Poikilothermic organisms are the ones whose internal temperature varies by a large degree depending upon the ambient environmental temperature. Most reptiles, fishes and amphibians are poikilothermic or cold blooded while most Mammals and birds are homeothermic or warm blooded.

This is the basic biology which we are taught in our primary classes. However, all this going to change with the discovery of the world’s first warm blooded fish – The Opah.

The process by which the Opah, a deepwater predatory fish keeps its body warm has been explained in a paper published today in Science. The process has been discovered by researchers from the National Oceanic and Atmospheric Administration (NOAA).

The secret is the unique design of the blood vessels in the fish’s gills which enables the fish to circulate warm blood throughout its body.

Studying the predatory habits of Opah gave the first sign that it was a unique fish. Heidi Dewar, a researcher at NOAA’s Southwest Fisheries Science Center and one of the paper’s authors explains. Opah resides in one of the coldest and darkest place of the ocean where the predatory fishes tend to be sluggish and wait for their prey rather than chase them.

However Opah has many features which are associated with quick moving and active predators. These include a large heart, lot of muscles and big eyes which make the Opah seem out of place in the cold depths of the ocean.

So when lead author Nicholas Wegner looked at a gill sample, he noticed something intriguing. Fishes have two kinds of blood vessels in their gills– vessels carrying blood from the body to pick up oxygen much like the veins in our body and vessels which carry oxygen rich blood back to the body. In the Opah , the incoming blood is warm after circulating across the body.

This happens because the Opah flaps its pectoral fins much faster to propel itself across the water and in the process generates a lot of heat. However the outgoing blood from the gills which has been in contact with the water is cold.

Wegner saw in the Opah gills, the two types of blood vessels were tightly packed together so that the incoming warm blood actually heats up the outgoing blood. It is known as “counter-current heat exchange” process and allows warm blood to be delivered all over the body.

Many fishes show regional endothermy and the warm blood may be localized to certain organs like the heart or the eyes. However Opah by warming the blood in its gills achieves whole body endothermy according to the paper’s authors. Various tests have revealed that the Opah is able to maintain a core body temperature which is 5 degrees Celsius warmer than the surrounding water.

There are skeptics of this theory too. Diego Bernal, an associate professor of biology at the University of Massachusetts Dartmouth believes the Opah is still a regional endotherm and it is just able to keep a larger part of its body warm than other fishes.

Bernal says, “It has a very warm core at the center of the body, but it gets cold as you go back into the body or back into the tail or as you go up or down.”

Temperature profile of the Opah has been included in the paper, shows that the warmest areas are core, including the heart, and the area around the eyes and brain, while the edges of the body are a little cooler.

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