I’d like an alien to be warm-blooded, but have external gills. Imagine a kind of giant crab or woodlouse (pillbug) which spends part of its time on land and part of the time in the sea. However, external gills will be an enormous heat loss for a warm-blooded creature whenever it enters the water. All that surface area, shedding heat into the water like it is going out of fashion!
The normal biological solution to heat loss in an extremity is to set up a countercurrent exchange system between the outgoing and incoming blood vessels. That would work fine to save heat; chill the blood as it leaves the body for the gill, warm it up again as it comes back. Diagram of countercurrent vs normal flow
But there is one teensy, weensy problem – the countercurrent would also do the same to oxygen, and not in a good way! The outgoing blood would pinch the oxygen from the returning blood and thus no oxygen would ever get from the gills to the body. Oops.
So, can anyone suggest an alternative, biologically plausible, method of saving heat in an external gill? Otherwise my aliens will be restricted to quick dips or only going swimming in the warmest of tropical seas.