Philobrya sublaevis Pelseneer, 1903

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 : 181-184

Philobrya sublaevis Pelseneer, 1903

( Figure 17 )

Philobrya sublaevis Pelseneer, 1903: 25 , 26, 42, 43, pl. 7, figs. 93‒95, pl. 8, figs. 96‒99; Nicol 1966 (in part): pl. 4, fig. 7 [not pl. 4, fig. 5 = P. limoides ]; M̧hlenhardt-Siegel 1989: 159, pl. 2, fig. 13 and pl. 5, fig. 13.

Type locality: Bellingshausen Sea, 64°24.7’S 64°19.2’W, 165 m.

Material examined: Neotype (here designated; MACN-In: 44476). Antarctic Peninsula : Bahía Paraíso , [64°51’S 62°54’W] (MACN-In 35081: 2 spec). South Shetland: 61°26’S 56°25’W, 350 m (MACN-In 35281: 1 spec). South Orkney: 60°58.89’S 43°26.71’W, 399 m (MLP-Ma 16134: 1 spec); 60°53’S 44°33’W, 218 m (MACN-In 44477: 1 spec). South Sandwich: 59°54.79’S 32°28.93’W, 518 m (MLP-Ma 16133: 5 spec, 2 v).

Distribution: Antarctica, from Bellingshausen Sea ( Pelseneer 1903; this study) to Antarctic Peninsula ( Nicol 1966), South Shetland (M̧hlenhardt-Siegel 1989), South Orkney (this study), and South Sandwich (this study); 165‒ 550 m.

Description: Shell medium-sized (maximum L = 9.0 mm), ovate, high (H/L = 0.96 ± 0.02, n = 18), compressed (W/H = 0.60 ± 0.05, n = 8), delicate; dorsal slope ill-defined. Umbo broad, almost at anterior end, rounded, only slightly outstanding from shell outline in smaller specimens, but projected in larger ones. Umbonal angle: 105‒ 120° (n = 8). Dorsal margin with the anterior portion long, straight, markedly obliquely forward and the posterior portion arched, forming a continuous curve with the posterior margin. Ventral margin short, sinuous all along the ontogeny.

Prodissoconch D-shaped, with posterior end higher than anterior end, markedly convex ventrally, of 700‒ 780 µm Lp, separated from the dissoconch by a prominent rim. Antero-dorsal and postero-dorsal areas poorly differentiated, sculptured with commarginal folds. Central area sculptured with 50‒54 fine radial ribs. Some ribs extending all along; other (shorter) ribs appearing exclusively distally, radiating or intercalated to the previous ones.

Dissoconch whitish, shiny. Sculpture cancellate, consisting of 10‒19 extremely faint radial ribs and delicate, regularly distributed commarginal treads.

Periostracum thin, pale yellow, extending up to 1 mm beyond calcified shell margin; forming delicate radial folds, raised commarginal lamellae, and long, thin setae. Commarginal lamellae extending up to the tip of the setae, usually with straight margin between contiguous setae, sometimes forming a shallow sinus.

Inner shell margin weakly crenulated, with 5‒8 prominent folds radiating from the umbo to the posterior portion of dorsal margin. Posterior adductor muscle scar located in the anterior half of the valves, dorsally displaced.

Hinge: Anterior and posterior series of G1b teeth forming an angle of 165‒170° (n = 8). All teeth in both series well-recognizable in specimens up to 2.3 mm L. Distal teeth of the posterior series partially disintegrated, forming pustules, in specimens longer than 4.6 mm. Distal teeth of the anterior series also forming pustules in specimens longer than 6.9 mm. Teeth and pustules completely lost in specimens of 9.0 mm. Resilifer long and narrow.

Remarks: The concept of Philobrya sublaevis has been largely misunderstood. The species was originally described from the Bellingshausen Sea at 550 m depth, based on a small-sized (4.5 mm L) specimen, whose type material is considered to be lost (Yves Samyn, in litt., August 2018). Several authors have used Pelseneer’s name to refer to specimens collected in the vicinity of the Antarctic Peninsula and the Scotia Arc islands, even when recognizing that their materials differs from Pelseneer’s (1903) description (e.g., Soot-Ryen 1951). Two other Antarctic species, Philobrya limoides Smith, 1907 , and Philippiella bagei Hedley, 1916 , were included by Dell (1964) in the synonymy of P. sublaevis . This synonymy was accepted and followed by all subsequent authors. However, the present study of a series of specimens from Bellingshausen Sea and the Scotia Arc islands, and the type material of Philobrya limoides and Philippiella bagei reveals that P. sublaevis is actually distinct from both, P. limoides and P. bagei . In order to stabilise the species concept, a neotype for P. sublaevis is here designated ( Fig. 17G, H ). The neotype comes from the same locality as the specimens described by Pelseneer (1903).

Philobrya sublaevis has a short ventral margin, with a deep sinus just behind the umbo, while in P. limoides ( Fig. 11 ) the ventral margin is long and straight, a condition that remains constant throughout ontogeny. In addition, the anterior portion of the dorsal margin is obliquely forward in P. sublaevis and vertical in P. limoides , thus resulting in a wider umbonal angle, posterior arch and higher profile in P. sublaevis than in P. limoides . Even when these two species show cancellate dissoconchs, the radial ribs and commarginal cords are weaker and the commarginal elements more regular in P. sublaevis than in P. limoides . Furthermore, P. sublaevis has internal radial folds (which are absent in P. limoides ), a greater number of radial ribs in the prodissoconch (50‒54 vs. 42‒47), and thinner periostracum.

Most of the bibliographic records assigned to “ P. sublaevis ” do not actually correspond to that species but to P. limoides (see remarks section of that species). In fact, only one of the figures provided by Nicol (1966: fig. 7) and one by M̧hlenhardt-Siegel (1989: fig. 13) may be attributed with certainty to P. sublaevis .

Dell (1964) also included Philippiella orbiculata Hedley, 1916 , in the synonymy of P. sublaevis . However, the syntypes of the former ( Fig. 7G‒L ) reveal a more subquadrate shell outline and higher and wider umbo than in P. sublaevis . In addition, P. orbiculata has a greater number of radial ribs (18‒27 vs. 10‒15) and thicker and darker periostracum. These differences, also recognizable in the specimen figured by Dell (1990: fig. 50, as “ P. sublaevis ”) suggest that P. orbiculata should be regarded as a different, valid species.

Other morphologically similar species to P. sublaevis are P. multistriata ( Fig. 13 ) and P. magellanica ( Fig. 12 ). However, P. multistriata differs from P. sublaevis by having a stronger dissoconch sculpture, shorter periostracal setae and larger prodissoconch (850‒950 µm L vs. 670‒750 µm L) and a lower number of radial elements (9‒13 vs. 50‒54, respectively). In addition, the resilifer of P. multistriata is short and wide, while that of P. sublaevis is long and narrow. Philobrya magellanica differs from P. sublaevis by having a narrower umbo, wider resilifer, a micropitted rather than radially sculptured prodissoconch and a periostracum without setae and commarginal lamellae (Appendix 1).

Huber (2010) interpreted Philobrya magellanica as a junior synonym of P. antarctica ( Philippi, 1868) . This opinion is not shared herein, where Philippi’s species is regarded as a nomen dubium (see below).

Figures

FIGURE 7. Philobrya spp. A‒D: syntype of Mytilus kerguelensis (NHMUK 1887.2.9.3156-7), E, F: paralectotype of Philobrya tumida (ZMB-Moll 63095b), G‒L: syntypes of Philippiella orbiculata (G, H: AM C.4651, I‒L: AM C.46522). A, E, G, I: outer views right valve (with periostracum), B, H, J, L: outer views left valve (with periostracum), C, K: inner views left valve, D, F: inner view right valve. Scale bars: 1 mm.

FIGURE 11. Philobrya limoides. A, B: syntypes of P. limoides (NHMUK 1905.7.10.73-82), C: syntype of Philippiella bagei (AM C.46517), D, O, P: specimens from South Orkney (D: MACN-In 35245, O, P: MACN-In 44444), E, F, H‒L, N, Q‒T: specimens from Puerto Melchior (MACN-In 35181), G, M: specimen from Bahía Paraíso (MACN-In 35081). A, D, E: outer views left valve (A, D: without periostracum, E: with periostracum), B, J: inner views left valve, C, F‒I: outer views right valve (C: with periostracum, F‒I: without periostracum), K‒N: inner views right valve, O: brooded embryo removed from the demibranch of an adult specimen, P‒R: hinge plates left valve (P: specimen of 10.2 mm L, Q: specimen of 6.4 mm L, R: specimen of 2.7 mm L), S, T: hinge plates right valve (S: specimen of 6.4 mm L, T: specimen of 2.7 mm L). Scale bars A‒N: 1 mm; O: 100 µm; P‒T: 500 µm.

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 13. Philobrya multistriata. A, I: syntype (MNHN-IM-2000-34158), B, G, H, N: specimens from Tierra del Fuego (MLP-Ma 7459), C‒F, J, K, O‒Q: specimens from Burdwood Bank (C‒F, Q: MACN-In 44468, J, K, O, P: MLP-Ma 7460), L, M: specimen from Isla de los Estados (MACN-In 44466). A‒G: outer views right valve (A, B: with periostracum, C‒G: without periostracum), H, L: outer view left valve (without periostracum), I, J: inner views left valve, K: inner view right valve, M: detail of prodissoconch (slightly tilt), N, O, Q: hinge plates right valve (N: specimen of 5.1 mm L, O: specimen of 3.4 mm L, Q: specimen of 7.1 mm L), P: hinge plate left valve (specimen of 3.4 mm L). Scale bars A‒K: 1 mm; L, N‒Q: 500 µm; M: 100 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.