Launches, squiggles and pounces, oh my! The water-land transition in mangrove rivulus (Kryptolebias marmoratus)

Mangrove rivulus (Kryptolebias marmoratus) are small fusiform teleosts (Cyprinodontiformes) with the ability to locomote on land, despite lacking apparent morphological adaptations for terrestrial movement. Rivulus will leave their aquatic habitat for moist, terrestrial environments when water conditions are poor, or, as we show here, to capture terrestrial insects. Specimens were conditioned to eat pinhead crickets on one side of their aquaria. After 2 weeks of conditioning, a barrier with a slope of 15 deg was partially submerged in the middle of the tank, forcing the fish to transition from water to land and back into water in order to feed. Kinematics during the transition were recorded using Fastec high-speed video cameras (125-250 frames s(-1)). Videos were analyzed using Didge and ImageJ software programs. Transition behaviors were characterized and analyzed according to their specific type. Body oscillation amplitude and wave duration were quantified for movements along the substrate, along with initial velocity for launching behaviors. Kryptolebias marmoratus used a diverse suite of behaviors to transition from water to land. These behaviors can be categorized as launches, squiggles and pounces. Prey were captured terrestrially and brought underwater for consumption. Kryptolebias marmoratus's suite of behaviors represents a novel solution to non-tetrapodal terrestrial transition, which suggests that fishes may have been able to exploit land habitats transiently, without leaving any apparent evidence in the fossil record.