Abstract
The life cycle of Periphylla periphylla (Peron & Lesueur, 1809) is described. it is the only known direct development of a coronate scyphozoa, showing a holopelagic life cycle without any sessile stage. It also lacks an ephyra stage that normally precedes the medusa stage and it is the first scyphozoan life cycle with sexual propagation where the planula stage is missing. The morphology and histology in the early development is described and a description of eight well recognised developmental stages, covering the period from egg to young medusa, is presented. A brief comparison of the population structure in 1992 and 1997 in Lurefjorden, Norway shows that there has been a dramatic change. The population in 1992 showed a normal size distribution with a median individual wet weight of 540 g, eggs and stages did not occur in the plankton and there was no significant recruitment. Eggs and larvae were first observed in 1993 and since then they have been present in all seasons. The population therefore showed a bimodal size distribution in 1997, composed of an aid cohort with a median wet weight of 760 g and a new and nume1ically dominating cohort with a median wet weight of 40 g. The abundance and population structure of P. periphylla indicate an exceptionally high age potential for a jellyfish and very low mortality in Lurefjorden.
