Philobrya capillata Dell 1964

Philobrya capillata Dell, 1964

(Figure 9)

Philobrya capillata Dell, 1964: 170 (in part), text figs. 2.5, 2.9, pl. 4, figs. 4, 5 [not text. fig. 2.8, pl. 4, fig. 9 = P. delli]. Philobrya magellanica. Jackson et al. 2015: figs. 2G, 3G. (Not Stempell, 1899: 230, pl. 12, figs. 13‒15).

Type locality: Shag Rocks, 53°43.67’S 40°57.00’W, 177 m.

Material examined: Photographs of the holotype (NHMUK 1962867). Shag Rocks: 53°23.80’S 42°42.03’W, 313 m (MLP-Ma 16117: 1 v). South Georgia: 54°30’S 35°50’W, 94 m (MACN-In 44439: 2 spec).

Distribution: Antarctic Peninsula (Dell 1964), Shag Rocks (Dell 1964; Jackson et al. 2015; this study) and South Georgia (this study); 93‒ 313 m.

Description: Shell large (maximum L = 10 mm), mytiliform, relatively high (H/L = 0.81 ± 0.01, n = 3), somewhat inflated (W/H = 0.67 ± 0.04, n = 3), solid; with a narrow, flat dorsal slope. Umbos broad, rounded, located at anterior end. Umbonal angle: 90‒100° (n = 3). Dorsal margin with the anterior portion relatively long, straight, slightly oblique backwards, and the posterior portion widely arched, forming a continuous curve with the posterior margin. Ventral margin slightly sinuated by byssal notch.

Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 470‒575 µm Lp (n = 3), separated from the dissoconch by a prominent rim. Surface microscopically pitted. Antero-dorsal and postero-dorsal areas ill-differentiate.

Dissoconch whitish, dull. Sculpture finely cancellate, consisting of 32‒40 faint radial ribs and numerous, densely packed commarginal cords. Radial and commarginal sculpture similar in strength.

Periostracum thick, yellowish straw, extending up to 0.5 mm from calcified shell margin; forming narrow radial folds, raised commarginal lamellae, and very short, thin setae. Commarginal lamellae extending up to the tip of the setae, but forming a deep sinuation between contiguous setae.

Inner shell surface with 36‒40 folds radiating from the umbo to shell margins, producing crenulations, those reaching the postero-dorsal margin stronger than the others. Posterior adductor muscle scar located in the posterior third of the valves, dorsally displaced.

Hinge: Anterior and posterior series of G1b teeth forming an angle of 170‒172° (n = 3). Teeth found along the entire range of sizes studied (i.e., 4.5‒10 mm). Resilifer moderately long and narrow.

Remarks: According to Dell (1964), the type material of P. capillata consists of the holotype (NHMUK 1962867) and two paratypes: one of them from Malvinas / Falkland Islands (NHMUK 1962868) and the other from Palmer Archipelago, Antarctic Peninsula (NHMUK 196582). The paratype from Malvinas / Falkland Islands is here reassigned to a new species described below: Philobrya delli.

Philobrya capillata is most similar to the East Antarctic P. tumida Thiele, 1912 (Fig. 7E, F; paralectotype), from which it differs by having smaller umbo, fewer radial ribs on the dissoconch (32‒40 vs. more than 50), thicker periostracum and stronger crenulations along the inner shell margin. Engl (2012, p. 48) speculated that the paratype from Palmer Archipelago “might be Philobrya tumida ”. However, the lack of arguments for such inference and the explicit statement by Dell (1964) that this paratype “agree well with the [holo]type” of P. capillata, led us to accept Dell’s (1964) identification.

Due to the mytiliform shape and densely packed periostracal setae, P. capillata resembles P. blakeana (Fig. 6), from which it differs by having the prodissoconch sculptured with microscopic pits (instead of radial ribs and commarginal cords). Other species similar in shape to P. capillata are Philobrya cf. barbata (Fig. 5B‒U) and P. olstadi (Fig. 14), although these two species have fewer radial ribs in the dissoconch (10‒15 in Philobrya cf. barbata and 18‒22 in P. olstadi vs. 32‒40 in P. capillata), a smooth inner shell margin and longer periostracal setae. Furthermore, the prodissoconch of Philobrya cf. barbata is commarginally sculptured, while that of P. olstadi is smooth (Appendix 1).

The specimens identified by Jackson et al. (2015: figs. 2G, 3G) as Philobrya magellanica actually correspond to P. capillata, while the identity of the specimen figured by these authors as P. capillata (Jackson et al. 2015: figs. 2E, 3E) remains uncertain.

The material studied herein provides the first record of P. capillata from South Georgia and extends its bathymetric range from 177 to 313 m.