Pseudomesochra

4.2. The genus Pseudomesochra View in CoL

The species of Pseudomesochra display an amalgam of plesiomorphic and derived character states hampering the assessment of their interspecific relationships. The problematic systematics and taxonomy of this group of copepods is fuelled also by the low number of individuals found in sediment samples which makes the assessment of intraspecific variability difficult, and by the fact that most species are known from the female only. Eight out of 23 species of Pseudomesochra ( P. affinis , P. meridianensis , P. gertwilleni , P. divaricata , P. media , P. similis , P. gemina , and P. tatianae ) had been described based on more than one female specimen, and the male is known only for P. gemina (one individual), P. perplexa (one individual), and P. bathyhabitatrix sp. nov. (one individual). Bodin (1968) believed that P. perplexa would not belong in Pseudomesochra . Willen (1996) shared Bodin’ s (1968) belief and was of the opinion that P. perplexa does not belong in Pseudomesochra , and suggested that the male CV specimens of P. meridianensis from the Laptev Sea are the only true records of males of Pseudomesochra (see above), but accepted that “the position of P. meridianensis sp. nov. has to remain open for the moment” ( Willen, 1996: 90).

The setation pattern of the rami on the postantennal mouthparts seems to be very variable and it has not been well-documented for several species. Noteworthy, two species, P. laptevensis and P. tatianae possess vestigial maxillipeds. On the other hand, due to its size and ease of observation, the segmentation pattern of the female antennule has been well-documented. The female antennule of Pseudomesochra possesses at most seven segments, with the main aesthetasc on the fourth segment ( Willen, 2000; but see also Table S1). Four species ( P. similis , P. gemina , P. laptevensis , and P. tatianae ) display the plesiomorphic segmentation of the female antennule with seven segments, and only P. media and P. abyssalis possess five-segmented female antennules; the other species possess six-segmented female antennules (Table S1). Taking the site of the main aesthetasc as landmark (it is always on the fourth segment), the reduction in the number of segments seems to be the result of fusion of either the sixth and seventh segments (as in the species with six-segmented female antennules) or fifth to seventh segments (as in the species with five-segmented antennules). The segmentation pattern of the female antennule seems to be a potential source of information to better understand the relationships among the species of Pseudomesochra , but this still needs to be evaluated.

Species groupings seldom reflect phylogenies, but are still useful for identification purposes. Some groupings of the species of Pseudomesochra are presented below based mainly on the segmentation and armature formula of A2 EXP, and armature formula of swimming legs. Due to the fact that most species have been described only from the female, and given that the position of the species for which the males have been described still needs to be verified, only the females were considered in these groupings. The position of the males attributed to Pseudomesochra was treated separately. These groupings are useful for species identification but their systematic values still need to be tested.

4.2.1. Group A

Only two species (see group A in Table S1), P. beckeri found off Morocco at 3920 m depth ( Becker and Schriever, 1979), and P. brucei from Svalbard archipelago found at 182–201 m depth ( Scott and Scott, 1901), possess an antennary exopod with three segments—the plesiomorphic condition in Pseudomesochra —of which the first and second segments bear two and one seta, respectively, and the distal segment possesses either one lateral, and one small and three well-developed distal setae ( P. beckeri ) ( Becker and Schriever, 1979), or one lateral, and one small and one well-developed apical element ( P. brucei ) ( Scott and Scott, 1901). Additionally, these two species possess six-segmented female antennules, and P1 EXP2 lacks inner armature—similar to what can be observed in species of group C (see below). Pseudotachidids bear a maximum of one lateral, and one reduced and two well-developed apical setae on the third exopodal segment of the antenna ( Willen, 2000). The pentasetose distal segment of the antennary exopod (one lateral plus four distal setae) of P. beckeri [ Becker and Schriever (1979) believed that the additional distal seta could represent a fourth segment] might be an observational error, and this segment is most probably armed with one lateral, and one small and two well-developed distal setae (the latter reinterpretation was used in Table S1); the distal segment of the antennary exopod of P. brucei was described with one lateral and two distal setae. The description of P. brucei is incomplete and it is uncertain which leg did Scott and Scott (1901) show in their plate III, Fig. 8 View Fig . If Becker and Schriever’ s (1979) description of the female P5 is correct, these two species differ also in the number of setae on the female P5 baseoendopod (four well-developed and one small outer seta in P. beckeri , but four well-developed setae in P. brucei ).

4.2.2. Group B and justification of P. bathyalis sp. nov., P. axa sp. nov., and P. bathysicola sp. nov

Pseudomesochra bathyalis View in CoL sp. nov., P. axa View in CoL sp. nov., and P. bathysicola View in CoL sp. nov., each known from one female only from the Gulf of Mexico, were attributed to that genus based on i) the rat-tail-like condition of the distal setae of the antennary endopod, ii) the shape of the distal armature elements of P1 ENP2 (with one outer spine, one medial longer spine, and one inner rat-tail-like seta), iii) the armature formula of P1 EXP3 (022, viz., with only two outer spines), iv) the rat-tail-like condition of the distal inner seta on P1 EXP3 and P1 ENP2, and v) the rat-tail-like condition of caudal setae IV and V. Pseudomesochra bathyalis View in CoL sp. nov., P. axa View in CoL sp. nov., and P. bathysicola View in CoL sp. nov. deviates from the groundpattern of Pseudomesochra View in CoL as detailed by Willen (1996) and Willen and Dittmar (2009) in the spiniform distal inner element of P2–P4 EXP3, and in the normal (not rat-tail-like) distal inner seta of P2–P4 ENP3.

Pseudomesochra bathyalis View in CoL sp. nov. and P. bathysicola View in CoL sp. nov. belong to a group of species with five-, six-, or seven-segmented female antennules, antennary exopod with two segments—derived condition in Pseudomesochra View in CoL —of which proximal with two, distal with four (two lateral, two distal, or one lateral and three distal as in P. aberrans View in CoL ) or—rarely—three setae (one lateral, two distal, as in P. gemina View in CoL , P. laptevensis View in CoL , and P. tatianae View in CoL ), and P1 EXP2 with—plesiomorphic—inner armature (group B in Table S1). To this group of species belong P. media View in CoL from Norway, P. similis View in CoL from Sweden, P. bathyalis View in CoL sp. nov. and P. bathysicola View in CoL sp. nov. from the Gulf of Mexico, P. gemina View in CoL from off North Carolina ( USA), P. laptevensis View in CoL from the Laptev Sea, P. tatianae View in CoL from Norway, P. scheibeli View in CoL from the Faroe Islands in the North Atlantic, P. aberrans View in CoL from the Bay of Biscay in the North Atlantic, and P. latifurca View in CoL from Norway [but see also Kim et al. (2021)]. All the species assigned to this group lack inner armature on P1 ENP1, and most of these species display four baseoendopodal setae on the female P5. Under this scheme, P. media View in CoL , seems to occupy an isolated position by i) the five-segmented female antennule, ii) the presence of one inner seta on P1 ENP1, and iii) the presence of eight armature elements—with three inner setae—on P2 EXP3 (the armature formulae of P3 and P4 have not been described yet). Within this group of species, only P. media View in CoL , P. aberrans View in CoL , and P. latifurca View in CoL display three setae on the endopodal lobe of the female P5. Six species ( P. similis View in CoL , P. bathyalis View in CoL sp. nov., P. bathysicola View in CoL sp. nov., P. gemina View in CoL , P. laptevensis View in CoL , and P. tatianae View in CoL ) share the armature formulae of P2–P4 EXP (0,1,223/0,1,323/0,1,323, respectively), and are here placed in group B’’ (see Table S1). Pseudomesochra similis View in CoL , P. bathyalis View in CoL sp. nov., and P. bathysicola View in CoL sp. nov. possess two lateral and two distal setae on A2 EXP2 and five setae on P3 ENP3, but P. bathyalis View in CoL sp. nov. possesses three exopodal setae on the female P5, and P. bathysicola View in CoL sp. nov. stands out by the lack of inner armature on P2 ENP1 and P4 ENP1, and by the lack of endopodal armature on the female P5. Within group B″, P. gemina View in CoL , P. laptevensis View in CoL , and P. tatianae View in CoL share the tetrasetose P3 ENP3. Within group B‴ (see Table S1), P. scheibeli View in CoL and P. aberrans View in CoL share the loss of one outer spine on P2–P4 EXP3 with the resulting armature formulae 0,1, 222; 0,1,322; 0,1,322, respectively; P. scheibeli View in CoL possesses four, P. aberrans View in CoL possesses three exopodal setae on the female P5. Pseudomesochra latifurca View in CoL stands out by i) the loss of one inner seta and by the presence of three outer spines on P2 EXP3, ii) the loss of one inner seta on P2 ENP3, iii) the loss of two inner setae on P3 and P4 EXP3 and presence of three outer spines, iv) loss of one inner seta on P3 ENP3, and v) the reduction of armature complements on P4 ENP3 from four to three.

Pseudomesochra axa sp. nov. from the Gulf of Mexico belongs to a group of species with five- or six-segmented female antennules, antennary exopod with two segments of which distal with four (two lateral, two distal) or—rarely—five setae (two lateral, three distal, as in P. gertwilleni ), and P1 EXP2 without inner armature (group C in Table S1). To this group of species belong also P. abyssalis from the North-east Atlantic, P. affinis and P. divaricata from Norway, P. meridianensis and P. gertwilleni from the Weddell Sea, P. minor from off Peru, and P. longifurcata from Scotland. Pseudomesochra abyssalis , P. meridianensis and P. minor (group C′ in Table S1) share the armature formulae of P1 ENP (1,121, i.e. with one inner seta on ENP2) and P2 ENP (1,1,221, i.e. endopod three segmented with two inner setae on third segment), but P. minor and P. meridianensis are morphologically more similar to each other than to P. abyssalis . Pseudomesochra minor and P. meridianensis share the six-segmented female antennule (it is five-segmented in P. abyssalis ) and possess three outer spines on P2–P4 EXP3 ( P. abyssalis underwent loss of one outer spine on those segments). Pseudomesochra meridianensis and P. minor share three outer spines on P3 EXP3, but the latter is different from the other two species in the loss of one inner seta, i.e. with only two inner setae on P3 EXP3. The three-segmented P3 ENP of P. meridianensis and P. minor displays the maximum number of armature elements on ENP3 (221); P. abyssalis differs in the loss of one inner seta on P3 ENP3, resulting in the armature 121. The armature formula of P4 EXP3 of P. abyssalis and P. meridianensis is the same as in P3, but P. minor displays only one inner seta (which is similar to the condition of P. gertwilleni ). The armature formula of P4 ENP3 of P. abyssalis , P. meridianensis , and P. minor is different: three elements (armature formula 111) are present in P. abyssalis , four (armature formula 121) in P. meridianensis , and five (armature formula 221) in P. minor . Three baseoendopodal setae are present on the female P5 of P. abyssalis (as in P. divaricata , P. longifurcata , P. affinis , and P. axa sp. nov.) and four in P. meridianensis and P. minor (as in P. gertwilleni ). Four species ( P. gertwilleni , P. divaricata , P. longifurcata , and P. affinis ; group C″ in Table S1) share the loss of the inner armature of P1 ENP1 resulting in the armature formula 0,121 (a similar condition is present in most species of group B; see Table S1). The P2 ENP and P4 ENP of P. divaricata has not been described yet. The P2 ENP of P. longifurcata and P. affinis , P3 ENP of P. divaricata , P. affinis and P. longifurcata , and P4 ENP of P. longifurcata and P. affinis have been described as two-segmented, and is the result of fusion of ENP2 and ENP3, and occupy an isolated position in group C but also in Pseudomesochra . The second swimming leg is similar in P. longifurcata and P. affinis , but differ in the armature formula of P2 EXP3 (with one inner seta in P. longifurcata , but unarmed in P. affinis ; P2 of P. divaricata remains unknown); the armature formula of P3 EXP/ENP (0,1,023/1221) is identical in P. divaricata , P. longifurcata , and P. affinis ; the armature formula of P4 EXP/ENP of P. longifurcata [0,1,023/1,1(2?)21] and P. affinis is most probably identical, but it is unknown for P. divaricata ; P. divaricata , P. longifurcata and P. affinis share also the six-segmented female antennule and the trisetose baseoendopodal part of the female P5. Pseudomesochra axa sp. nov. occupies an isolated position in group C. In fact, this species is unique in Pseudomesochra in the trisetose P1 ENP2 with one inner, one distal, and one outer element. The armature formula of P2–P4 EXP is similar to those of some species of group C’’ [P2 EXP (armature formula 0,1,023) is similar to that of P. affinis ; P3 EXP (armature formula 0,1,023) is similar to that of P. divaricata , P. affinis , and P. longifurcata ; P4 EXP (armature formula 0,1,023) is similar to that of P. longifurcata , and P. affinis ]. The endopod of the second swimming leg (armature formula 1,1,121) is similar to that of P. gertwilleni ; P3 and P4 ENP (armature formulae 1,1,121) is similar to those of P. abyssalis and P. gertwilleni , and P. meridianensis and P. gertwilleni , respectively. The trisetose baseoendopodal part of the female P5 is shared with P. divaricata , P. longifurcata and P. affinis , and P. abyssalis .

The description of P. crispata is very incomplete and the armature formulae of A2 EXP and swimming legs remain undescribed. Also, Lang (1948: 642) believed that the armature complement of P2 and P 3 in Brady (1910) might be erroneous. The species is, therefore, regarded here as taxon inquirendum and has been excluded from the analyses above.