Numerous global reports of the species Udonella caligorum, currently thought to be a species complex, suggests that the group may be species-rich. Herein we describe Udonella fugu n. sp., previously described as U. caligorum, found on the parasitic copepod Pseudocaligus fugu infecting Takifugu spp. from Japan. Using morphological data U. fugu can be distinguished from the current valid species by at least one of the traditionally used characters in udonellid taxonomy, and phylogenetic analyses of ssrDNA sequence data for U. fugu and other udonellids confirm that U. fugu forms a distinct clade from other udonellids including U. caligorum. Variable regions in the ssrDNA demonstrated a range of between 2.75 and 5.5% difference between currently recognized species of Udonella. These differences in ssrDNA sequences are phylogenetically useful when distinguishing between morphologically similar udonellids and can be used in conjunction with other data (morphology, phylogeography and fish host) to help clarify udonellid systematics. Udonella fugu was also found to cause significant damage to farmed tiger puffers through their feeding activities. Individual skin lesions were round in shape but merged with adjoining lesions to form more extensive lacerations. In some of the specimens from P. fugu infecting Takifugu niphobles, the protozoan ciliate Trichodina was found on the udonellid body surface and in their intestinal contents. We conclude that the udonellids are a more species-rich group than currently recognized, that early descriptions of new species may have been synonymized with U. caligorum in error and that the frequent global reports of U. caligorum may actually represent new species. This has led to a wide range of morphological descriptions for U. caligorum, blurring the usefulness of morphological data for the group.
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