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Abstract Background Species belonging to the genus Lernaea are cosmopolitan parasites that can infect many different freshwater fish hosts. Due to a high degree of morphological intraspecific variability and high levels of interspecific similarities, their classification is extremely difficult and controversial. Although the suitability of the shape of cephalic horns has been questioned decades ago by some experimental infection studies, this character still plays the central role in the identification of Lernaea spp. Methods We used the nominal species Lernaea cyprinacea and Lernaea cruciata to test the hypothesis that the shape of the anchor can exhibit host-induced morphological variability, and that the two taxa may be synonymous. Results We examined 517 wild or farmed specimens of five host fish species (four cyprinids and a mosquitofish), and found that all 16 parasite specimens collected from mosquitofish could be morphologically identified as L. cruciata, whereas the remaining 25 parasite specimens were all identified as L. cyprinacea. We experimentally infected goldfish and mosquitofish specimens with offspring (copepodids) of a single L. cyprinacea specimen: the adult parasites from goldfish were morphologically identified as L. cyprinacea, and those from mosquitofish as L. cruciata. We then used molecular data to corroborate that all these specimens are conspecific. Conclusions Our results suggest that L. cyprinacea and L. cruciata may be synonyms, misidentified as different species as a result of host-induced morphological variation. Given the current shortage of molecular data for the genus Lernaea, in order to resolve the taxonomy of this genus (determine the exact number of species), future studies should aim to sequence as much molecular data as possible, and conduct further experimental infections.

Abstract Background Isopoda is a highly diverse order of crustaceans with more than 10,300 species, many of which are parasitic. Taxonomy and phylogeny within the order, especially those of the suborder Cymothoida Wägele, 1989, are still debated. Mitochondrial (mt) genomes are a useful tool for phylogenetic studies, but their availability for isopods is very limited. To explore these phylogenetic controversies on the mt genomic level and study the mt genome evolution in Isopoda, we sequenced mt genomes of two parasitic isopods, Tachaea chinensis Thielemann, 1910 and Ichthyoxenos japonensis Richardson, 1913, belonging to the suborder Cymothoida, and conducted comparative and phylogenetic mt genomic analyses across Isopoda. Results The complete mt genomes of T. chinensis and I. japonensis were 14,616 bp and 15,440 bp in size, respectively, with the A+T content higher than in other isopods (72.7 and 72.8%, respectively). Both genomes code for 13 protein-coding genes, 21 transfer RNA genes (tRNAs), 2 ribosomal RNA genes (rRNAs), and possess a control region (CR). Both are missing a gene from the complete tRNA set: T. chinensis lacks trnS1 and I. japonensis lacks trnI. Both possess unique gene orders among isopods. Within the CR of I. japonensis (284 bp), we identified a repetitive region with four tandem repeats. Phylogenetic analysis based on concatenated nucleotide sequences of 13 protein-coding genes showed that the two parasitic cymothoids clustered together and formed a basal clade within Isopoda. However, another parasitic cymothoid, Gyge ovalis Shiino, 1939, formed a sister group with the suborder Limnoriidea Brandt & Poore in Poore, 2002, whereas two free-living cymothoid species were located in the derived part of the phylogram: Bathynomus sp. formed a sister group with the suborder Sphaeromatidea Wägele, 1989, and Eurydice pulchra Leach, 1815 with a clade including Bathynomus sp., Sphaeromatidea and Valvifera G. O. Sars, 1883. Conclusions Our results did not recover the suborders Cymothoida and Oniscidea Latreille, 1802 as monophyletic, with parasitic and free-living cymothoidans forming separate clades. Furthermore, two parasitic cymothoidans formed the sister-clade to all other isopods, separated from Epicaridea Latreille, 1825, which challenges currently prevalent isopod phylogeny. Additional mt genomes of parasitic and free-living isopods might confer a sufficient phylogenetic resolution to enable us to resolve their relationships, and ultimately allow us to better understand the evolutionary history of the entire isopod order.