Resolving taxonomic and nomenclatural problems in the genus Caligus O. F. Müller, 1785 (Copepoda: Caligidae) Author Boxshall, Geoffrey A. Department of Life Sciences, The Natural History Museum, Cromwell Road, London SW 7 5 BD, UK Author Bernot, James P. Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington DC, 20560, USA Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269, USA text Zootaxa 2023 2023-10-30 5360 4 545 567 https://www.mapress.com/zt/article/download/zootaxa.5360.4.5/52133 journal article 10.11646/zootaxa.5360.4.5 1175-5326 10084656 EA1BE6F9-88E2-4357-895E-8ED415206592 Caligus hyalinae Heegaard, 1966 and Caligus chelifer Wilson, 1905 Caligus hyalinae was originally described from material found in plankton samples. Heegaard’s (1966) material came from plankton tows taken in the Gulf of Mexico off Rockport, Texas . Both sexes were found but Heegaard (1966) noted that the genital complexes of the females did not contain eggs and considered that the very slender shape of the female genital complex may change considerably in “mature” females. The males were adult as indicated by the possession of secondary sexual characters, such as the large myxal process on the maxilliped. Unfortunately, Heegaard’s description lacks certain important details and is almost certainly inaccurate as C. hyalinae apparently exhibits multiple extremely unlikely character states, such as the possession of only 1 inner seta on the second endopodal segment of leg 2, the lack of both outer margin spines on the third exopodal segment of leg 2, and the lack of outer margin spines on the second and third exopodal segments of leg 3. Given this level of inaccuracy, it is necessary to interpret this description with caution. Caligus hyalinae is very similar in gross morphology to C. chelifer and the type and only locality of C. hyalinae falls within the known distribution range of C. chelifer in the western Atlantic. Comparison of C. hyalinae with more recent redescriptions of C. chelifer , such as that of Kabata (1972) , reveals numerous similarities between these two species. The overall body shape is very similar in both species although the free abdomen in both sexes shows a trace of a subdivision in Kabata’s figures of C. chelifer whereas no such subdivision is indicated in Heegaard’s (1966) figures of C. hyalinae . The caudal rami are elongate in both species. Both species show an unusual configuration of the armature elements on the distal exopodal segment of leg 1, namely, spines 1 to 3 are all subequal in size, seta 4 is more than twice as long as the spines and is longer than the segment, and the 3 plumose setae on the posterior margin are all reduced (i.e. are shorter than the segment). The sternal furca has weakly divergent tines with rounded tips in both species. The maxilliped of the male carries an unusually long, slender myxal process with a divided tip opposing the subchela in both species. This is a robust and rare character state shared by the males of C. hyalinae and C. chelifer . There are differences between the descriptions; namely, the exopod of leg 4 is shown as 3-segmented by Heegaard (1966) but as only 2-segmented by Kabata (1972) , and the myxal process on the female maxilliped is not shown in the in situ figure of Heegaard (1966) . But, given the numerous inaccuracies in the description of C. hyalinae pointed out above, the similarities between these two species provide sufficient evidence for us to propose to recognise Caligus hyalinae Heegaard, 1966 as a junior subjective synonym of Caligus chelifer Wilson, 1905 . Caligus hyalinae was proposed as a possible member of the Caligus undulatus -species group by Ohtsuka et al. (2020) . We concur and consider its senior subjective synonym, C. chelifer , to be a member of the group.