Pseudomesochra bathyhabitatrix, Gómez & Yáñez-Rivera & García-Vázquez, 2025

4.3. Pseudomesochra perplexa View in CoL , P. gemina , and P. bathyhabitatrix sp. nov.: doubtful records of Pseudomesochra ?

Before the present study, P. perplexa and P. gemina were the only two species from which the males were known. Pseudomesochra perplexa is known from one male only ( Bodin, 1968) and both sexes have been described for P. gemina ( Coull, 1973a) . An additional male specimen from the Laptev Sea identified with P. gemina was reported by Willen (1996). Assignment of single males to Pseudomesochra without knowing the females has been always controversial. Sexual dimorphism in the single male of P. perplexa was observed in the subchirocerate antennule, P5 and P6, P2 ENP ( Bodin, 1968), but sexual dimorphism in the male of P. gemina (the female of this species has been fully described) was documented in the “slightly haplocer” antennule, rostrum, P2 ENP2–3, and P5 ( Coull, 1973a). Bodin (1968) was not certain about the belonging of P. perplexa in Pseudomesochra mainly because of the reduction of the outer armature of P2–P4 EXP3 from three to two outer spines. However, the females of some other species ( P. scheibeli , P. aberrans , and P. abyssalis ) also possess two outer spines on P2–P4 EXP3. Willen (1996) believe that P. perplexa could not belong in Pseudomesochra because the distal inner seta on the third exopodal and endopodal segment of swimming legs, and caudal setae IV and V are not rat-tail-like, and because the distal armature of P1 ENP2 consists of a distal outer normal spine, and one distal medial and one distal inner plumose seta, instead of one outer short and one distal medial long spine, and one inner rat-tail-like seta [but see the redefinition of Pseudomesochra by Willen and Dittmar, (2009: 297)]. Willen (1996) reported on female and male CV copepodids of P. meridianensis , and showed the habitus, A1, P2 and P3 ENP, P5, P6 of the male, and A1, P2 and P3 ENP of the female. Willen (1996) noted that these individuals fully agreed with the diagnosis for the genus and suggested that this could be the only true record of the male of the species of Pseudomesochra . The adult female of P. meridianensis possesses five armature elements on P2 and P3 ENP3 (two inner setae, one distal inner rat-tail-like seta and one distal outer spine, and one outer spine). The same armature formulae of P2 and P3 ENP3 are present in the female and male CV [see Willen, (1996: 105–106, Figs. 19 View Fig and 20 View Fig )], but the male P2 ENP3 shows a slightly modified outer spine. She noticed also that the degree of development of the outer process on the male and female P3 ENP2 is different, but she was not certain about its true nature. Pseudomesochra gemina and P. meridianensis share the same armature formulae of the female P2–P4. As noted above, the armature complement of the female and male CV of P. meridianensis is the same as in the adult, and sexual dimorphism is already expressed in the outer spine of the male CV P2 ENP3, suggesting that, besides the other differences observed by Willen (1996) between the male of P. gemina and the generic diagnosis—distal setae on A2 ENP, swimming legs and caudal rami not rat-tail-like, and different shape of the armature elements on P1 ENP2—these two species seem to follow the same pattern of sexual dimorphism of the male P2 ENP3. A notable difference, however, is the over elongate distal outer process on P2 ENP2 of the male of P. gemina as shown by Coull (1973a: 600, Fig. 27 View Fig ). As noted above, Willen (1996) believe that P. perplexa does not belong in Pseudomesochra given the non-rat-tail-like setae of the A2 ENP and caudal rami, and shape of the distal armature of P1 ENP2. The male of P. perplexa and P. bathyhabitatrix sp. nov. share the elongate antennulary aesthetasc, the non-rat-tail-like setae of the antennary endopod, the elongate setiform elements on P1 EXP2 (the distal outer element is a spine of comparable length to that of the two outer spines in the other species of Pseudomesochra ), armature and shape of the modified P2 ENP3 [with three inner setae and an outer distal apophysis, the latter homologous to the outer spine of the female P2 ENP3 as noted by Willen (1996) for P. meridianensis ], and shape of the P3 ENP2 (with distal inner margin extended), suggesting that these two species might be related. The caudal setae of P. perplexa were not described, and they were broken off in the only male specimen of P. bathyhabitatrix sp. nov. Interestingly, Bodin (1968) was not able to see the postantennal mouthparts of P. perplexa “correctement” Bodin (1968: 65), and Coull (1973a) omitted any comment on the male mouthparts of P. gemina . Later, Willen (1996) noted that the mouthparts were extremely reduced in the males of her P. gemina from the Laptev Sea. The mouthparts of the only male specimen of P. bathyhabitatrix sp. nov. are also very reduced. The reduction of mouthparts in harpacticoids has been documented for deep-sea and subtidal Argestidae , (e.g., Mesocletodes fladensis Wells, 1965 , M. angolaensis Menzel and George, 2009 , M. unisetosus Gomez ´, 2018a, M. nudus Vakati, Thistle and Lee, 2017 ; Aegisthidae Giesbrecht, 1893 (e.g., Nudivorax Lee and Huys, 2000 ; Scabrantenna Lee and Huys, 2000 ; Andromastax Conroy-Dalton and Huys, 1999 ), Pseudotachidiidae (e.g., Paranannopus arndwilleni Willen, 2005 ), and Cletodidae Scott T., 1905 [e.g., several species of Stylicletodes Lang, 1936c ; see Cho et al. (2022)] and are potential indicators of common ancestry among the species and genera within their respective families ( Cho et al., 2022; Menzel and George, 2009). It seems that the genus Pseudomesochra is composed of several evolutionary lineages, but the description of the males of the known species, and the description of both sexes of new species are needed to evaluate and assess the phylogenetic importance of the different kinds of sexually dimorphic characters.