Scientists from the USA have provided evidence to show that the Opah, a large pelagic marine fish, heats up its brain and eyes so they are warmer than the waters in which it lives.
The Opah or Moonfish, Lampris guttatus, is a massive, colourful and deep-bodied fish found in the open oceans and makes repeated dives into the depths, where the water is much colder.
The lower water temperatures there are believed to affect fish vision and neural function, so a number of fish groups have indepdently evolved the ability to heat up their eyes and brains to reduce the effects. (See Swordfish warm up their eyes for better hunting)
Rosa Runcie from California State University and her team of coauthors, studied the trait, which is known as cranial endothermy, by measuring cranial temperatures in 40 live and decked opah around 90-100cm in length.
Runcie's findings, which have been published in the latest issue of the Journal of Experimental Biology, show that the Opah has evolved a number of features that can help produce heat and prevent it from escaping into the surrounding cool water.
Picture kindly supplied by Kathy Dickson.
In some cases the tissue behind the eyes of the Opah examined was 6C warmer than the normal muscle tissue temperature - with the mean temperature being 2.1C higher.
Runcie and her coauthors said: "The proximal region of the paired lateral rectus extraocular muscle appears to be the primary source of heat.
"This muscle is the largest extraocular muscle, is adjacent to the optic nerve and brain and is separated from the brain only by a thin layer of bone.
"The proximal lateral rectus muscle is darker red in color and has a higher citrate synthase activity, indicating a higher capacity for aerobic heat production, than all other extraocular muscles."
The authors believe that the muscle, which has a layer of fat insulating it from the cold water in the gill cavity, and the network of arteries and veins within forms a "putative counter-current heat exchanger" which they think helps the Opah maintain an elevated cranial temperature.
The species is widely distributed and has been recorded worldwide in tropical and temperate waters. It reaches a size of up to 2m/6'6" and can weigh as much as 270kg.
A related species, Lampris immaculatus, known as the Southern opah, occurs in waters in the southern hemisphere and reaches a size of around 1m/39".
For more information see the paper: Runcie RM, Dewar H, Hawn DR, Frank LR and KA Dickson (2009) - Evidence for cranial endothermy in the opah (Lampris guttatus). Journal of Experimental Biology 212, 461-470 (2009).