Revision of the genus Oxyarcturus (Isopoda, Valvifera, Antarcturidae), with a description of a new deep-sea species from Argentina

A new antarcturid isopod, Oxyarcturus holoacanthus sp. nov. , is fully described based on seven specimens collected in the Mar del Plata submarine canyon at 2950 m depth, during the “Talud Continental III” expedition on board the Argentinian RV “Puerto Deseado”. Oxyarcturus holoacanthus sp. nov. is closely related to O. spinosus (Beddard, 1886), from which it can be distinguished by the body spine pattern. The penial plate, a novel character for the genus Oxyarcturus , as well as for the family Antarcturidae, is described in detail. The species O. dubius (Kussakin, 1967) and O. beliaevi (Kussakin, 1967) are considered as incertae sedis until further morphological and molecular data can clarify their taxonomic position. An update of the geographic and bathymetric records of the genus Oxyarcturus is provided.


Introduction
The valviferan isopod family Antarcturidae Poore, 2001 is one of the most diverse families of the suborder in the deep-sea (Poore and Bruce 2012).Currently, it contains approximately 100 nominal species, assigned to 17 genera, most of them distributed in the Southern Hemisphere (Poore 2001;Boyko et al. 2008;Pereira et al. 2019).In particular, the genus Oxyarcturus, which is herein studied, was erected by Brandt (1990) to include three species from the Subantarctic Region and off South Africa (Beddard 1886;Kussakin 1967;Kensley 1977Kensley , 1978;;Brandt 1990).
The deep-sea fauna off the Argentine coast is still scarcely known.Ten species of Antarcturidae were reported from this area during the 19 th and 20 th centuries; all these species were collected by foreign surveys, and each one reported from a single station (Beddard 1886;Kussakin 1967;Kussakin and Vasina 1998;Doti et al. 2020).In 2012 and 2013 the Argentine RV "Puerto Deseado" carried out three expeditions to the Mar del Plata submarine canyon (Talud Continental I-III).These surveys resulted in a remarkable improvement in our knowledge of the benthic invertebrates from this area, i.e., cold-water corals, echinoderms, crustaceans, etc. (Martinez et al. 2014;Doti 2017;Pereira and Doti 2017;Bernal et al. 2018;Risaro et al. 2020;Flores et al. 2021;Pereira et al. 2021;Roccatagliata 2023;among others).In particular, for the family Antarcturidae, three species collected by the above mentioned surveys were recently described: Xiphoarcturus kussakini Pereira, Roccatagliata & Doti, 2019, X. carinatus Pereira, Roccatagliata & Doti, 2019, and Fissarcturus argentinensis Pereira, Roccatagliata & Doti, 2020, bringing to 13 the number of species of Antarcturidae recorded off Argentina (Pereira et al. 2019(Pereira et al. , 2020)).
In the current contribution a new deep-sea species of the family Antarcturidae, Oxyarcturus holoacanthus sp.nov. is fully described and illustrated based on seven specimens collected in the Mar del Plata submarine canyon.In addition, O. dubius (Kussakin, 1967) and O. beliaevi (Kussakin, 1967) are considered as incertae sedis until their taxonomic status is resolved.Finally, the geographic and bathymetric distributions of the genus are updated.

Material and method
The specimens were obtained with a bottom otter trawl during the "Talud Continental III" expedition on board the RV "Puerto Deseado", in 2013.Afterwards, the material was fixed in 96% ethanol.
The described specimens were stained with Chlorazol Black E, and the appendages were dissected and temporarily mounted in glycerol.Drawings of the whole animal were prepared with a Leica MZ8 stereoscopic microscope, those of the dissected appendages were prepared using a Carl Zeiss (Axioskop) compound microscope, both equipped with a camera lucida.Line drawings were rendered in a digital format using a Wacom tablet and the Adobe Illustrator program (Coleman 2003).
For the sake of clarity, in those appendages with a large number of setae, some of them were omitted in the figures.Therefore, the number of setae drawn may not match with the numbers mentioned in the text.
Habitus photographs were taken with a digital camera Nikon D7500 equipped with a macro lens Sigma 105 mm f2.8 EX.Appendages' photographs were taken with a digital camera Sony Cyber-shot DSC-WX1 mounted on the compound microscope.
Total body length was measured in dorsal view from the frontal margin of head to the tip of the pleotelson (in flexed specimens, total length was estimated by the sum of individual body parts).Appendage lengths were measured after Hessler (1970).For the terminology of the body parts, spines and setae, see Pereira et al. (2019).The eye size was estimated by the ratio between the eye diameter (e) and the distance between the eye and the anterior margin of the head (d) following Pereira et al. (2020).
The type material is deposited in the Invertebrate collection of the Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" (MACN), Buenos Aires.
Remarks.Brandt (1990) in a comprehensive work of the Antarctic Valvifera revised the old genus Antarcturus zur Strassen, 1902, and transferred 17 of the species previously placed in this genus to Acantharcturus Schultz, 1981, Chaetarcturus Brandt, 1990, Litarcturus Brandt, 1990, Tuberarcturus Brandt, 1990and Oxyarcturus Brandt, 1990. Brandt (1990) erected the genus Oxyarcturus to include three species: O. spinosus (Beddard, 1886), O. dubius (Kussakin, 1967) and O. beliaevi (Kussakin, 1967); and stated that members of this genus have "caudal margin of pleotelson with 3 acute spines, two long laterocaudal ones and one shorter medial spine"; being this the only feature characterizing the genus.Although this medial spine is evident in O. spinosus (type species) and O. holoacanthus sp.nov.(see Beddard (1886); current study Figs 1A, B, 2C), the presence of this spine in O. dubius and O. beliaevi is uncertain.Unfortunately, Kussakin (1967) did not include figures in dorsal view of O. dubius and O. beliaevi.However, he wrote that O. dubius has "very small, weakly expressed spinules, largest of which were disposed in midline near posterior margin of pleotelson".Regarding O. beliaevi, although Kussakin (1967) did not mention the presence of a medial spine in this species, Kensley (1977Kensley ( , 1978) ) reported this spine in specimens from South Africa.However, Wägele (1991) and Kussakin and Vasina (1995) questioned Kensley's identification.
O. dubius (Kussakin, 1967) is clearly not a member of Oxyarcturus, and O. beliaevi (Kussakin, 1967) probably neither.Therefore, these two species are herein removed from the genus Oxyarcturus and placed as incertae sedis, until further morphological and molecular data can clarify their taxonomic position.
On the other hand, the specimens from South Africa identified by Kensley (1977) as Antarcturus beliaevi most probably belong to a new species of Oxyarcturus.If confirmed to be a new species, its description should be completed, one specimen should be designated as holotype, and a formal name should be adopted.
Description.Adult male (habitus based on the holotype MACN-In 44318; appendages based on the paratype MACN-In 44319-a).
Mandibles (Fig. 3D, E) asymmetrical, without palp.Incisor processes with 4 strong sclerotized teeth.Molar processes with grinding surface and indented margins; right molar process narrower than left one, and with 9 setae on lower surface.Left lacinia mobilis with 3 rounded teeth and 1 stout serrate seta; right lacinia mobilis with 4 acute teeth and a few small setae.
Maxilliped (Fig. 4C) basal endite short and broad, with 18 setulate setae on distal and lateral margins.Palp of 5 articles; article 1 0.4× article 3 length, with 11 simple setae; article 2 0.6× article 3 length, with 24-25 simple setae; article 3 longest, with 40 simple and serrulate setae on inner margin, and 4 long setae (3 of them broken) on outer distal angle; article 4 0.9× article 3 length, with 30 simple and serrulate setae on inner margin, and 8 long simple and serrulate setae along outer margin; article 5 shortest, with 12 simple and serrulate setae.Epipod long, oval, surpassing distal margin of palp article 1, with few simple setae on ventral surface and inner margin.Basis and epipod, ventral surfaces with small setae and cuticular combs.
Pereopods II-IV (Fig. 5) alike, pereopod IV longest.Bases gradually changing from short and smooth (pereopod II) to long and with spines (pereopod IV).Note: spines are broken or worn out in the specimen drawn.These spines are better preserved in the holotype (see Figs 1A, 2A).Ischium, merus and carpus with 1 long distal spine (except on carpus of pereopod IV).Distal corner of flexor margin of basis and flexor margins of ischium to propodus with two rows of filter setae.Dactylus (excluding claws) 0.2-0.3×propodus length, with 2 distal claws, ventral claw 0.4-0.6×dorsal claw length (see Fig. 5C).
All articles with small setae scattered on extensor and flexor margins.
Pleopod I (Figs 7B, 8B) protopod longer than those of remaining pleopods, with 9 coupling setae on inner margin, and 10 stout spines along outer margin.Endopod 1.1× exopod length, inner and distal margins with 55 plumose setae; outer margin with many setules.Exopod, inner and distal margins with 33 plumose setae; outer margin with 49-50 short distally plumose setae (some of them arranged in a second row on posterior surface).Posterior surface groove narrowing distally, and ending on lateral margin, overlapped by a thin layer; groove slightly projected distally, followed by a group of tiny setae (see detail).All plumose setae shorter than rami.
Etymology.The specific epithet combines two Greek words holos = "entire, complete" and akantha = "spine", referring to the many long and acute spines that cover the body surface of this species.
Distribution.Only known from the Mar del Plata submarine canyon, off Buenos Aires Province, Argentina, at 2950 m depth (Fig. 11).
Remarks.Oxyarcturus holoacanthus sp.   and pleon with many long dorsal spines on both sexes (male with small tubercles and female with small spines in O. spinosus).

Discussion
Oxyarcturus is one of the most poorly known genera of the family Antarcturidae.Currently, this deep-sea genus is composed of two nominal species and one probable new species from South Africa, each one with few records and few specimens reported (Beddard 1886;Kensley 1977Kensley , 1978;;present study).This is also true for the new species herein studied, which was described based on only seven specimens obtained in a single station at the Mar del Plata submarine canyon.
Valviferans occur in 34 of 41 samples examined from the Talud Continental I-III expeditions (Pereira et al., in prep.).The single record of Oxyarcturus holoacanthus sp.nov.herein reported may reflect a patchy distribution of this species rather than an artifact of the sampling methods.It is worth noting that among the isopod species, singletons (species recorded only once) have also been reported from other deep-sea areas (Brandt et al. 2005;2007a, b, 2015;Brix et al. 2018).
Although just a few specimens of Oxyarcturus holoacanthus sp.nov.were collected, it was possible to perform a detailed description of the body and appendages.The penial plate, which appears to be atypical for the family, is herein also described (Figs 7A, 8A).Pereira et al. (2020) described the penial plate of Fissarcturus patagonicus (Ohlin, 1901) as well, and discussed the importance of this character in the systematics of the family Antarcturidae.These findings suggest that the morphology of the penial plate should be included in future descriptions and phylogenetic studies.However, we are aware that this is not at all times possible, since deep-sea species are usually collected in low numbers of specimens, and adult males are not always available.
The spine pattern of Oxyarcturus holoacanthus sp.nov.showed slight variations among the specimens examined.However, since we have few specimens and some of them are damaged, we are not able to confirm whether these differences are related to size and/ or sex.It is worth noticing that intraspecific variations in the spine pattern have been reported to some species of Antarcturidae (Nordenstam 1933;Kussakin 1967).Beddard (1886) mentioned that females of O. spinosus show more abundant and longer spines than males.Furthermore, sexual variation in the spine pattern has also been reported for other antarcturids, mainly in the genus Fissarcturus Brandt, 1990 (Brandt 1990(Brandt , 2007;;Pereira et al. 2020).
The members of the genus Oxyarcturus are known only from the Southern Hemisphere (Fig. 11, Table 1) i.e., O. spinosus was recorded off Marion Island and off Heard Island, O. holoacanthus sp.nov. off Argentina (Mar del Plata submarine canyon, ca.38°S, 53°W), and Antarcturus beliaevi sensu Kensley, 1977 off South Africa.Regarding the bathymetric distribution (Table 1)  Kensley, 1977 Off South Africa 2500-3000 Kensley (1977Kensley ( , 1978) ) all the species of Oxyarcturus were collected at deep waters between 1580-3000 m.Thus, the members of Oxyarcturus seem to be restricted to the lower slope.