Metacaligus Thomsen 1949

Özak, Argun Akif, Kurt, Tuba Terbiyik, Kamanli, Seyit Ali, Akbulut, Gürkan, Yanar, Alper & Boxshall, Geoffrey A., 2024, Two new species of planktonic Caligus O. F. Müller 1785 (Copepoda: Caligidae) from Türkiye with an updated review and checklist of planktonic caligids, Zootaxa 5437 (3), pp. 301-335 : 325-326

Validity of Metacaligus Thomsen 1949

Metacaligus was originally established by Thomsen (1949) as a subgenus of Caligus in order to accommodate a new species, Caligus (Metacaligus) uruguayensis Thomsen, 1949 , which was based on material collected from Trichiurus lepturus ( Linnaeus, 1758) caught in the estuary of the Rio de la Plata, Uruguay. His species lacked a sternal furca and Thomsen (1949) transferred three other species, Caligus rufus Wilson, 1908 , C. afurcatus Wilson, 1913 and C. enormis Wilson, 1913 , into his new subgenus because they apparently shared the lack of a sternal furca. Ho & Bashirullah (1977) proposed to raise Metacaligus to generic status and, in addition to the type species M. uruguayensis ( Thomsen, 1949) , they included two other species in the genus, M. rufus ( Wilson, 1908) and M. hilsae ( Shen, 1957) . Metacaligus rufus was collected from an ariid catfish, Bagre marinus ( Mitchill, 1815) ( Wilson, 1908) and M. hilsae from a dorosomatid clupeiform host, Tenualosa reevesii ( Richardson, 1846) (as Hilsa reevesii ). After re-examination of the type material of Caligus afurcatus and C. enormis, Ho & Bashirullah (1977) excluded both species from Metacaligus even though the sternal furca is absent, because they lacked the “ Metacaligus type ” of leg 1 (with a large spine 1 and progressively smaller spines 2 and 3 (both lacking an accessory process), a long seta 4, plus 3 reduced plumose setae along the posterior margin of the distal exopodal segment). In C. afurcatus spine 1 is minute and spines 2 and 3 each have an accessory process, and in C. enormis spine 1 is apparently absent and spines 2 and 3 each have an accessory process. Boxshall & El-Rashidy (2009) placed the latter species in the Caligus productus -species group due to the loss of 2 plumose setae and the reduction of the third from the posterior margin of the same segment.

Pillai (1985) recognized that Caligus hilsae Shen, 1957 was a junior synonym of Caligus unguidentatus Rangnekar & Murti, 1950 but placed this species in Caligus due to Thomsen’s proposal of a new subgenus having “apparently failed to get accepted”. Caligus unguidentatus was transferred to Metacaligus by Ho & Lin (2002) who considered it to be a valid genus, and they added a fourth species, M. latus Ho & Lin, 2002 , taken from the same host species, Trichiurus lepturus , as the type species. Hayes et al. (2012) recognized that Caligus trichiuri Krøyer, 1863 , originally reported from the same host (as Trichiurus haumala ), was a senior synonym of Metacaligus uruguayensis and transferred it as Metacaligus trichiuri ( Krøyer, 1863) . Finally, Suárez-Morales et al. (2012b) added another species, M. yucatanensis , found in the plankton in a coastal lagoon in the Gulf of Mexico. In their analysis of the systematics of caligid genera, Dojiri & Ho (2013) treated Metacaligus as a valid genus and noted four diagnostic character states the combination of which could be used to separate this genus from Caligus , namely: 1, the lack of a sternal furca; 2, the lack of accessory processes on spines 2 and 3 on the distal margin of the second exopodal segment of leg 1; 3, the 3 plumose setae on the posterior margin of the same segment are short (i.e. shorter than the segment); and 4, there is only 1 outer spine on the third exopodal segment of leg 2. As pointed out by Dojiri & Ho (2013), all these character states can be found individually in particular species of Caligus , but the full combination is found only in species of Metacaligus . [The description of M. yucatanensis only appeared in 2012, presumably after Dojiri & Ho (2013) ’s analysis of caligid systematics had gone to press, and therefore, this species was not included in their discussion.]

Metacaligus , as currently constituted, is heterogeneous: M. yucatanensis shares characters 1 and 2 of Dojiri & Ho (2013) but the plumose setae on the exopod of leg 1 (character 3) are long, and it retains 2 outer spines on the third exopodal segment of leg 2 (character 4). The four other Metacaligus species share additional character states, including the lack of a posterior process on the proximal segment of the female antenna and the major reduction or loss of the tine of the postantennal process. However, these two character states are widespread within Caligus and are shared with members of the C. diaphanus -group, which is also characterized by the lack of accessory processes on spines 2 and 3 on the distal margin of the second exopodal segment of leg 1 ( Boxshall 2018), another Metacaligus characteristic regarded as diagnostic by Dojiri & Ho (2013). Based on the numerous detailed similarities between Metacaligus yucatanensis and the two new species Caligus described here, we propose to transfer Metacaligus yucatanensis Suárez-Morales, Kim & Escamilla, 2012 , to Caligus as C. yucatanensis ( Suárez-Morales, Kim & Escamilla, 2012) comb. nov. We recognize C. yucatanensis as a member of the C. undulatus -group.

The characters listed by Dojiri & Ho (2013) as supporting the recognition of Metacaligus as a valid genus are not robust. The loss of the sternal furca has occurred at least three times independently within the genus Caligus : in C. enormis (a member of the C. productus -group), in C. yucatanensis ( C. undulatus -group), and in C. afurcatus . The reduced length of the 3 plumose setae on the posterior margin of the distal exopodal segment of leg 1 is typical of members of the C. confusus -group and is also found in some other species such as C. pauliani Nuñes-Ruivo & Fourmanoir, 1956 , a member of the C. bonito -group (as C. biseriodentatus Shen, 1957 ). The absence of accessory processes from spines 2 and 3 on the distal exopodal segment of leg 1 is found in members of the C. diaphanus - group and in numerous other Caligus species. The loss of the proximal outer spine of the third exopodal segment of leg 2 is rare in Caligus , however, it is lacking in C. chinglonglini Ohtsuka & Boxshall, 2019 and is reduced in size in members of the C. pseudorhombi -group and C. undulatus -group ( Ohtsuka et al. 2020; Ohtsuka & Boxshall 2019; present account). In the light of this, we consider the case for maintaining Metacaligus as a separate genus is extremely weak and we therefore propose to treat Metacaligus Thomsen, 1949 as a synonym of Caligus O.F. M̧ller, 1785. We also propose to transfer its species to Caligus : Metacaligus trichiuri , M. rufus , and M. unguidentatus all return to their original combinations as Caligus trichiuri , Caligus rufus , and Caligus unguidentatus , respectively. Transferring Metacaligus latus would create a secondary homonym of Caligus latus Byrnes, 1987 , so we propose a replacement name Caligus cultellus nom. nov. So, Metacaligus latus Ho & Lin, 2002 becomes Caligus cultellus nom. nov., with the new name derived from the Latin cultellus which is the root of the English word cutlass, referring to the cutlass fish host ( Trichiurus lepturus ) of this species.

Excluding Caligus yucatanensis ( Suárez-Morales, Kim & Escamilla, 2012) comb. nov., which is a member of the C. undulatus -group, this cluster of the remaining four species formerly placed in Metacaligus represents a new species group, the Caligus trichiuri -group, characterized by the following combination of character states: the lack of a posterior process on the proximal segment of the female antenna; the reduction or loss of the tine of the postantennal process; the lack of a sternal furca; the lack of accessory processes on spines 2 and 3 on the distal margin of the second exopodal segment of leg 1; the small size of the 3 plumose setae on the posterior margin of the same segment (i.e. they are shorter than the segment); the presence of only 1 outer spine on the third exopodal segment of leg 2; leg 4 is 3-segmented with a 2-segmented exopod armed with I, IV spines. The C. trichiuri -group shares several of these character states with the C. diaphanus -group but can readily be distinguished by the state of leg 4, which is 4-segmented with a 3-segmented exopod armed with I, I, III spines in the latter group.