Norwegian fjords as production sites for the coronate deep-water medusa,
Periphylla periphylla

Periphylla periphylla has a global distribution, usually confined to deep water and with a low abundance, seldom reported higher than one per 1,000 m³. However, in Lurefjorden, western Norway, anecdotal stories by fishermen describe mass occurrence of this species causing problems for the fisheries. The current studies were initiated in 1991 and have given new information about the life cycle, nutritional ecology, bioluminescence physiology and chemistry, and population dynamics. Lurefjorden is hydrographically an untypical fjord, by having a very narrow (200 m) and shallow (20 m) sill entrance, which restricts Atlantic water from filling up the 450-meter deep basin. There is neither any main freshwater supply from rivers, thereby reducing the fjord circulation.
P. periphylla has a true direct development, representing the only known scyphozoan species of that kind. Eggs are released in deep water and the embryo is gradually developed in the deep water without feeding for several months. Our results indicate an exceptionally high potential age (exceeding 10 years), abundance of the old individuals (recruited long before 1993) of 5–40 per 1,000 m³, and with the young medusae at more than 100 per 1,000 m³. However, there has been a main change in the population dynamics in recent years, with no recruitment before 1993 and continuous recruitment thereafter and signs of a population explosion.

Reasons for the high abundance of P. periphylla are suggested and problems related to quantification of gelatinous macrozooplankton are discussed. Future plans for a newly introduced deep-water ROV is also outlined.

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