(Fig. 4b; Fig. 6)
Crypthelia formosa Cairns 1983a: 133–136, figs. 31g, 39a–f, 40a–c, map 12; Cairns 1983b: 431, table 1; Cairns 2015: 310; Río Iglesias et al. 2012: 191; Bax & Cairns 2014: 107, table 1, 109, 110, map 3
Distribution. Patagonia, 42° S to 48° S; Scotia Arc from Tierra del Fuego to South Georgia, 483–1841 m; Burdwood Bank, 1647–2044 m. New record off Mar del Plata, 877–1398 m.
Material examined. USNM 60207 (holotype) off Burdwood Bank, Eltanin St. 1592 (54° 43–45’ S, 55° 30–37’ W); USNM 60087 (paratype) off Scotia Arc, Vema St. 17–61 (54° 44’ S, 55° 39’ W); MACN-In 40647 off Mar del Plata, Argentina, St. 59 (37° 49.688’ S, 54° 5.236’ W), 1398 m, September 2013; MACN-In 40648 off Mar del Plata, Argentina, St. 42 (37° 59.110’ S, 54° 41.136’ W), 877 m, May 2013; MACN-In 40649 off Mar del Plata, Argentina, St. 59 (37° 49.688’ S, 54° 5.236’ W), 1398 m, September 2013; MACN-In 42501 off Mar del Plata, Argentina, St. 37 (37° 59.848’ S, 54° 24.206’ W), 1275 m, May 2013.
Description. Fragments delicate with uniplanar growth. Presence of polychaete tube on posterior face induces anastomosis and thickening of branches bearing it (Fig. 4b) or deviation of plane of growth. Largest fragment 3.8 cm wide and 3.8 cm tall with basal branch 4.0 mm thick. Branches round in cross section. Distal branches, 0.6–1.0 mm thick, bear cyclosystems considerably wider, resembling beads on a string (Fig. 6a). Branching dichotomous, two branches originating laterally from one cyclosystem. Coenosteum white and porcellaneous, with a linear-imbricate texture especially conspicuous along slender branches (Fig. 6c), that turns reticulate-imbricate to reticulate-granular in cyclosystems and polychaete tube. Coenosteal strips convex, 58–104 µm wide and bordered by longitudinal rows of short slits pierced by one or two deep pores.
Cyclosystems round to slightly elliptical, 2.0– 3.3 mm wide (average 2.6 mm, n=66, σ=0.3), placed on anterior face, at a level with coenosteum surface or slightly raised from it. There are 14–21 dactylotomes per cyclosystem (mode 18, n=66). They are 0.1–0.2 mm wide and occupy about 1/3 of gastropore tube total heigth. Pseudosepta concave with maximum width of 0.2–0.3 mm. Abcauline margin of cyclosystems bears a slightly concave lid as a result of fusion of two or more pseudosepta (Fig. 6b). Far end of lid 0.5–1.5 mm wide and proximal end 0.8–1.1 mm wide. An additional lid can develop in adcauline margin of cyclosystem, facing the initial one. Lid can also be forked due to presence of a dactylotome. One or two round nematopores 40–67 µm in diameter may be present on lids as well, or on pseudosepta and cyclosystem wall.
Gastropores round, 0.8–0.9 mm wide. Gastropore tube double-chambered and about 1.6 mm deep, with gastropore ring constriction almost at bottom of tube, resulting in a very shallow basal chamber (Fig. 6b).
Ampullae placed inside cyclosystem wall and lid. Male ampullae occupy interior of lid and surround cyclosystem in a semicircle centred at lid, with three or four slightly sunken efferent pores on cyclosystem wall (Fig. 6b). Female ampullae occupy interior of lid and extend backwards forming a conspicuous bump (Fig. 6a). Female efferent pore at base of lid and opens towards interior of gastropore tube (Fig. 6d, e).
Discussion. Specimens from study area coincide with Cairns’ (1983a) description of C. formosa in all characters, except the diameter of cyclosystems, which is larger in specimens from the study area. This could be due to variability within the species. Cairns (1983a) only described female specimens of C. formosa, but in 2015 he established the ampullar configuration for this species as B1-C1 (B1=female efferent pore beneath lid; C1=male ampullae surround cyclosystem and efferent pores apical), which coincides with the sexual characteristics of the specimens described here.
Crypthelia formosa is the only species of the genus found hitherto in the SWA. Previous records are from Cairns (1983a) based on specimens from Scotia Arc, Burdwood Bank and South Georgia, and from Río Iglesias et al. (2012), based on specimens collected from Patagonia, from 42° S to 48° S within a depth range of 200–1500 m (specific depth not informed). In the Antarctic zone there are only two other species, also present in New Zealand: C. fragilis and C. studeri (see Bax and Cairns 2014). Specimens from the study area differ from C. fragilis in having a larger cyclosystem and gastropore diameter and higher number of dactylotomes per cyclosystem, and they differ from C. studeri in cyclosystem shape, often irregular in C. studeri, and in pseudosepta width, much lower in C. studeri. Besides, neither C. fragilis nor C. studeri develop an additional lid, as occurs in specimens from study area and in C. formosa. Identification of C. formosa specimens from off Mar del Plata provides an extension of the known distribution of this species within SWA.