Caligus hamatus Heegaard, 1955

Boxshall, Geoffrey A. & Bernot, James P., 2023, Resolving taxonomic and nomenclatural problems in the genus Caligus O. F. Müller, 1785 (Copepoda: Caligidae), Zootaxa 5360 (4), pp. 545-567 : 561-562

Caligus hamatus Heegaard, 1955

Caligus hamatus is known from a single male collected in a plankton sample taken at a depth of 20 m in the surface layers of the Bonny River, an arm of the Niger River delta in southern Nigeria, during the Atlantide Expedition ( Heegaard, 1955). It has never been reported since and no host association has ever been reported for C. hamatus . The whereabouts of the type specimen is unknown ( Margolis et al., 1975) but, fortunately, Heegaard’s (1955) description, although inadequate by modern standards, contains sufficient detail to allow comparison with related congeneric species, specifically with Caligus undulatus Shen & Li, 1959 .

Caligus undulatus was also originally described from zooplankton samples, collected in Qingdao (as Tsingtao) Harbour on the eastern coast of China, and the type material comprised numerous ovigerous females and males ( Shen & Li, 1959). Since its description, C. undulatus has been reported widely from across the Indo-West Pacific ( Pillai, 1966; Venmathi Maran & Ohtsuka, 2008; Venmathi Maran et al., 2012a, b; Venmathi Maran et al., 2016; Moon & Park, 2019) and western Atlantic ( Montú, 1982; Suárez-Morales et al., 2012a, b; Ortega et al. 2017; Kim et al., 2019). All of these records were based on specimens caught in the plankton and it wasn’t until 2020 that this caligid was finally found on a host, the clupeid Sardinella zunasi (Bleeker, 1854) caught in Japanese waters ( Ohtsuka et al., 2020). It has since been recorded on a second clupeid host, Konosirus punctatus (Temminck & Schlegel, 1846) landed at a fish market in southern Korea ( Lee et al., 2022).

Comparison of C. hamatus with C. undulatus reveals that the males of these two species share numerous character states including: broad marginal membrane surrounding the dorsal cephalothoracic shield; first and second free abdominal somites both about twice as long as wide and with the first slightly shorter than the second; male maxilliped with prominent myxal process opposing tip of subchela; sternal furca with parallel to weakly divergent tines separated by narrow gap; second exopodal segment of leg 1 armed with plumose setae on posterior margin that are almost as long as first exopodal segment, and with seta 4 much longer than second segment; leg 2 with outer spine on first exopodal segment passing obliquely across surface of ramus, with outer spine on second segment much smaller and lying close to lateral margin, and with at least one outer spine on third segment apparently missing; leg 3 with weakly curved outer spine on first exopodal segment not reaching articulation between second and third segments; leg 4 with 2-segmented exopod bearing 4 subequal spines on distal segment. On the basis of these similarities in fine details, and in the absence of any significant morphological differences, we conclude that C. hamatus and C. undulatus are conspecific. Unfortunately, the widely used species name C. undulatus was established after C. hamatus , which therefore has priority. However, a case has been submitted to the ICZN to propose that Caligus undulatus Shen & Li, 1959 be given precedence over the virtually unused Caligus hamatus Heegaard, 1955 .