Caligus mebachii Marukawa, 1927

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 : 560-561

Caligus mebachii Marukawa, 1927

Marukawa (1927) established C. mebachii based on material collected from Thunnus obesus (Lowe, 1839) (as “mebachii ” in Japanese) caught in neritic waters of Japan ( Nagasawa et al., 2018). The original description was an entry in the Encyclopedia of the Fauna of Japan and was very brief. This species has not been reported since, although the original description was reprinted in subsequent editions up to and including the final 1947 revised and enlarged edition ( Marukawa, 1947) and again in the 1956 abridged version for students. According to Damkaer & Onbé (2018), Marukawa’s section on copepods was unchanged through all these editions, still reporting the new species of Caligus . It was included in the catalogue of Caligus species ( Margolis et al., 1975) and was listed by Nagasawa et al. (2010) in their annotated list of Caligus species recorded from Japan and by Nagasawa et al. (2018) in their list of parasitic copepods from tuna species.

Translated from the original Japanese, the original description states: “ Body flattened, translucent tinged with yellowish. Body length: female 6-7 mm; male 4-5 mm. Cephalothorax oval, separated from third [= fourth] pediger. Fourth pediger wider than long and reduced, with leg 4, located in front of genital complex. Genital complex of female nearly quadrate, gonopore located anteriorly [dubious character state]; that of male widest at mid-length, slightly longer than wide, having four spiniform elements on each side. Abdomen two-segmented in both sexes; anal somite produced posteriorly. Caudal ramus nearly quadrate, with three plumose setae and one outer spine which is longer in male than in female.” The descriptive text is accompanied by two illustrations ( Fig. 7A, B ), both dorsal habitus views, presumably of the male and female as identified by Marukawa (1927). We consider that these two specimens are not conspecific. The smaller specimen ( Fig. 7A ) (presumably the male of Marukawa) exhibits an indentation in the lateral margin of the dorsal cephalothoracic shield that is characteristic of species of Euryphorus Milne Edwards, 1840 . In addition, this figured specimen clearly has a biramous leg 4 and lacks lunules ( Marukawa, 1927). This specimen appears to be a young male of E. brachypterus (Gerstaecker, 1853) , a common parasite of scombrids, including Thunnus obesus , in Japanese waters ( Nagasawa et al., 2018).

In contrast the larger specimen ( Fig. 7B ) has paired lunules and a 4-segmented uniramous leg 4 and clearly belongs to the genus Caligus . Although Marukawa (1927) identified this specimen as the “female”, the internal structures figured within its genital complex suggest that it is a male containing developing spermatophores. The shape and general proportions of the genital complex and the short, clearly 2-segmented abdomen are also in accord with those of a male. The figure appears to show that the first exopodal segment of leg 3 is armed with a spine which is directed laterally and does not lie across the surface of the ramus as is typical for Caligus species. The unusual orientation of this spine is typical of C. coryphaenae (e.g., Ho & Lin, 2004: Fig. 81), also a common parasite of scombrids in Japanese waters, including Thunnus obesus ( Nagasawa et al., 2018) . The body length for this specimen given by Marukawa (1927) was 6–7 mm and overlaps with the known body length of male C. coryphaenae , given as 4.5 to 6.0 mm by Kabata (1979). The configuration of the setae on the caudal ramus of male C. coryphaenae is also unusual ( Fig. 7C, D ). There are 6 caudal setae, as typical for the genus; 3 long pinnate setae distally, 1 short inner seta and 2 short outer setae. The innermost long pinnate seta is located ventral to the other setae and is ornamented with two rows of pinnules, the pinnules along the inner margin of the seta are particularly long and densely set. The pinnule row along the outer margin does not lie in the same plane as the inner row and is set at about 150° relative to the inner row so it projects ventrolaterally rather than laterally. To the best of our knowledge, the broad extent of the pinnules on this inner distal seta is unique to male C. coryphaenae and this arrangement is visible in Marukawa’s (1927) illustration of the “female” (cf. Fig. 7B ). However, Marukawa’s (1927) figure includes the outlines of two elongate structures originating on the genital complex which we presume represent egg sacs. We suspect that these structures, which are not the correct proportions for egg sacs, were added to the drawing perhaps to indicate the presumed sex of the specimen. We, therefore, tentatively conclude that Marukawa’s “female” is a male of C. coryphaenae which was mistakenly identified as the female.

The type material of C. mebachii cannot be traced and without it we cannot resolve the confusion surrounding the sex of the so-called “female”. The two specimens illustrated in Marukawa (1927) are effectively syntypes and we here designate the male ( Fig. 7A ) as the lectotype. Subsequent designation of a lectotype by means of an illustration is permitted under Article 74.4. of the International Code of Zoological Nomenclature ( ICZN, 1999), even when the specimens can no longer be traced. The newly designated lectotype of C. mebachii is identifiable as a young male of Euryphorus brachypterus and we formally propose to treat Caligus mebachii Marukawa, 1927 as a junior subjective synonym of Euryphorus brachypterus (Gerstaecker, 1853) .

Figures

FIGURE 7. A, B, Dorsal habitus views of male (A) and “female” (B) Caligus mebachii Marukawa, 1927, illustration from the 1956 abridged edition of Marukawa (1947) [Copyright expired in 2008]; C, Anal somite and caudal rami of male Caligus coryphaenae Steenstrup & Lütken, 1861, dorsal view using CLSM; D, Same, using light microscopy.