Jellyfish distribute vertically according to irradiance

We tested the hypothesis that the coronate jellyfish Periphylla periphylla distributes vertically according to a preferential range of absolute light intensities. The study was carried out in Lurefjorden, Norway, a fjord characterized by mass occurrences of this jellyfish. We collected data on the vertical distribution of P. periphylla medusa during day, dusk and night periods from video observations by a remotely operated vehicle in relation to estimated ambient light levels. Our results suggest that large P. periphylla (average size in catches ~9 cm diameter) avoided total irradiance levels above 5×10(−3) µmol quanta m(−2) s(−1). Nearly two-thirds of the population stayed above irradiance of 10(−7) µmol quanta m(−2) s(−1) during daytime, while some individuals occupied much darker water. Thus, part of the population appeared to distribute vertically and undertake diel vertical migration (DVM) according to a preferential range of light intensities.