Constant temperature and fluctuating temperature have distinct effects on hypoxia tolerance in killifish (Fundulus heteroclitus).

Climate change is leading to rapid change in aquatic environments, increasing the mean and variability of temperatures, and increasing the incidence of hypoxia. We investigated how acclimation to constant temperatures or to diel temperature fluctuations affects hypoxia tolerance in mummichog killifish (Fundulus heteroclitus). Killifish were acclimated to constant cool (15°C), constant warm (25°C) or a diel temperature cycle (15°C at night, 25°C during day) for 6 weeks. We then measured hypoxia tolerance (time to loss of equilibrium in severe hypoxia, tLOE; critical O2 tension, Pcrit), whole-animal metabolism, gill morphology, haematology and tissue metabolites at 15°C and 25°C in a full factorial design. Among constant temperature groups, tLOE was highest and Pcrit was lowest in fish tested at their acclimation temperature. Warm-acclimated fish had lower metabolic rate at 25°C and greater gill surface area (less coverage of lamellae by interlamellar cell mass, ILCM), but cool-acclimated fish had greater brain glycogen stores. Therefore, effects of constant temperature acclimation on hypoxia tolerance were temperature specific and not exhibited broadly across test temperatures, and they were associated with different underlying mechanisms. Hypoxia tolerance was less sensitive to test temperature in fish acclimated to fluctuating temperatures compared with fish acclimated to constant temperature. Acclimation to fluctuating temperatures also increased haemoglobin-O2 affinity of the blood (decreased P50) compared with constant temperature groups. Therefore, acclimation to fluctuating temperatures helps maintain hypoxia tolerance across a broader range of temperatures, and leads to some distinct physiological adjustments that are not exhibited by fish acclimated to constant temperatures.

Novel spikey ionocytes are regulated by cortisol in the skin of an amphibious fish.

Cortisol is a major osmoregulatory hormone in fishes. Cortisol acts upon the gills, the primary site of ionoregulation, through modifications to specialized ion-transporting cells called ionocytes. We tested the hypothesis that cortisol also acts as a major regulator of skin ionocyte remodelling in the amphibious mangrove rivulus (Kryptolebias marmoratus) when gill function ceases during the water-to-land transition. When out of water, K. marmoratus demonstrated a robust cortisol response, which was linked with the remodelling of skin ionocytes to increase cell cross-sectional area and Na+-K+-ATPase (NKA) content, but not when cortisol synthesis was chemically inhibited by metyrapone. Additionally, we discovered a novel morphology of skin-specific ionocyte that are spikey with multiple cell processes. Spikey ionocytes increased in density, cell cross-sectional area and NKA content during air exposure, but not in metyrapone-treated fish. Our findings demonstrate that skin ionocyte remodelling during the water-to-land transition in amphibious fish is regulated by cortisol, the same hormone that regulates gill ionocyte remodelling in salinity-challenged teleosts, suggesting conserved hormonal function across diverse environmental disturbances and organs in fishes.