MEDUSAE, SIPHONOPHORES, AND CTENOPHORES AS PLANKTIVOROUS PREDATORS INCHANGING GLOBAL ECOSYSTEMS

Medusae, siphonophores, and ctenophores are planktivorous predators op erating at higher trophic levels in marine ecosystems of a wide range of productivity. It has been hypothesized that high-productivity ecosy stems such as areas of upwelling tend towards food chains dominated by larger phytoplankton, large copepods, and ultimately many species of fish rather than gelatinous predators; ecosystems with lower productiv ity are characterized by small flagellate phytoplankton, small copepod s, and ultimately numerous medusae and ctenophores. Evidence is provid ed that medusae, siphonophores, and ctenophores are actually important predators in both sorts of planktonic ecosystems, although uneven rep orting in the literature may be cause for underestimates of the import ance of these carnivores in some systems. As world fisheries begin to experience serious declines, it is relevant to recognize that the carn ivorous ''jellyfishes'' are ubiquitous and are thus opportunistically positioned to utilize secondary production that is ordinarily consumed by fish. (C) 1995 International Council for the Exploration of the Se a