We were all taught in school that humans and animals are warm-blooded, while reptiles, fish, and amphibians are cold-blooded creatures. While some fish (like tuna) are able to partially warm the blood that goes to their eyes and heart muscle; most fish that live in the coldest depths of the oceans are slow because of their cold body temperature. The Moonfish, also called an opah, lives deep in the ocean, and it is a predator fish. Rather than just lurking and waiting for the next meal to swim slowly past; these fish are active hunters because they are found to be completely warm-blooded fish. They have two parts to their gills. The blood that comes back from the body to the gills for fresh oxygen is warmer blood, and it warms the cooler blood that is going from the gills into the body. The opah swims by rapidly moving its fins rather than wriggling its body, and this is what generates the extra heat to keep the blood warmer, even in the cold ocean. http://www.smh.com.au/environment/a...fish-found-in-deep-ocean-20150514-gh259l.html
Hmmmmm It's not rare or newly discovered, this is a new discovery about a already known fish that they hadn't bothered to study as much before.
"The surprise came when the lizards reached the reproductive time of the year, from September to December. During the cold early hours of the morning in that season, their breathing and heart rates rose and their temperatures reached as much as 10 °C above those of their burrows." "reptiles that can stay warmer than the environment, such as female pythons that shiver to heat their eggs" "In 2015 it became the first fish discovered to have a warm heart, and it can maintain its body temperature at 5C above surrounding waters." Note that the reptiles are still called cold blooded, even though they have had measured temperature 10deg above surrounding temp, while the so-called fish only has a measured temp, sometimes, 5deg above surrounding temp. (from mechanical/muscle activity, not metabolic or so it is reported) What would be their temperature in so-called myth tarturas ? (whatever someone wants it to be)
...in other words, temporarily achieving a temperature slightly higher that ambient temperatures (air or water) is not by itself evidence of being endothermic.