Philobrya cf. barbata Thiele, 1912

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 : 159-161

Philobrya cf. barbata Thiele, 1912

( Figure 5B‒U )

Material examined: Burdwood Bank: 54°23.32’S 59°41.18’W, 65 m (MACN-In 44416: 45 spec, 9 v).

Description: Shell medium-sized (maximum L = 6.4 mm), ovate (in small specimens) to mytiliform (in larger specimens), relatively high (H/L = 0.85 ± 0.04, n = 20), somewhat inflated (W/H = 0.68 ± 0.03, n = 20), solid; with a narrow, depressed dorsal slope. Umbos broad, bluntly pointed, anteriorly located, ventrally directed; only slightly outstanding from shell outline in smaller specimens, but prominent and markedly recurved in larger specimens. Umbonal angle: 90‒100° (n = 10). Dorsal margin with the anterior portion relatively long and straight, nearly vertically, and the posterior portion broadly curved in small-sized specimens but flattened in larger specimens. Dorsal margin forming a continuous curve with the posterior margin. Ventral margin nearly straight in smaller specimens, becoming increasingly concave with growth, due to the widening of the byssal notch.

Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 465‒485 µm Lp (n = 15), raised from dissoconch; sculptured with low and wide, regularly distributed, closely packed commarginal cords.

Dissoconch whitish, dull. Sculpture cancellate, consisting of 12‒15 thin radial ribs, and numerous, moderately separated commarginal cords. Commarginal sculpture weaker than radial sculpture.

Periostracum very thick, straw-yellowish, extending up to 1 mm beyond calcified shell margin; forming fine radial folds, raised commarginal lamellae, and extremely long, thin setae. Commarginal lamellae extending up to the tip of the setae, but forming a deep sinuation between setae.

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 165‒170° (n = 5). All teeth in both series clearly recognizable in specimens up to 1.3 mm L. Distal teeth of both series partially disintegrated, forming pustules, in specimens of 1.7‒3.0 mm L. Teeth and pustules almost completely lost in specimens longer than 5.5 mm. Resilifer long, moderately (in smaller specimens) to very wide (in larger specimens).

Remarks: Thiele (1912) described Philobrya barbata from Observatory Bay [49°25’S 69°53’E] in the Kerguelen Islands, and Hedley (1916) described Philobrya hamiltoni (under Philippiella ) from Lusitania Bay [54°41’S 158°54’E] and Nuggets [54°31’S 158°57’E], in the Macquarie Island. Engl (2012: 46, pl. 5, fig. 6) designated a lectotype for P. barbata . As part of this study, we had the opportunity to examine photographs of five syntypes of Philippiella hamiltoni (three from Lusitania Bay, AM C.46560, and two from the Nuggets, AM C.47253; one of them is shown in Figure 5A ). The great similarity between the types of Philobrya barbata and P. hamiltoni is unquestionable.

A series of specimens from the Burwood Bank studied herein, also appear similar to the types of Philobrya barbata and P. hamiltoni by having mytiliform shape and extremely long periostracal setae. However, the Burdwood Bank specimens show thin radial sculpture on the dissoconch, a character not mentioned by Thiele (1912) or Hedley (1916) for the species they described. In addition, the Burdwood Bank specimens have a higher shell outline and less projected and wider umbo than the Macquarie Island specimen figured by Hedley (1916: figure 8). The above-mentioned differences suggest that the Burdwood Bank specimens could correspond to a new species. However, due to the imperfect knowledge of P. barbata and P. hamiltoni , we prefer to postpone introducing a new name for this entity until the identity of the other two taxa is properly clarified.

Due to the mytiliform shape and hairy periostracum, Philobrya cf. barbata also resembles P. capillata ( Fig. 9 ), P. olstadi ( Fig. 14 ) and the large specimens of P. blakeana ( Fig. 6 ) (see comparisons under those species and Appendix 1).

Figures

FIGURE 5. Philobrya hamiltoni and Philobrya cf. barbata. A: syntype of P. hamiltoni (AM C.46560), B‒U: Philobrya cf. barbata, specimens from Burdwood Bank (MACN-In 44416). A, D: outer views left valve (A: with periostracum, D: without periostracum), B, C, E‒H, O: outer views right valve (B: with periostracum, C, E‒H, O: without periostracum), I‒L, N: inner views right valve, M: inner view left valve, P: detail of prodissoconch, Q‒S: hinge plates left valve (Q: specimen of 4.7 mm L, R: specimen of 5.5 mm L, S: specimen of 1.7 mm L), T, U: hinge plates right valve (T: specimen of 1.7 mm L, U: specimen of 5.5 mm L). Scale bars A‒N: 1mm; O, Q‒U: 500 µm; P: 100 µm.

FIGURE 6. Philobrya blakeana. A, I: lectotype of Brachidontes (Hormomya) blakeanus (MMUM EE 7674), B, J‒L, O, P: specimens from Tierra del Fuego (B: MACN-In 44420, J, K, O, P: MLP-Ma 16106, L: MLP-Ma 16108), C‒H, M, N: specimens from Burdwood Bank (MACN-In 44422). A‒C, E‒H: outer views right valve (A, B: with periostracum, C, E‒H: without periostracum), D: outer view left valve (without periostracum), I, J: inner views right valve, K: inner view left valve, L: brooded embryo removed from the demibranch of an adult specimen, M‒O: hinge plates right valve (M: specimen of 7.7 mm L, N: specimen of 5.2 mm L, O: specimen of 3.5 mm L), P: hinge plate left valve (specimen of 3.5 mm L). Scale bars A‒K: 1 mm; L: 100 µm; M‒P: 500 µm.

FIGURE 9. Philobrya capillata. A, B: holotype (NHMUK 1962867), C‒F: specimens from South Georgia (MACN-In 44439). A, C: outer views right valve (with periostracum), B: inner view right valve, D: detail of prodissoconch, E: hinge plate left valve (specimen of 4.5 mm L), F: hinge plate right valve (specimen of 4.4 mm L). Scale bars A‒C: 1 mm; D: 100 µm; E, F: 500 µm.

FIGURE 14. Philobrya olstadi. A, I: holotype (NHM.UIO D 28607), B‒H, J‒T: specimens from South Orkney (B‒G, J‒ T: MACN-In 44469, H: NMS Z 1921.143.699). A‒F: outer views right valve (A, E: with periostracum, B‒D, F: without periostracum), G, H: outer views left valve (G: without periostracum, H: with periostracum), I‒L, N: inner views right valve, M: inner view left valve, O: detail of prodissoconch, P, S, T: hinge plates left valve (P: specimen of 5.5 mm L, S: specimen of 5.8 mm L, T: specimen of 3.7 mm L), Q, R: hinge plates right valve (Q: specimen of 3.7 mm L, R: specimen of 5.8 mm L). Scale bars A‒N: 1 mm; O: 100 µm; P‒T: 500 µm.