Philobrya magellanica ( Stempell, 1899 )

Urcola, Matias Ricardo & Zelaya, Diego Gabriel, 2024, The Genus Philobrya J. G. Cooper, 1867 (Bivalvia: Philobryidae) In Patagonia And Adjacent Antarctic Waters, Zootaxa 5437 (2), pp. 151-192 : 171-173

Philobrya magellanica ( Stempell, 1899)

( Figure 12 )

Avicula (Meleagrina) magellanica Stempell, 1899: 230 , pl. 12, figs. 13‒15.

Stempelleria magellanica . Clasing 1918: 22-26.

Avicula (Stempellia) magellanica . Odhner, 1922: 221.

Philobrya capillata . Linse 2002: fig. 9.1.2-15; Jackson et al. 2015: fig. 3E. (Not Dell, 1964: 170: text figs. 2.5, 2.9, pl. 4, figs. 4, 5).

Type localities: Punta Arenas , [53°10’S 70°54’W], 10 fathoms [18.3 m] and Bahía San Nicolás , [53°50’S 71°07’W], 15 fathoms [27.4 m], Strait of Magellan .

Material examined: Photographs of a syntype from Punta Arenas (ZMB-Moll 51991). Strait of Magellan: Punta Arenas , [53°10’S 70°54’W] (MACN-In 12342-1: 1 spec) ; Canal David, 53°38.48’S 72°22.86’W, 40 m (MLP- Ma 16119: 10 spec, 1 v) ; Bahía Nash, 53°42’S 72°21’W, 30 m (MLP-Ma 16120: 1 spec) ; Cabo Holand, 53°48.63’S 71°37.46’W, 15‒25 m (MLP-Ma 16121: 1 spec) ; Bahía Rosa, 53°52.75’S 71°13.33’W, 25‒30 m (MLP-Ma 16122: 2 v). Beagle Channel: Dos Lomos , 54°51’S 68°19’W, 5‒10 m (MLP-Ma 16123: 1 spec) ; Bahía Ushuaia, 54°50’S 68°15’W, 18‒21 m (MLP-Ma 16124: 1 spec) ; Isla Conejo, 54°50.88’S 68°14.67’W, 16 m (MACN-In 44452: 2 spec, 6 v) ; Pozo Ángel, 54°50’S 68°14’W, 21 m (MLP-Ma 16125: 3 v) ; Puerto Carelo, 54°52’S 68°13’W, 12‒20 m (MLP-Ma 16126: 1 spec) ; Punta Segunda: 54°52’S 68°18’W, 74‒98 m (MLP-Ma 16127: 1 spec) , 54°52’S 68°07’W, 21‒23 m (MLP-Ma 16128: 1 spec) , 54°50.90’S 68°02.80’W, 12 m (MACN-In 44453: 4 spec, 3 v) , 54°51.27’S 68°02.15’W, 14 m (MACN-In 44454: 1 spec) , 54°51.27’S 68°02.12’W, 19 m (MACN-In 44455: 4 spec, 4 v) ; Isla Becasses , 54°57’S 67°01’W, 30‒40 m (MLP-Ma 16129: 6 spec, 1 v) ; Paso Richmond , [55°16’S 66°45’W] (MACNIn 24951: 1 spec) ; Monte Moat , 55°02’S 66°42’W, 15‒20 m (MLP-Ma 16130: 4 spec, 13 v) ; Cabo San Pío : 55°03’S 66°37’W, 30‒35 m (MLP-Ma 16131: 8 v), 55°06’S 66°29’W, 65‒80 m (MLP-Ma 16132: 2 spec, 3 v). Atlantic coast of Patagonia : Santa Cruz Province: Puerto Deseado: 47°45.30’S 65°52.90’W, intertidal (MACN-In 44445: 7 v), 47°45.41’S 65°52.64’W (MACN-In 44446: 1 spec) , 47°45.42’S 65°52.60’W (MACN-In 44447: 1 spec) , 47°45.54’S 65°52.20’W (MACN-In 44448: 1 spec) , [ 47°45’S 65°55’W] (MACN-In 44449: 1 spec) , 47°46.90’S 65°52.80’W, 6 m (MACN-In 44450: 1 spec, 1 v), 47°47.30’S 65°52.50’W, 8‒9 m (MACN-In 44451: 1 spec, 7 v). Tierra del Fuego Province : Río Grande [53°47’S 67°41’W] (MACN-In 12622: 1 spec) . Isla de los Estados : Puerto Cook, [54°46’S 64°03’W] (MACN-In 35219: 1 spec) ; Puerto Vancouver , 54°52.44’S 64°03.37’W, 271 m (MACNIn 44456: 5 v). Burdwood Bank: 54°31.68’S 61°27.98’W, 137 m (MACN-In 44457: 21 spec, 34 v) ; 54°28.49’S 60°56.46’W, 102 m (MACN-In 44458: 43 v); 54°29.74’S 60°12.59’W, 95 m (MACN-In 44459: 3 spec, 8 v); 54°16.67’S 59°57.79’W, 96 m (MACN-In 44460: 2 spec, 27 v); 54°30.39’S 59°48.65’W, 105 m (MACN-In 44461: 17 v); 54°24.21’S 59°32.49’W, 86 m (MACN-In 44462: 4 spec, 39 v); 54°28.83’S 59°11.67’W, 122 m (MACN-In 44463: 2 spec, 14 v); 54°23.82’S 58°30.53’W, 135 m (MACN-In 44464: 1 spec, 5 v); 54°11.54’S 58°19.88’W, 90 m (MACN-In 44465: 1 spec, 10 v).

Distribution: Atlantic coast of Patagonia, from Santa Cruz Province (this study) to Cape Horn ( Linse 2002), Isla de los Estados (this study), Burdwood Bank (this study), Strait of Magellan ( Stempell 1899; this study) and Beagle Channel (this study); intertidal to 430 m (living specimens: 5‒137 m).

Description: Shell large (maximum L = 11 mm, fide Stempell, 1899), ovate to subcircular, high (H/L = 0.91 ± 0.03, n = 30), compressed (W/H = 0.53 ± 0.03, n = 30), moderately solid; with a wide, flat dorsal slope. Umbo narrow, pointed, subterminal, markedly ventrally curved; not outstanding from shell outline in smaller specimens, but somewhat projected in larger specimens. Umbonal angle: 100‒115° (n = 21). Dorsal margin with the anterior portion long, straight to slightly convex, markedly oblique forward and the posterior portion broadly arched, forming a continuous curve with the posterior margin. Ventral margin nearly straight in smaller specimens, becoming increasingly concave with growth.

Prodissoconch D-shaped, with posterior end higher than anterior end, moderately projected ventrally, of 400‒ 430 µm Lp (n = 10), raised from dissoconch. Antero-dorsal and postero-dorsal areas sculptured with densely packed commarginal folds. Central area sculptured with microscopic pits. Pits ovate, forming numerous arcs that radiate from the apex and intersect at right angles, forming a quincuncial pattern.

Dissoconch whitish, dull, sculptured with 15‒20 faint radial ribs.

Periostracum thick, yellow to yellowish brown, extending up to 2 mm beyond calcified shell margin; forming low commarginal and radial folds; setae missing.

Inner shell surface and shell margins smooth. Posterior adductor muscle scar located in the antero-posterior half of the valves, dorsally displaced.

Hinge: Anterior and posterior series of G1b teeth forming an angle of 164‒172° (n = 8). All teeth in both series well-recognizable in specimens up to a length of 1.6 mm. Distal teeth of the posterior series partially disintegrated, forming pustules, in specimens longer than 2.0 mm. Distal teeth of the anterior series also forming pustules in specimens longer than 4.3 mm. Resilifer long and wide.

Remarks: The narrowly pointed umbo and the periostracum with commarginal and radial folds, but lack of setae make Philobrya magellanica an easily recognizable species. Another species lacking periostracal setae is P. quadrata ( Fig. 15 ), which, however, differs from P. magellanica by having a wider umbo and lack of radial periostracal folds. In addition, P. magellanica have larger umbonal angle (100‒115° vs. 85‒95°, respectively) and less evident posterior area of shell than P. quadrata .

In general shell shape P. magellanica also resembles P. sublaevis ( Fig. 17 ), from which it differs by its hooked umbo, pitted prodissoconch and lack of periostracal lamellae and setae (see Appendix 1). The specimens figured by Jackson et al. (2015: figs. 2G, 3G) as P. magellanica correspond to P. capillata .

Figures

FIGURE 12. Philobrya magellanica. A, K: syntype (ZMB-Moll 51991), B: specimen from Tierra del Fuego (MACN-In 44454), C‒J, L‒S: specimens from Burdwood Bank (C, J: MACN-In 44460, D, E, G, I: MACN-In 44463, F, H, R, S: MACN-In 44457, L, M, P, Q: MACN-In 44462, N: MACN-In 44459, O: MACN-In 44465). A‒E: outer views right valve (A, B: with periostracum, C‒E: without periostracum), F‒K, M: inner views right valve, L: inner view left valve, N: detail of prodissoconch (slightly tilt), O, Q, R: hinge plates right valve (O: specimen of 4.6 mm L, Q: specimen of 1.7 mm L, R: specimen of 4.3 mm L), P, S: hinge plates left valve (P: specimen of 1.7 mm L, S: specimen of 4.3 mm L). Scale bars A‒M: 1 mm; N: 100 µm; O‒S: 500 µm.

FIGURE 15. Philobrya quadrata. A, I: Philippiella quadrata original figures by Martens & Pfeffer (1886: figs. 6a, b), B, C, M: specimens from South Georgia (B, C: neotype, MACN-In 44471, M: MACN-In 44472), D‒H, K, L, N‒R: specimens from Burdwood Bank (D‒F, N: MACN-In 44474, G, H, K, L, O‒R: MACN-In 44475), J: Philippiella ungulata original figure by Martens & Pfeffer 1886: fig. 7). A, B, D‒G: outer views right valve (A, B: with periostracum, D‒G: without periostracum), C, H: outer views left valve (C: with periostracum, H: without periostracum), I‒K: inner views left valve, L: inner view right valve, M: brooded embryo removed from the demibranch of an adult specimen, N, Q, R: hinge plates left valve (N: specimen of 3.3 mm L, Q: specimen of 1.9 mm L, R: specimen of 3.0 mm L), O, P: hinge plates right valve (O: specimen of 1.9 mm L, P: specimen of 3.0 mm L). Scale bars A‒L: 1 mm; M: 100 µm; N‒R: 500 µm.

FIGURE 17. Philobrya sublaevis. A, P: original figures of P. sublaevis by Pelseneer (1903: pl. 7, figs. 93, 94), B‒E, I‒O, Q‒W: specimens from South Sandwich (MLP-Ma 16133), F: specimen from South Orkney (MLP-Ma 16134), G, H: Neotype (MACN-In 44476). A‒G, I, Q: outer views right valve (A‒D, I, Q: without periostracum, E‒G: with periostracum), H, L‒P: inner views right valve, J: outer view left valve (without periostracum), R: detail of prodissoconch (slightly tilt), S, V, W: hinge plates right valve (S: specimen of 6.6 mm L, V: specimen of 6.6 mm L, W: specimen of 4.6 mm L), T, U: hinge plates left valve (T: specimen of 6.6 mm L, U: specimen of 4.6 mm L). Scale bars A‒ Q: 1 mm; R: 100 µm; S‒W: 500 µm.