Trophic relationships of the freshwater jellyfish Craspedacusta sowerbyi Lankester 1880

Abstract

The medusae of the hydrozoan Craspedacusta sowerbyi appear in September in a variety of ponds and lakes in Wisconsin. Short term laboratory feeding trials in about 1 liter of water indicate that the medusae (11.6‐mm diam) consume zooplankton ranging in size from about 0.2 to 2.0 mm and kill but do not eat nekton up to 8.8 mm long. Clearance rate coefficients are as high as 0.9 liter·medusa−1·d−1 for ingested prey and up to 64 liters·medusa−1·d−1 for larger nekton which were killed but not ingested. Neither invertebrate planktonic predators nor fish eat the medusae. Crayfish readily eat medusae which are resting on the bottom of an aquarium. The medusae probably do not reduce zooplankton stocks enough to compete with fish. If Craspedacusta has an effect on zookplankton stocks, it is by reducing the density of other invertebrate predators such as the rotifer Asplanchna.