The amphibious capacity in an annual fish

Amphibious fishes are characterized by their ability to emerge out of the water as a part of their normal life history. This enables them to occupy ecological niches unavailable to fully aquatic species. We used morphological, physiological, and behavioural analyses to investigate the amphibious capabilities of the annual killifish Millerichthys robustus, because voluntary emersions from water were frequently observed in the laboratory. We found that Millerichthys is capable of breathing atmospheric air for several days of emergence, maintaining similar or higher respiratory rates than in water. The interlamellar region of the gill tissue was remodelled in response to exposure to air, probably through hyperplasia and hypertrophy, to avoid dehydration and prevent the collapse of tissue of the gill epithelia. The reduction of water volume and intraspecific interactions trigger fish emersion. We identified and described specific behaviours that allow Millerichthys to emerge from water (caudal propulsion after intraspecific aggression, and launching during voluntary emersions) and move on land (side placement, tail-flip, squiggles, and lateral slides). We demonstrated that the hermaphrodite annual killifish M. robustus expresses a true amphibious capacity, so far unknown in any annual killifish. We discuss the adaptive potential of amphibious capacity during seasonal water loss in temporary pools of México.