insulation. Despite of this, to maintain a high body temperature, they use their gills (like a radiator), in which there is a counter-current circulation of the blood. So, blood warmed in muscles of pectoral fins goes to gills to get oxygen, but avoid the loose of heat with a counter-current system.

Moreover, they have a secondary circuit that maintain the temperature in the brain and eyes.


  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. McGraw Hill (13 ed).
  • Hill, Wyse & Anderson (2006). Fisiología animal. Editorial Medica Panamericana (1 ed)
  • Wegner, N; Snodgrass, O; Dewar, H & Hyde, JR (2015). Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science. Vol. 348 no. 6236 pp. 786-789, DOI: 10.1126/science.aaa8902



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