Philobrya blakeana

Philobrya blakeana (Melvill & Standen, 1914) (Figure 6) Brachidontes (Hormomya) blakeanus Melvill & Standen, 1914: 129, pl. 7, figs. 4, 4a: Trew 1987: 27; McGhie 2008: 8. Philobrya blakeana. Trovant et al. 2013: 1847, 1850, Supplementary material, fig. S1. Philobrya crispa. Linse, 2002: 118, 119, pl. 18a, figs. 9.1.2-9‒12, pl. 18b, figs. 9.1.2-13, 14; Jackson et al. 2015: figs. 2F, 3F. Type localities: Roy Cove, Malvinas / Falkland Islands [51°33’S 60°23’W] (Brachidontes (Hormomya) blakeanus); Punta Rico, Isla Picton, Beagle Channel, 55°07.30’S 66°52.78’W, 25 m (Philobrya crispa). Material examined: Lectotype of Brachidontes (Hormomya) blakeanus (designated herein; MMUM EE 7674). Atlantic coast of Patagonia: Chubut Province: Comodoro Rivadavia, [45°53’S 67°29’W] (MACN-In 6834: 5 v). Santa Cruz Province: Puerto Deseado, 47°47.30’S 65°52.50’W, 8‒9 m (MACN-In 44417: 1 spec, 3 v). Beagle Channel: Bahia Lapataia, 54°51’S 68°29’W, 20‒31 m (MLP-Ma 16103: 1 spec); Bahía Ushuaia: 54°49’S 68°16’W, 24‒32 m (MLP-Ma 16104: 1 spec), 54°50’S 68°15’W, 18‒21 m (MLP-Ma 16105: 1 v); Isla Conejo, 54°50.88’S 68°14.67’W, 16 m (MACN-In 44418: 3 spec, 3 v); Pozo Ángel, 54°50’S 68°14’W, 21 m (MLP-Ma 16106: 3 spec, 1 v); Isla Marian, 54°51.55’S 68°13.52’W, 10 m (MACN-In 44419: 1 spec); Puerto Carelo, 54°52’S 68°13’W, 12‒20 m (MLP-Ma 16107: 2 spec); Isla Despard, 54°52’S 68°11’W, 64‒73 m (MLP-Ma 16108: 1 spec); Punta Segunda, 54°51.27’S 68°02.12’W, 14 m (MACN-In 44420: 6 spec); Isla Becasses, 54°57’S 67°01’W, 30‒40 m (MLP-Ma 16109: 7 spec, 5 v); Monte Moat, 55°02’S 66°42’W, 15‒20 m (MLP-Ma 16110: 13 v); Cabo San Pio: 55°03’S 66°37’W, 30‒35 m (MLP-Ma 16111: 1 spec, 21 v), 55°06’S 66°29’W, 65‒ 80 m (MLP-Ma 16112: 1 v). Isla de los Estados: Puerto Basil Hall, [54°45.18’S 64°09.25’W], 86 m (MACN-In 22248/1: 1 spec, 1 v); Puerto Vancouver, 54°52.44’S 64°03.37’W, 271 m (MACN-In 44421: 6 v). Burdwood Bank: 54°31.68’S 61°27.98’W, 137 m (MACN-In 44422: 34 spec, 88 v); 54°28.49’S 60°56.46’W, 102 m (MACN-In 44423: 4 v); 54°29.74’S 60°12.59’W, 95 m (MACN-In 44424: 1 v); 54°30.39’S 59°48.65’W, 105 m (MACN-In 44425: 61 v); 54°24.21’S 59°32.49’W, 86 m (MACN-In 44426: 1 v); 54°28.83’S 59°11.67’W, 122 m (MACN-In 44427: 2 v); 54°23.82’S 58°30.53’W, 135 m (MACN-In 44428: 9 v); 54°11.54’S 58°19.88’W, 90 m (MACN-In 44429: 1 spec, 15 v). Distribution: Atlantic coast of Patagonia, from Chubut Province to Tierra del Fuego Province (this study), Isla de los Estados (this study), Malvinas / Falkland Islands (Melvill & Standen 1914) and Burdwood Bank (Jackson et al. 2015; this study), extending to the Pacific coast of Patagonia (Linse 2002); 8‒665 m (living specimens: 8‒137 m). Description: Shell medium-sized (maximum L = 8.2 mm), modioliform (in small specimens) to mytiliform (larger specimens), relatively high (H/L = 0.76 ± 0.03, n = 28), somewhat inflated (W/H = 0.65 ± 0.05, n = 28), very thick; with a flat, poorly differentiated dorsal slope. Umbo broad, located at the anterior end, only slightly outstanding from shell outline, somewhat dorsally displaced in smaller specimens, but ventrally located in larger specimens. Umbonal angle: 90‒110° (n = 10). Dorsal margin with the anterior portion long, straight to slightly convex, obliquely directed backwards, and the posterior portion broadly curved, forming a continuous curve with the posterior margin. Ventral margin straight in small specimens, becoming increasingly concave with growth, due to enlargement of byssal notch. Prodissoconch D-shaped, with posterior end higher than anterior end, moderately projected ventrally, of 500‒ 550 µm Lp (n = 10), separate from the dissoconch by a prominent rim. Surface sculptured with strong, regularly distributed commarginal cords, and four radial ribs in the central area. Dissoconch whitish, dull. Sculpture cancellate, consisting of 20‒35 faint radial ribs, and closely packed commarginal cords. Radial and commarginal sculpture similar in strength. Periostracum very thick, yellowish, extending up to 0.5 mm beyond calcified shell margin; forming thin radial folds, raised commarginal lamellae, and short, delicate setae. Commarginal lamellae extending up to the tip of the setae, but forming a deep sinuation between contiguous setae. Inner shell surface with 30‒40 folds, radiating from the umbo to shell margins, where produce strong crenulations, all folds and crenulations similar in strength. 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 155‒160° (n = 6). Distal teeth of the posterior series partially disintegrated, forming pustules, in specimens longer than 2.5 mm. Distal teeth of the anterior series also forming pustules in specimens longer than 3.2 mm. Resilifer long and narrow. Remarks: The original description of P. blakeana does not provide information on the number of specimens on which the species was based on. Trew (1987), McGhie (2008) and Trovant et al. (2013) regarded the single typespecimen housed at the Manchester Museum as the holotype, although it is not identical with that figured by Melvill & Standen (1914: pl. 7, figs. 4, 4a). Considering that no other specimen is currently known, and in accordance with ICZN Art 74.6, that individual should be actually regarded as lectotype (Fig. 6A, I). The original description of P. crispa is accompanied by excellent scanning electron microscope photographs (Linse 2002: pl. 18a, figs. 9.1.2-9–12, pl. 18b, figs. 9.1.2-13, 14). Linse (2002) described the prodissoconch of P. crispa as having six radial ribs. However, only four ribs are actually recognizable in the photographs she provided, being the other two radial elements the limits of the antero-dorsal and postero-dorsal prodissoconch areas. In all aspects, Philobrya crispa is indistinguishable from the lectotype of P. blakeana (Fig. 6A, I) and thus considered a junior synonym. Philobrya blakeana shows variation in shell outline with growth: from a modioliform shape (in small-sized specimens) to a mytiliform shape (in large-sized specimens). Hence, the outline of small-sized specimens of P. blakeana appear similar to P. atlantica (Fig. 4), P. brattstromi (Fig. 8), P. sanjuani (Fig. 16) and P. wandelensis (Fig. 18), while larger specimens resemble Philobrya cf. barbata (Fig. 5), P. capillata (Fig. 9) and P. olstadi (Fig. 14). However, the thick shell of P. blakeana and its peculiar prodissoconch sculpture, consisting of radial and commarginal, elements, allow an easy distinction of this species. Other distinctive characters of P. blakeana are the presence of a great number of radial ribs of the dissoconch and the high density of periostracal setae (see Appendix 1). Another morphologically similar species to Philobrya blakeana is P. kerguelensis (E.A. Smith, 1885), described from Kerguelen Islands (Fig. 7A‒D). However, the latter differs from P. blakeana by having a more elongated shell outline, a narrower dorsal slope, and weaker crenulations along the inner shell margins. Although Philobrya blakeana was mentioned from the Strait of Magellan, in the ecological contribution by Rios et al. (2003), these records were regarded as misidentifications by Trovant et al. (2013). No additional specimens of P. blakeana were found in that area, as part of our study.

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