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The types of parasite life cycles encountered in aquatic ecosystems vary in complexity from simple direct life cycles (a) in which transmission occurs between two definitive hosts (DH) of the same species, to more complex, indirect life cycles that involve transmission of parasites between multiple obligate intermediate hosts (IH) of different species (b–d). (This figure is available in black and white in print and in color at Integrative And Comparative Biology online).
