Is the Opah a warm blooded fish?
Did you say warm-blooded?

Despite the definition you cite from Merriam-Webstre Dictionnary, the term warm-blood is very unclear. The correct terms are endo-, exo-, poikilo- and homeo- therm. In short&hellip;

Source of heat 


endo = inside
exo = outside


Variation in inside temperature


Poikilo = varies
homeo = does not vary


Please note, that these terms refer to extreme cases. There are organism which temperature are extremely constant (homeotherm), organisms which temperature are extremely variable (poikilotherm) and anything in between.

Any combination of these two axes exist. For example: If the temperature in the environment never varies you can be homeotherm without needing to be endotherm. Also, some organisms manage to keep a quasi constant body temperature via behavioural actions (moving to the sun when cold, going into the water when it's hot). A fun example (although debated to what I've heard) are the large dinosaurs that are thought to be homeotherms because their metabolism produce some heat and they are so large that they remain warm thanks to this heat source. However, they were probably not able to regulate actively their temperature. Therefore, I would tend to qualify dinosaurs as homeo-exo-therm but I wouldn't be surprised if someone prefers to call large dinosaur homeo-endo-therm individuals.

Opah fish

From what your second quote, it sounds like the Opah fish is endotherm. It is unclear from the quotes whether Opah fish are homeothermic.

Wegner et al. (2015) is a great read on the subject. Below is their abstract but I recommend reading the whole paper, it is easy to read and is very short (3 pages).


  Endothermy (the metabolic production and retention of heat to warm body temperature above ambient) enhances physiological function, and whole-body endothermy generally sets mammals and birds apart from other animals. Here, we describe a whole-body form of endothermy in a fish, the opah (Lampris guttatus), that produces heat through the constant “flapping” of wing-like pectoral fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike other fish, opah distribute warmed blood throughout the body, including to the heart, enhancing physiological performance and buffering internal organ function while foraging in the cold, nutrient-rich waters below the ocean thermocline.


In the whole, paper, I could not find a mention of how much variation in body temperature there is. They highlight however that the point of such adaptation is for foraging in cold waters suggesting that they would just make a boost of temperature in extreme cases but not necessarily actively maintained a constant temperature otherwise. I would tend to think that Opah fish are much more variable than birds or mammals. To reinforce my believes, in the wikipedia > Opah fish it is written (without reference unfortunately) "The opah is not homeothermic like birds and mammals".



FYI, this answer was written by partially recycling this other answer of mine.