Philobrya olstadi (Soot-Ryen, 1951)

(Figure 14)

Philippiella olstadi Soot-Ryen, 1951: 12, 13, pl. 1, figs. 9, 10; Ageitos de Castellanos 1960: fig. 5.

Philobrya quadrata . Melvill & Standen 1907: 124, 145. (Not Philobrya quadrata (Pfeffer, 1886: 119, 120, pl. 4, figs. 6a, b)). Philobrya olstadi . Engl 2012: 47, figs. 5a, b.

Type locality: Deception Island, [62°57’S 60°39’W], South Shetland, 75 m.

Material examined: Photographs of the holotype (NHM.UIO D 28607). South Orkney: [60°44.3’S 44°44.1’W], intertidal (MACN-In 44469: 16 spec, 1 v); Scotia Bay, [60°46’S 44°40’W], intertidal (NMS Z 1921.143.699: photographs of 1 spec) . Antarctic Peninsula: Puerto Melchior, [64°19’S 62°58’W], intertidal (MACN-In 44470: 1 spec, 1 v) .

Distribution: South Orkney (Engl 2012; this study), South Shetland (Soot-Ryen 1951; Engl 2012; this study), Antarctic Peninsula (Castellanos 1960; this study), and Gough Island? (Dell 1964); intertidal to 75 m.

Description: Shell medium-sized (maximum L = 6.2 mm), mytiliform, relatively high (H/L = 0.82 ± 0.04, n = 16), somewhat inflated (W/H = 0.74 ± 0, 05, n = 16), moderately solid; with a narrow, ill-defined dorsal slope. Umbos broad, located at anterior end, ventrally directed, slightly (in smaller specimens) to markedly (in larger specimens) projected, bluntly pointed. Umbonal angle: 85‒90° (n = 10). Dorsal margin with the anterior portion long, straight to slightly convex, markedly oblique backwards, and the posterior portion arched to flattened, forming a continuous curve with the posterior margin. Posterior margin evenly arched in smaller specimens, bluntly pointed in larger specimen. Ventral margin nearly straight in small-sized specimens, becoming increasingly concave with growth due to the enlargement of byssal gap.

Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 500‒550 µm Lp (n = 12), separated from the dissoconch by a prominent rim. Postero-dorsal areas poorly defined. Central area smooth in appearance.

Dissoconch whitish, dull. Sculpture finely cancellate, consisting of 18‒22 faint radial ribs and numerous, moderately separated commarginal cords. Commarginal and radial sculpture similar in strength.

Periostracum very thick, yellowish straw, only slightly projected beyond shell margin; forming fine radial folds, raised lamellae, and short, thin setae. Commarginal lamellae extending up to the tip of the setae, but forming a deep sinuation between contiguous setae.

Inner shell surface and shell margins smooth. 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 152‒156° (n = 5). All teeth in both series well-recognizable in specimens of up to 2.0 mm L. Distal teeth of both series partially disintegrated, forming pustules, in specimens of 2.5‒5.8 mm L. Teeth and pustules completely lost in specimens longer than 6.0 mm L. Resilifer long and narrow.

Remarks: Philobrya olstadi characterizes by having mytiliform shape, with well-projected umbo, the prodissoconch smooth, the periostracum showing numerous short, densely packed setae, and the inner shell surface smooth.

Philobrya olstadi closely resembles P. capillata (Fig. 9) and large-sized specimens of P. blakeana (Fig. 6), which however differ by having less projected umbos, crenulated inner shell margins and a distinctly sculptured prodissoconch ( P. blakeana showing radial and commarginal sculpture, and P. capillata microscopic pits). Furthermore, P. olstadi has a lower number of radial ribs in the dissoconch than P. capillata (18‒22 vs. 32‒40, respectively).

Philobrya cf. barbata (Fig. 5B‒U) is another species similar to Philobrya olstadi, although the latter has a more oblique anterior portion of dorsal margin, more projected posterior end and a flattened ventral portion of the posterior margin (particularly in larger specimens), thus resulting in a more elongate shell outline. In addition, P. olstadi has shorter periostracal setae than Philobrya cf. barbata (Appendix 1).

Melvill & Standen (1907) identified as Philobrya quadrata a specimen collected in Scotia Bay, South Orkney. The study of this material (Fig. 14H) reveals that this specimen actually corresponds to P. olstadi .

Linse (2002: 120) identified as P. olstadi specimens collected in the Drake Passage, off Cape Horn. However, the illustration provided by the author (Linse 2002: fig. 9.1.2-16) suggests that this material most probably represents P. brattstromi .

Dell (1964) identified as P. olstadi specimens from Gough Island, although the identity of this material deserves further studies.