New genus and species of calanoid copepods (Crustacea) belonging to the group of Bradfordian families collected from the hyperbenthic layers off Japan

Abstract A new genus and species of calanoid copepods belonging to the group of Bradfordian families, Pogonura rugosagen. et sp. nov., is described from the deep-sea hyperbenthic layers off Nagannu Island, Okinawa Prefecture, southwestern Japan. Pogonuragen. nov. is similar to another Bradfordian genus Procenognatha in sharing the following characteristics: (1) segmentation of the antennule, fused segments II–IV, X–XI, XXVII–XXVIII in females and II–IV, X–XII, XXVII–XXVIII, right XXII–XXIII in males; (2) retained setae on the ancestral segments I–IV of the antennary exopod; (3) setules on the mandibular gnathobase; (4) 3 sclerotized setae on the maxillary endopod; (5) absence of sensory seta on the maxilliped; (6) large spinules on the posterior surface of the rami of legs 2 and 3; and (7) setation and segmentation of female leg 5. Pogonuragen. nov. is distinctly distinguished from Procenognatha by the following features: (1) reduction of a seta on the ancestral segment IX of the antennary exopod, (2) 8 setae (7 in Procenognatha) on the maxillular exopod, (3) 5 brush-like setae (6 in Procenognatha) on the maxillary endopod, and (4) reduction of right endopod of male leg 5. The systematic position of Pogonuragen. nov. in the Bradfordian families is also discussed. Although this new genus shares synapomorphies with some diaixid genera, an assignment of this genus to any Bradfordian family should be pending until the taxonomy of this family group is clearly settled.

introduction Some clausocalanoidean families of calanoid copepods are characterized by the presence of chemosensory setae on the maxillary endopods, and in some taxa, on the maxillules and maxillipeds (Bradford 1973;Nishida and Ohtsuka 1997) and are called the Bradfordian families (Ferrari and Steinberg 1993). These groups are distributed in various marine habitats including both the pelagic realm and the hyperbenthic layers of the oceans (Bradford-Grieve 2004). Recently, many new families and genera collected from deep-sea hyperbenthic layers were established in the Bradfordian group (Ferrari and Markhaseva 1996;Ohtsuka et al. 2002Ohtsuka et al. , 2003Markhaseva et al. 2008;Markhaseva and Schulz 2009). So far, seven families of the Bradfordian group have been recognized: Diaixidae Sars, 1902; Kyphocalanidae Markhaseva & Schulz, 2009, Parkiidae Ferrari & Markhaseva, 1996Phaennidae Sars, 1902;Rostrocalanidae Markhaseva, Schulz & Martinez Arbizu, 2008;Scolecitrichidae Giesbrecht, 1892;and Tharybidae Sars, 1902. According to Bradford (1973), the Scolecitrichidae and Phaennidae are families that are well defined by the armature of sensory elements on the maxillae, whereas the Diaixidae and Tharybidae are not as clearly diagnosed. Markhaseva et al. (2014) considered only Tharybis as a member of Tharybidae based on an autapomorphic character, an enlarged and vaulted arthrite of the maxillulary praecoxa which can differentiate between Tharybidae and Diaixidae. Undinella and Brodskius were conventionally included in Tharybidae (e.g., Markhaseva et al. 2014). Recently, analyses of relationships among the Bradfordian genera were performed by Markhaseva and Ferrari (2005) and Laakmann et al. (2019). Laakmann et al. (2019) also conducted a molecular-based phylogenetic analysis of these families but concluded that relationships between these seven families or 15 genera were not supported, except for the closeness between Procenognatha (conventionally assigned to Diaixidae) and Tharybis (Tharybidae).
The present paper deals with a description of a new genus and species of calanoid copepods belonging to the Bradfordian family collected from the deep-sea hyperbenthic layers off Nagannu Island, Okinawa Prefecture, southwestern Japan. The systematic position of this new genus is also discussed.

Materials and methods
Copepods were collected from the deep-sea hyperbenthic layer off Nagannu Island, west of Okinawa Prefecture, southwestern Japan (26°19.23'N, 127°26.35'E, depths of 595-627 m) on May 21, 2011, using a sledge net (mouth area of 1450 × 326 mm, mesh size of 0.33 mm; see Ohtsuka et al.1992) towed along the sea bottom for 30 minutes at 2 knots. Samples were fixed with 10% neutralized formalin seawater immediately after capture. Type specimens are deposited at the National Museum of Nature and Science, Tsukuba, Japan (NMST-Cr 27413-27415). The morphological terminology follows Huys and Boxshall (1991).
Remarks. Because Pogonura gen. nov. has brush-like sensory setae on the maxillary endopod, it can be assigned to one of the Bradfordian families. The new genus can be tentatively included in Diaixidae because it fits the familial diagnosis proposed by Markhaseva et al. (2014), except for the proximal basal endite of the maxillule with 3 setae (vs. 4 setae typical for the Diaixidae) and 2-segmented exopods of both legs 5 of the male.
The present new genus also shares the following characteristics with the diaixid genus Procenognatha (Markhaseva and Schulz 2010): the maxilliped carries no specialized sensory setae; legs 2 and 3 carry 3 large spinules on the posterior surface; and leg 5 of the female is uniramous, 2-segmented, with the distal segment having 3 processes and 1 articulated spine.
The male of the present new species has complex structures on leg 5, which can be seen in other diaixid genera such as Anawekia and Diaixis. These three genera have rows of setules and/or spinules on the left exopod of leg 5 [cf. figs 7 and 9 in Othman and Greenwood (1994); figs 11 and 12 in Andronov (1979)], and these setulae and/ or spinules seem to be homologues in position and shape. However, Anawekia and Diaixis have some derivative characteristics: (1) the posterior corner of the prosome, leg 4, and the urosome of both sexes are asymmetrical; (2) female leg 5 is totally reduced; and (3) the left endopod of male leg 5 is reduced.
Etymology. The new generic name is derived from two Greek words pogon, meaning "beard", and oura, meaning "tail", to denote the paired setae like moustache on the genital double-somite of the female. Gender feminine.
Seta and spine formulae of legs 1-4 are shown in Table 1. Leg 1 (Fig. 4A) with medial long setules on coxa and basis; von Vaupel Klein organ (Vaupel Klein 1972) distinct on anterior surface of endopod; distal seta of basis twice as long as endopod;  Leg 5 (Fig. 4E) uniramous; coxae and intercoxal sclerite fused to form common base; basis broad, about 1.3 times as long as wide; exopod 1-segmented, ca. 4.4 times as long as wide, with 3 lateral processes and 1 terminal bipinnate spine.
Etymology. The specific name of the new species is derived from a Latin word rugosa, meaning "rugose", to denote leg 5 of the adult male with many foliaceous armatures.

Discussion
The females of Pogonura rugosa gen. et sp. nov. have a ventrolateral pair of spiniform setae on their genital double-somite (Figs 1, 8). These armatures are similar to leg 6 possessed by females of podoplean copepods in the position (symmetrical armatures on the ventrolateral of the genital double-somite) (cf. Huys and Boxshall 1991). However, these spiniform setae do not seem to be a homologue of leg 6. Generally, leg 6 comprises a symmetrical pair of basal processes with a few elements at the tip (at most 3 setae or spines on each process) and is connected to the inner muscles at the base (Boxshall 1982;Huys and Boxshall 1991). However, the spiniform setae of P. rugosa gen. et sp. nov. lack basal processes and are not connected with inner muscles like the podoplean's leg 6. Some groups of calanoids generally have the genital operculum on their genital double-somite of adult females, which is considered a homologue of leg 6 (Bradford-Grieve et al. 2010).
Pogonura gen. nov. has 3 setae on the terminal exopodal segment of the maxilla, although almost all copepods have at most 2 setae on this segment (Ferrari and Ivanenko 2008). In the Clausocalanoidea, however, some genera of the family Aetideidae have 3 setae on their counterparts (cf. Pseudeuchaeta vulgaris Markhaseva, Mohrbeck & Renz, 2017;Paracomantenna profunda Markhaseva & Renz, 2019). This retention in setae can be considered as an ancestral state in the clausocalanoideans. Markhaseva and Ferrari (2005) and Laakmann et al. (2019) attempted to morphologically classify the Bradfordian genera into three main groups by considering the setations of the maxillary endopods, the antennary exopods and the maxillipedal praecoxal endites [=syncoxal endites sensu Huys and Boxshall (1991)], viz., Group A (Diaixidae and Tharybidae), Group B (Phaennidae and Parkiidae), and Group C (Scolecitrichidae). Laakmann et al. (2019) simultaneously conducted a molecular phylogenetic analysis of the Bradfordian genera but failed to assign different families or genera into any robust group except for Procenognatha and Tharybis. Pogonura gen. nov. shares some plesiomorphies with Group A sensu Markhaseva and Ferrari (2005) in the setation of the maxillipedal syncoxa. In addition, members of Group A have the following other plesiomorphies: all setae on the ancestral segments I-IV of the antennary exopod are retained and no specialized chemosensory seta is observed on the maxillipedal syncoxa. As mentioned in Remarks, Pogonura gen. nov. shares the following synapomorphies with some diaxid genera: with Procenognatha, posterior spinules are present on legs 2 and 3 (Markhaseva and Schulz 2010) and with Anawekia and Diaixis, a row of spinules is found on the left exopod of male leg 5 (Andronov 1979;Connell 1981;Othman and Greenwood 1994). These synapomorphies of Pogonura gen. nov. imply their close relationships with these diaixids. According to Laakmann et al.'s (2019) molecular analysis, Procenognatha comprises a robust clade with Tharybis (Tharybidae). In the present study, an assignment of Pogonura gen. nov. to any Bradfordian family should be pending until the taxonomy of this family group is clearly settled.