Paramacrochiron thailandicum n. sp. (Copepoda: Cyclopoida: Macrochironidae) is described from the rhizostome medusa Rhopilema hispidum occurring in the Gulf of Thailand. The new species is distinguishable from other congeners mainly by detailed features of the fourth legs and the urosome in both sexes. Copepodid and adult stages of the genus Paramacrochiron seem to be host-specific to rhizostome medusae, but may spend periods free in the plankton or benthos during the absence of the host. A cladistic analysis shows that the Macrochironidae comprises two main lineages. Paramacrochiron is recovered as a monophyletic group, located on one lineage together with Pseudomacrochiron, which appears as a paraphyletic taxon with the limited character set used. The other main lineage comprises the genera Macrochiron and Sewellochiron, but the sole species of the latter genus, Sewellochiron fidens, is nested within Macrochiron. This analysis provides little support for maintaining Sewellochiron as a separate genus.
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