Air Breathing and Gill Ventilation Frequencies in Juvenile Tarpon, Megalops atlanticus: Responses to Changes in Dissolved Oxygen, Temperature, Hydrogen Sulfide, and PH

Abstract

This study quantified the air-breathing frequency (ABf in breaths h⁻¹) and gill ventilation frequency (Vf in ventilations min⁻¹) of tarpon Megalops atlanticusas a function of PO₂, temperature, pH, and sulphide concentration. Ten tarpon held at normoxia at 22–33°C without access to atmospheric oxygen survived for eight days, and seven survived for 14 days (at which point the experiment was terminated) suggesting that the species is a facultative, rather than an obligate, air breather. At temperatures of 29°C and below ABf was highest and Vf was lowest at low oxygen partial pressures. Tarpon appear to switch from aquatic respiration to air breathing at PO₂levels of roughly 40 torr. The gills were the primary organ for oxygen uptake in normoxia, and the air-breathing organ the primary mechanism for oxygen uptake in hypoxia. At 33°C, both ABf and Vf were elevated but highly variable, regardless of PO₂. There were no mortalities in tarpon exposed to total H₂S concentrations of 0–232 µM (0–150.9 µM H₂S); however, high sulfide concentrations resulted in very high ABf and Vf near zero. Vf was reduced when pH was acidic. We conclude that air breathing provides an effective means of coping with the environmental conditions that characterize the eutrophic ponds and sloughs that juvenile tarpon typically inhabit.