Trophodynamics of the two scyphozoan jellyfishes, Aurelia aurita and Cyanea capillata, in western Norway

Abstract

Predation of two species of semaeostome scyphomedusae, Aurelia aurita and Cyanea capillata, which regularly occur in high abundance during the summer in coastal waters of the north-east Atlantic, was experimentally studied under controlled laboratory conditions during two consecutive years. Ephyra larvae of A. aurita showed a linear increase in predation rate over the tested range in prey concentration (Artemia nauplii) 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 high prey abundance can support a daily food ration (DR) of more than 100%. For A. aurita medusae, predation is facilitated by a large prey size, with low DR for small zooplankton, maximum DR, exceeding 250%, for big zooplankton and somewhat lower DR for small fish C. capillata medusae, 40–120 mm bell diameter, show a functional predation response on zooplankton prey abundance, with an initial linear increase up to 25–50 prey l⁻¹ and maximum predation rate positively related to the size 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 fractions of zooplankton, three size classes of small fish, and different sizes of A. aurita as food, indicate that mixed zooplankton is not the optimum prey type for C. capillata. Both species of medusa can catch and ingest fish larvae and small fish, C. capillata somewhat more efficiently 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-prey relationship invites a speculative hypothesis of population control of the latter species by the former.