Responses to Light In Two Eyeless Cave Dwelling Amphipods (Niphargus ictus and Niphargus frasassianus)

Two amphipod species, Niphargus ictus and N. frasassianus, which are endemic to Frasassi Caves, Italy, possess morphological features typical of most troglobitic species. Most notably they lack eyes. Although cave waters flow directly into the adjoining Sentino River and N. frasassianus is found within two meters of the resurgence, neither species is present at or further outside the mouth of the cave, which raises the question of how they avoid leaving the cave. It was hypothesized that these animals might be able to detect light, and could use light cues to remain inside the cave. Individuals of both species exhibited greater activity levels in the presence of bright vs. low light levels. Neither species exhibited the dorsal light reflex, but both showed weak negative phototaxis when exposed to bright light directed at them from above and below. N. frasassianus, tested in an apparatus that permitted them to travel freely between bright or low lighted areas, demonstrated negative phototaxis. The results show that both species can detect light, and suggest light cues may be utilized to remain in the caves.