Phenotypic flexibility in respiratory traits is associated with improved aerial respiration in an amphibious fish out of water

Abstract

Amphibious fishes have evolved multiple adaptive strategies for respiring out of water, but there has been less focus on reversible plasticity. We tested the hypothesis that when amphibious fishes leave water, enhanced respiratory performance on land is the result of rapid functional phenotypic flexibility of respiratory traits. We acclimated four isogenic strains of Kryptolebias marmoratus to air for 0, 1, 3 or 7 days. We compared respiratory performance out of water with traits linked to the O 2 cascade. Aerial O 2 consumption rate was measured over a step-wise decrease in O 2 levels. There were significant differences between strains, but time out of water had the largest impact on measured parameters. Kryptolebias marmoratus had improved respiratory performance [lower aerial critical oxygen tension (P crit ), higher regulation index (RI)] after only 1 day of air exposure, and these changes were strongly associated with the change in hematocrit and dorsal cutaneous angiogenesis. Additionally, we found that 1 h of air exposure induced the expression of four angiogenesis-associated genesvegfa, angpt2, pecam-1 and efna1in the skin. After 7 days in air, respiratory traits were not significantly linked to the variation in either aerial P crit or RI. Overall, our data indicate that there are two phases involved in the enhancement of aerial respiration: an initial rapid response (1 day) and a delayed response (7 days). We found evidence for the hypothesis that respiratory performance on land in amphibious fishes is the result of rapid flexibility in both O 2 uptake and O 2 carrying capacity.Bold values denote a significant relationship between the two parameters. Day 1: change between 0 and 1 day of air exposure; Day 3: change between 0 and 3 days of air exposure; Day 7: change between 0 and 7 days of air exposure. Hct, hematocrit; n RBC , number of red blood cells.

Hypoxia-seeking behaviour, metabolic depression, and skeletal muscle function in an amphibious fish out of water

“…Furthermore, prolonged air exposure in both strains resulted in lower O 2 consumption rates compared with active fish (35% depression), which was accentuated (51% depression) when fish were exposed to aerial hypoxia acutely. The fact that fish exposed to aerial hypoxia demonstrated lower O 2 consumption rates than air-exposed fish (which were fasted and inactive) is suggestive of active metabolic rate depression, particularly because O 2 consumption measurements were obtained above P crit for K. marmoratus in air (Blanchard et al, 2019), indicating that this was not simply an oxyconforming response. We also tested the hypothesis that chronic hypoxia exposure in air would protect endogenous energy reserves and skeletal muscle integrity, thereby maintaining locomotor performance, possibly owing to hypoxic hypometabolism.…”

“…Fish were acclimated to the experimental chamber for a 2 h period in air, during which time the chamber was left open to maintain P O2 at approximately 21 kPa. A 2 h acclimation has previously been shown to be sufficient for K. marmoratus to recover from handling stress (Blanchard et al, 2019). Following the 2 h acclimation period, the chamber was sealed and the decline in P O2 was measured for 1 h using O 2 -sensing optodes, which were calibrated weekly.…”

“…The P O2 in the O 2 consumption trials performed under aerial hypoxia started at 9.3 ±0.2 kPa and declined to 8.4±0.2 kPa. The P O2 in all trials remained above the lower aerial critical oxygen tension (P crit ) for K. marmoratus strain 1 (SLC8E) and strain 2 (50.91) (Blanchard et al, 2019). All O 2 experiments were conducted at 25°C, and between 12:00 and 18:00 h to minimize the effects of diel metabolic rhythms (Rodela and Wright, 2006).…”

Prolonged survival out of water is linked to a slow pace of life in a selfing amphibious fish

“…Furthermore, the scope of metabolic depression after 21 days in air in Honduras fish was larger (58% reduction) than that of either the Belize (31% reduction) or Florida (44% reduction) strains, which probably allowed Honduras fish to conserve protein stores. Previous studies have suggested that K. marmoratus maintain or increase metabolic rate for several days after moving from water to land, but those experiments did not examine fish that were out of water for longer than 7 days (Ong et al, 2007;Blanchard et al, 2019). While probably helpful for survival during prolonged emersion, the scope of metabolic depression we found in K. marmoratus is smaller than has been measured in classically aestivating amphibious fishes such as P. aethiopicus, Synbranchus marmoratus and Lepidogalaxias salamandroides, which reduce O 2 consumption by 65-80% during months-long aestivation in mud (Guppy and Withers, 1999).…”

“…Kryptolebias marmoratus can survive more than 66 days out of water in leaf litter or packed nose-to-tail within rotting mangrove logs (Taylor et al, 2008). There is no evidence that K. marmoratus aestivates when on land (Ong et al, 2007;Blanchard et al, 2019), but they are largely inactive (Turko et al, 2014(Turko et al, , 2017 and do not eat (Pronko et al, 2013;Wells et al, 2015). We first measured energy use and oxygen uptake in isogenic strains acclimated to terrestrial conditions for 21 days.…”