The distribution pattern and nocturnal surface behaviour of the deep-sea medusa Periphylla periphylla in a Norwegian fjord was studied. Medusa abundance, size distribution and condition were determined, using surface collections, net tows and ROV-based video profiles. Only larger, mature medusae reached the surface and aggregated into small groups of both sexes, whereas juvenile medusae remained in deeper waters. Observations on the behaviour and cytology of aggregated medusae suggested a mating strategy. We hypothesize that this behaviour is the by-product of a holopelagic life history, developed in a more oceanic deep environment with low species abundance, as surface aggregation increases the chance of encounter and mating.
