Morphological and Physiological Correlates of Evolutionary Reduction of Metabolic Rate among Amblyopsid Cave Fishes

Abstract

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 μ 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.