Causes for mass occurrences of the jellyfish Periphylla periphylla: a hypothesis that involves optically conditioned retention

The mesopelagic coronate medusa Periphylla periphylla has been very abundant (20–320 individuals m−2), with as many as 2.5 individuals m−3, for more than a decade in certain Norwegian fjords. These abundances are two to three orders of magnitude higher than reported from open ocean environments. Comparisons of the size, density and behavior of this jellyfish in three fjords (Lurefjorden, Sognefjorden and Halsafjorden) support the hypothesis that retention of P. periphylla is related to basin topography, light attenuation and photosensitivity. Furthermore, we hypothesize that the abundances of P. periphylla in Lurefjorden and Halsafjorden, but not in Sognefjorden, have been stimulated by increased light absorbance of water masses formed in the North Sea.