Aspects of the Physiology of Terrestrial Life in Amphibious Fishes

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Abstract
Aspects of the physiological adaptations for terrestrial life possessed by the Chinese mudskipper fish, Periophthalmus cantonensis have been studied. The basic amphibious features were comparable to those found earlier in the East African mudskipper species, Periophthalmus sobrinus. The fish can survive for up to days out of water, under moderate environmental conditions, and did not drown in aerated water. Durations of voluntary periods out of contact with water, in nature, were short. Evaporative water loss rates while out of water were relatively low, substantially below those of frogs of much larger size. The ‘diving syndrome’ was absent, metabolic rates, heart rates, and blood lactic acid levels all being unaffected by shifts of fish between water and air. During slow desiccation of intact fish, water was lost from the internal organs in the following sequence (from greatest to least proportional water losses): heart, blood, white muscle, brain, liver. Rapid desiccations withdrew water from the blood and heart, leaving hydration levels of other organs unaffected. Metabolic rates were only moderately sensitive to temperature in the range 10° – 20 °C (Q10 ≅ 1·5), but were highly temperature sensitive in the range 20° – 30 °C (Q10 ≅ 2·6). Measurements of routine metabolic rates of fish in water indicate that active metabolism at 20 °C can reach at least 5 times the standard rate. Measurements of blood levels of ammonia and urea, and of ammonia and urea excretion rates, showed that starvation for 9·5 days did not affect blood levels of these compounds, but did reduce urea excretion rates by 40% below rates for recently fed fish. Ammonia excretion was unaffected. Similar measurements on fish starved for 9·5 days, then kept out of water for 16·5 h, then returned to water, indicated that waste nitrogen produced by fish out of water was retained in their bodies until they returned to water. Fish out of water produced waste nitrogen 40% less rapidly than fish in water, and strongly shifted toward ureotelism. These experiments, however, did not permit fish to retain and renew water in their mouths, as they would have done in nature. The water in mudskipper burrows was found to be anoxic or nearly so. Mudskipper tolerance for hypoxia was, however, limited. Fish became inert at dissolved oxygen levels of about 20% saturation.