TROPHODYNAMICS OF THE 2 SCYPHOZOAN JELLYFISHES, AURELIA-AURITA AND CYANEA-CAPILLATA, IN WESTERN NORWAY

Predation of two species of semaeostome scyphomedusae, Aurelia aurita and Cyanea capillata, which regularly occur in high abundance during t he summer in coastal waters of the north-east Atlantic, was experiment ally studied under controlled laboratory conditions during two consecu tive years. Ephyra larvae of A. aurita showed a linear increase in pre dation rate over the tested range in prey concentration (Artemia naupl ii) of 15-200 prey l(-1). The results indicate that the ephyra larvae can fully utilize periods and/or patches of high prey abundance, and h igh prey abundance can support a daily food ration (DR) of more than 1 00%. For A. aurita medusae, predation is facilitated by a large prey s ize, with low DR for small zooplankton, maximum DR, exceeding 250%, fo r big zooplankton and somewhat lower DR for small fish. C. capillata m edusae, 40-120 mm bell diameter, show a functional predation response on zooplankton prey abundance, with an initial linear increase up to 2 5-50 prey l(-1) and maximum predation rate positively related to the s ize of the medusa. A high predation rate on mixed zooplankton (ca. 200 prey h(-1)) is sustained by medium-sized medusae at least over 4 days , although comparative predation experiments with several size fractio ns of zooplankton, three size classes of small fish, and different siz es of A. aurita as food, indicate that mixed zooplankton is not the op timum prey type for C. capillata. Both species of medusa can catch and ingest fish larvae and small fish, C. capillata somewhat more efficie ntly than A, aurita; both species also cause a considerable mortality to encountered fish without ingesting them. C. capillata can catch and ingest A. aurita in impressively high quantities and this predator-pr ey relationship invites a speculative hypothesis of population control of the latter species by the former.