Stylaster laevis Studer 1878: 635 Boschma 1957: 12 Stylaster obliquus Studer 1878: 635–636 Moseley 1881: 88 Conopora tenuis Moseley 1879: 503 Moseley 1881: 82 Stylaster laevis Moseley 1881: 81 Conopora laevis : Zibrowius 1981: 274–277 Cairns 1983b: 490–492 Stylasterids (Hydrozoa: Stylasteridae) from Mar del Plata submarine canyon and adjacent area (southwestern Atlantic), with a key to the species off Argentina Bernal, M. C. Cairns, S. D. Penchaszadeh, P. E. Lauretta, D. Zootaxa 2021 2021-05-12 4969 3 401 452 [151,315,151,177] Hydrozoa Stylasteridae Conopora Animalia Anthoathecata 8 9 Cnidaria species undetermined  ( Fig. 4a, d; Fig. 5)      Stylaster laevis Studer 1878: 635, pl. 2, figs. 5a–b;  Boschma 1957: 12    Stylaster obliquus Studer 1878: 635–636, pl. 2, figs.7a–d;  Moseley 1881: 88    Conopora tenuis Moseley 1879: 503;  Moseley 1881: 82, 88, pl. 12, figs. 5–6   Stylaster laevis(?):  Moseley 1881: 81    Conopora laevis: Zibrowius 1981: 274–277, pl. 3, figs. 8–11, pl. 4, figs. 1–5;  Cairns 1983b: 490–492, figs. 21a–d; Cairns 1991:73–75, pl. 58e–g, 59a–g; Cairns 2015: 204(in key), 207, table 8, 211–212, figs. 104k, 121f–i, 123; Lindner et al. 2008: fig. 3, table S1   Distribution.New Caledonian region, 200–1140 m; Indo-West Pacific Ocean, 95–1035 m. New record off Mar del Plata, 1275–1398 m, should specimens be confirmed as  C. laevis.   Material examined.  USNM 60036off Burdwood Bank,  EltaninSt.1592 (54° 43–45’ S, 55° 30–37’ W); MACN-In 40653 off Mar del Plata, Argentina, St. 37 ( 37º 59.848’ S, 54º 24.206’ W),  1275 m,  May 2013; MACN-In 42510 off Mar del Plata, Argentina, St. 59 ( 37º 49.688’ S, 54º 5.236’ W),  1398 m,  September 2013.   Description.Colonies delicate and more or less uniplanar. Largest specimen (MACN-In 40653) 3.5 cmwide, 3.2 cmtall and thickest branch (probably basal branch) 4.0 mm wide. Branches round in cross section. Thinnest branches 1.0 mm wide. Branching dichotomous with anastomosis at points where a polychaete tube is present ( Fig. 4a), on anterior or posterior side. Coenosteum white, porcellaneous in branches and coarser in polychaete tubes. Seen at high magnification, cyclosystems and branches bear nematopores about 35–40 µmin diameter, raised on conical stumps ( Fig. 5a, d). Pseudosepta also bear nematopores, flush and in a row of two or three ( Fig. 5e). Surface of polychaete tubes coarse due to abundant hemispherical bumps with raised circular pores that are probably nematopores. Linear-imbricate texture, especially conspicuous in thinnest branches ( Fig. 5d). Strips 67–104 µmwide, delimited by rows of deep, short slits or pores. Texture more reticulate and slits shallower on cyclosystem surfaces. Cyclosystems placed sympodially on thin branches and aligned laterally on thicker ones. Cyclosystems round to slightly oval shaped at their opening, 1.0– 1.3 mmwide (average 1.2 mm, n=15, σ=0.1) based on the measurement of terminal cyclosystems. Each newly formed cyclosystem is raised on a markedly conical structure that narrows towards its base ( Fig. 5a). The sympodial arrangement of cones is visible in thinner branches, whereas cyclosystems are more flush in thicker ones.Terminal cyclosystems bear 12–16 dactylotomes (mode 13, n=15). Basal cyclosystems often bear less dactylotomes in their adcauline margin, which is more sunken in coenosteum. Dactylotomes 77–122 µmwide and occupy about one third of the total gastropore tube length. Gastropores round, 0.6–0.7 mmwide at base of pseudosepta. Gastropore tube short and double-chambered, widening towards opening and bears constriction at about 1/3 distance from base ( Fig. 5b). Ampullae may be present as a bulge adjacent to cyclosystem, around 0.6 mmwide ( Fig. 5c), or as numerous bulges partially or completely sunken in branch and polychaete tube coenosteum. Spherical cavities on surface of polychaete tube wall correspond with ampullae ( Fig. 4d). Internal diameter of five ampullae from a cross section of two branches is 0.5–0.6 mm.   Discussion.  Conopora laeviswas originally described by Studer (1878)as  Stylaster laevisbased on a specimen from New Zealand. Zibrowius (1981)stated that Studer´s (1878)original descriptions of  Stylaster laevisand  Stylaster obliquusbased on holotypesfrom New Zealandbelong to the same specimen. He also mentioned that those holotypes, together with syntypesof  Conopora tenuisfrom station 170 of the Challengerexpedition to Kermadec islands, described by Moseley (1879), are very similar in cyclosystem structure and branch texture and he suggested that they all belong to the same species. Zibrowius (1981)examined specimens from station 171 of the Challengerexpedition, identified doubtfully (with an interrogation mark) by Moseley (1881)as  Stylaster laevis, and stated that they are  Conopora verrucosa. Cairns (1991)examined specimens from New Zealand, providing new records of  Conopora laevisand Cairns (2015)described new specimens of  C. laevisfrom New Caledonia. Specimens from study area are similar to  Conopora verrucosaand  Conopora laevisin growth pattern and delicacy, presence of a polychaete tube and sympodial arrangement of cyclosystems. Presence of numerous conical nematopores on surface of branches and cyclosystems, similar to those described by Cairns (2015)for  C. verrucosa, suggests specimens could belong to this species, since  C. laevisoften lacks or bears very few and/or inconspicuous nematopores ( Cairns 2015). Presence of diastemas also supports this hypoteses. However, the presence of numerous internal and partially internal ampullae in branches and in polychaete tube coenosteum of specimens suggests they could belong to  C. laevis, since  C. verrucosaonly bears ampullae around cyclosystems and efferent pores on pseudosepta ( Cairns 2015). Regarding pores on ampullae from polychaete tube of studied specimens, these could be apical efferent pores instead of nematopores, which may suggest they belong to male specimens of  C. laevis, although further examination of more specimens is necessary to confirm this. The ampulla adjacent to cyclosystem seen in Fig. 5chas the characteristics of a female ampulla of  C. laevisalthough it lacks the lateral efferent pore described by Cairns (2015)(it might not have developed yet). Regarding pores present on pseudosepta, it is improbable they are efferent pores since they do not differ in size with the nematopores at surface of branches, no ampullae have been found surrounding cyclosystems and there is more than one pore on each pseudoseptum, in contrast with  C. verrucosawhere there is only one large efferent pore per pseudoseptum and its size is considerably larger than that of pores in pseudosepta of specimens from study area. These are therefore most probably  C. laevis, although it is necessary to study the typematerial of this species to confirm this. Should they belong to  C. laevis, this record is a novelty for the SWA. 2013-05 USNM Argentina Mar del Plata 1275 -37.997467 Burdwood Bank 1 -54.403435 Eltanin St. 8 9 USNM 60036 1 2013-09 USNM Argentina 1398 -37.828133 Mar del Plata 1 -54.087265 8 9 1