Ontogeny of phototaxis in early life stages of mahi-mahi (Coryphaena hippurus)

Larval fish orientation remains a poorly understood mechanism. It is critical for studying larval dispersal but has, surprisingly, never been tested in any pelagic species. This laboratory experiment investigates the ability of Coryphaena hippurus L. larvae, hereafter ‘mahi-mahi’, to orient in relation to a sun-like light source through ontogeny. Larvae were tested using the Drifting In Situ Chamber (DISC), which was initially developed to tackle larval fish orientation in situ. It is equipped, among others, with a GoPro that captures the position of the larvae every second during a 10-minute observation period, and three compasses to track their directionality. Here, the DISC was adapted to conduct laboratory in a large (3 m3) circular aquarium filled with seawater. As such, the stimulus perceived by the larvae could be thoroughly controlled and the larvae’s behavioral response identified at both the individual, age and population levels. A total of 82 individual larvae, ranging from 7 days to 23 days post-hatching, were exposed to a directional and alternating (by 180°) light, meant to mimic the sun’s changing position in the sky. The results show that larvae respond to the light over any other directional stimuli and that this ability is absent until the flexion stage, and prominent afterwards. This strong phototaxism implies that early in their development, mahi-mahi larvae may be able to use the sun as a compass to orient themselves in the open ocean, providing them with an efficient mechanism to optimize foraging.