[Phys-L] biophysics: warm-blooded fish!

[John Denker <jsd@av8n.com>]

In thermodynamic engineering, there is such a thing
as a counter-current heat exchanger.  See e.g.
  http://en.wikipedia.org/wiki/Dilution_refrigerator

It is reported that the opah (Lampris guttatus) has
figured this out.
  http://www.sciencemag.org/content/348/6236/786

Abstract:

> 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.

Mammals and birds have similar structures, applied to
extremities (not lungs), allowing the extremities to
get cold while the core temperature remains high.
  http://www.newscientist.com/blog/lastword/1997/02/why-dont-penguins-feet-freeze.html