On the ciliate fauna associated with the marine species of the amphipod genusGammarusJ. G. fabricius

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Abstract
The present paper describes the ciliate epifauna found on the marine species of the amphipod genus Gammarus in Scandinavian waters. Twentyfive species of ciliates were found which, ecologically, could be placed in the following four groups: Suspension feeders, herbivores, carnivores, and parasites. It is shown that the forming of swarmers in sessile commensals (peritrichs and chonotrichs) is correlated with the moulting of the host. While most of the epizoans found on Ganunarus are specific for the genus, the distribution of the ciliates within the host genus is controlled mainly by environmental factors. especially salinity. The marine and brackish water Gammarus species have no ciliates in common with the fresh-water species G. lacustris. Even G. duebeni, when living in freshwater, has an impoverished marine fauna of ciliates. Brackish water populations of Gammarus species were found to harbour the greatest number of ciliate species. This is in contrast to the general concept that a smaller number of species is found in brackish water than in similar localities in the sea and in fresh-water. This is explained by the fact that Gammarus forms much denser populations and has a much wider distribution in brackish water than in marine environments.