Sudden phototactic reversal induced by hypochlorous acid exposure in the bioluminescent ostracod crustacean, Vargula hilgendorfii.

The reversal of phototaxis has been observed in a wide range of animal species. However, environmental chemicals that can cause a quick reversal of phototaxis have rarely been reported. Here we identified hypochlorous acid (HClO) as an inducer of phototactic reversal in Vargula hilgendorfii , also known as sea fireflies. This species shows innate photophobic swimming behavior away from light sources and 0.1% HClO triggers rapid positive phototactic responses, that is reversible upon washout. Phototactic assays using monochromatic light irradiation revealed the highest photophobic responses at 460 nm, while the HClO-induced photophilic responses were observed widely, even in UV light.