Morphological and physiological correlates of evolutionary reduction of metabolic rate among amblyopsid cave fishes

Synopsis

I report on multiple correlates of reduction in metabolic rate during evolutionary adaptation to caves in the fish family Amblyopsidae. The family includes six species with one surface-dweller, Chologaster cornuta, a facultative cave dweller, Chologaster agassizi, and four obligate cave-dwellers that, on the basis of eye degeneration, have been isolated in caves for increasing times: Typhlichthys subterraneus < Amblyopsis spelaea < Amblyopsis rosae < Speoplatyrhinus poulsoni. of those traits I examined, the strongest correlates of reduction in whole fishes metabolic rate were reduction in ventilation frequency and volume > brain metabolic rate > gill surface area. Relative amplitude declined and the estimated ventilation minute-volume decreased 5.9 fold from a value of 1.12 to 0.19 ml. Log brain rate of oxygen consumption was directly related to log body mass, b = +0.75, and decreased from 1.5 to 0.06 μl per mg dry mass per hour among species. No single gill trait accounted for the reduction in total lamellar surface area from 228 to 103 mm² g^-1. There was no reduction in muscle metabolic rate or histological indices of thyroid activity. Log muscle rate of oxygen consumption was inversely related to log body mass, b = -0.60, but did not differ among species. The mean total thyroid follicle volume among species, from 0.010 to 0.020 mm³ g^-1 of adult fishes, showed no relation to interspecific differences in whole fish metabolic rate. In the general discussion I show that no trait that could contribute to a lowered metabolic rate is in the same rank order as metabolic rate, though collectively the reductions were in the same order as metabolic rates. I explain that this is as expected with convergent evolution of complex traits. Finally I discuss the literature about central nervous system control of metabolic rate.