Mammals including birds have always been considered as warm blooded animals and fish and reptiles as cold blooded however a new discovery challenges this idea.

Located deep beneath the world's oceans in the Eastern Pacific are incredibly large fish called opahs or moonfish where they apparently thrive in frigid temperatures within hundreds of feet beneath the surface.

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Fish are considered to be cold blooded creatures however these opahs have the ability to keep their bodies relatively warm.

According to lead author of the study, Nicholas Wegner from the National Oceanic and Atmospheric Administration, these creatures change the way scientists think about endothermy which is the warming of the body. This was widely thought about mammals and birds but apparently this is being observed now with fish.

A fully grown opah can weigh up to 100 pounds however they can grow double that size. Wegner also adds that they swim using their large pectoral fins which is reminiscent of bird wings where they resemble penguins while swimming.Their pectoral muscles are also disproportionally large as opposed to other fish.

These large pectoral muscles are also the main source of heat for these fish when they swim as most species have a difficult time of retaining heat in cold waters. Wegner also adds that fish cannot maintain body warmth for long since their blood flow is within close proximity with cold water as it moves through the gills.

However, these opahs developed a system how to solve this problem by minimizing heat loss. This is accomplished by maintainig the blood vessels that carry warm blood from the muscles along with blood vessels that carry cold blood from the gills before it gets absorbed by the surrounding water.

Wegner explains this adaptation where this process allows them to increase the whole body temperature that includes vital organs like the heart that is not possible with other fish. 

However, the opahs cannot do this effectively like other endothermic animals, as they cannot achieve high body temperatures like mammals and birds can but the fact that they can accomplish this is already a feat and makes for an exciting, breakthrough discovery.

This study is published in the journal, Science Advances. 

Tagsopah, first warm blooded fish, NOAA