Navigation in the dark: early behavioural adaptation of Europe's only native cave fish

This study analyses behavioural adaptations in a recently discovered European cave fish. Navigation in standardized labyrinth conditions was compared between this cave fish, its epigean ancestor, the European stone loach, Barbatula barbatula, and hybrid specimens. Loach behaviour was tracked and recorded using an infrared camera. In open field tests, in both dark and light conditions, cave loaches showed reduced thigmotaxis compared to the other two groups. Hybrid loaches generally showed strong avoidance of the open field, whereas epigean loaches avoided the open field more in light than in dark conditions. Other tests were performed in darkness only. Cave loaches showed more consistent lateralization in turning behaviour during exploration of the Y-maze than the other two groups, indicating a search strategy optimized for nonvisual navigation. In all groups, complex maze learning was evident in the first half of the 15 trials, with a reduction in latency of departure from the starting chamber, and in both time and track length to reach the food chamber. However, cave loaches were far more successful in finding food than were epigean loaches and hybrid loaches. When the path to the food chamber was altered, epigean loaches generally responded by changing their route, whereas cave loaches tended to continue following a previously learned path and hybrid loaches exhibited intermediate behaviours. In conclusion, the enhanced ability of cave loaches to navigating labyrinths in the dark compared to their epigean ancestor suggests the evolution of behavioural traits that optimize survival in caves.