Terrestrial emergence reflects lower thermal preferences in the mangrove rivulus (Kryptolebias marmoratus)

Many fishes exploit the terrestrial environment. Some of these amphibious species use it as an ecological release, others for feeding, while others use land to escape aquatic stressors. Another hypothesis is that amphibious fishes leave water to thermoregulate, but there is little corroborating data. One of the most intriguing fish that emerges onto land is the mangrove rivulus (Kryptolebias marmoratus), which can survive on land for up to 75 days. The mangrove rivulus emerges to escape from warm water and initially benefits from enhanced evaporative cooling on land. However, emergence imposes an immediate hypoxaemia due to reduced gas exchange over the gills and an accumulation of CO2 that impairs haemoglobin-O2 binding. In other ectothermic species, hypoxia is known to decrease thermal preference. Before physiological acclimation restores respiratory function, it is therefore plausible that fish that have emerged onto land will seek cooler temperatures beyond what is achieved by simple evaporative cooling. To test this idea, two strains of laboratory-raised mangrove rivulus were placed in a thermal gradient that provided the option of land and water to determine how their preferred temperature was informed by choice of the surrounding environment. We hypothesized that selecting land would be associated with anapyrexia, or a purposeful selection of lower temperatures. In trials where thermal gradients were provided in aquatic, terrestrial or combined settings, mangrove rivulus selected cooler temperatures only when on land. These findings support the premise that terrestrial emergence serves an active thermoregulatory strategy.