Taylor, E. W., S. Egginton, S. E. Taylor and P. J. Butler (1997).  Factors which 
may limit swimming performance at different temperatures. Cambridge University 
Press, Cambridge (Uk).
	
As thermal diffusion is an order of magnitude more rapid than molecular 
diffusion, it is clear that the same design features that make the gills of fish 
well suited for respiratory gas exchange from water (large surface area, active 
convection of water and blood at appropriate ventilation/perfusion ratios across 
a functional counter-current) also provide for very effective branchial heat 
exchange. This is reinforced by the relatively high heat capacity of water which 
is more than 3000 times that of air, so that for most fishes, and indeed all 
other water-breathing ectotherms, body temperature equilibrates rapidly to any 
change in environmental temperature. Consequently, in the absence of specific 
anatomical specialization to maintain thermal gradients, temperature throughout 
the body of fishes is in equilibrium with the environment to within a fraction 
of a degree. Thus, large changes in body temperature may be experienced: 
diurnally, by coastal fish subjected to tidal variations; or by vertically 
migrating pelagic species, particularly if they cross a thermocline; or 
seasonally by eurythermal temperate zone fish. Over evolutionary time, 
speciation of tropical and polar fishes has resulted in species with widely 
different thermal ranges within the accepted biological temperature range 
(between the freezing point of water and the temperature for protein 
denaturation), which do not overlap.