Philobrya quadrata
Creators: name: Urcola, Matias Ricardo
name: Zelaya, Diego Gabriel
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Arcida
Philobryidae
Philobrya
Philobrya quadrata

description: Philobrya quadrata (Pfeffer, 1886)
(Figure 15)
Philippiella quadrata Pfeffer in Martens & Pfeffer, 1886: 119, 120, pl. 4, figs. 6a, b.
Philobrya quadrata. Dell 1964: 167, 168, text fig. 2.6.
Philippiella ungulata. Pfeffer in Martens & Pfeffer, 1886: 120, 121, pl. 4, fig. 7; Soot-Ryen, 1951: 11.
Type locality: South Georgia, 54°30’S 35°50’W, 94 m [here restricted].
Material examined: South Georgia: 54°30’S 35°50’W, 94 m (MACN-In 44471: Neotype, here designated; MACN-In 44472: 1 spec). Burdwood Bank: 54°16.67’S 59°57.79’W, 96 m (MACN-In 44473: 2 spec, 2 v); 54°23.32’S 59°41.18’W, 65 m (MACN-In 44474: 7 spec, 4 v); 54°24.21’S 59°32.49’W, 86 m (MACN-In 44475: 6 spec, 7 v).
Distribution: Only known with certainty from South Georgia (Pfeffer in Martens & Pfeffer 1886; Soot-Ryen, 1951; Dell 1964; this study) and Burdwood Bank (this study); 18‒ 94 m. Philobrya quadrata was also mentioned from some other sub-Antarctic and Antarctic localities, although the identity of this material deserves further studies (see Remarks section).
Description: Shell small (maximum L = 5.3 mm), ovate, high (H/L = 0.93 ± 0.04, n = 10), compressed (W/H = 0.56 ± 0.06, n = 10), delicate; with a narrow, depressed dorsal slope. Umbo bluntly pointed, located at anterior end, only slightly outstanding from shell outline in smaller specimens but prominent, bluntly pointed and recurved in larger specimens. Umbonal angle: 85‒95° (n = 11). Dorsal margin with the anterior portion long, vertically straight to somewhat arched, and the posterior portion flatted to curved, forming a continuous curve with the posterior margin. Ventral margin almost straight in small-sized specimens to markedly sinuate in larger specimens.
Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 315‒385 µm Lp (n = 4), raised from dissoconch. Antero-dorsal and postero-dorsal areas raised, sculptured with densely packed commarginal folds. Central area sculptured with microscopic, ovate pits, forming quincuncial pattern.
Dissoconch whitish, shiny; smooth in appearance, but microscopically sculptured with 35‒45 faint radial striae and low growth marks.
Periostracum thick, yellowish straw, extending up to 1 mm beyond the calcified shell margin; forming low, irregular commarginal folds, not projected into 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 160‒165° (n = 5). Distal teeth of both series partially disintegrated, forming pustules, in specimens of 1.9 mm L. Teeth and pustules completely lost in specimens longer than 3.0 mm. Resilifer long and narrow.
Remarks: Until now, there have been great difficulties to properly understand the concept of Philobrya quadrata. The poor original description and illustration of this species (here reproduced in Fig. 15A, I) and the fact that its type material has remained missing since the beginning of the 20 th century (Thiele 1912; Dell 1964) indubitably contributed to this fact. In addition, good series of specimens were never available (Dell 1964). All these issues led to consider P. quadrata as a species of uncertain status (Engl 2012). As part of the present study, we have had access to topotypic specimens of P. quadrata, as well as a series of additional specimens from Burdwood Bank. This material allowed us to properly redescribe the species. Based on topotypic specimens, we here designate a neotype for P. quadrata, with the express purpose to fix the species concept (Fig. 15B, C).
Together with Philobrya quadrata, Pfeffer in Martens & Pfeffer, 1886, described another philobryid species from South Georgia: Philippiella ungulata (Fig. 15J). The author distinguished these two species based on few and subtle morphological characters: 1) the subquadrate (in P. quadrata) vs. subcircular (in P. ungulata) shell shape; 2) the slightly concave (in P. quadrata) vs. strongly concave (in P. ungulata) ventral margin and 3) the slightly arched (in P. quadrata) vs. markedly recurved (in P. ungulata) umbo. However, when considering a series of specimens, the above-mentioned “differences” appears as part of the intraspecific variability. In fact, the umbo becomes more prominent and “hooked” in larger specimens, associated with an increment of the ventral margin concavity and the deepening of the byssal notch. In fact, Philobrya ungulata also exhibits the diagnostic characters mentioned above for P. quadrata. Soot-Ryen (1951, p. 11) already commented that these two philobryids are closely related “and perhaps only forms of one species”, he named as Philobrya ungulata. Dell (1964) ratified this synonymy although referring to the species as P. quadrata. Dell (1964) should be regarded as the first revisor (in the sense of ICZN Article 24.2), and consequently P. quadrata should be regarded as the name to apply to the species.
The ovate shell outline with prominent umbo, the dissoconch only sculptured with faint radial striae and the periostracum only showing low commarginal folds make Philobrya quadrata an easily recognizable species.
Philobrya quadrata resembles P. magellanica (Fig. 12), although the latter shows narrower and markedly pointed umbo, larger umbonal angle (100‒115° vs. 85‒95°, respectively) and the periostracum with commarginal and radial folds (Appendix 1).
Melvill & Standen (1907) identified as Philobrya quadrata specimens collected in South Orkney. However, these specimens (NMS Z 1921.143.699) actually correspond to P. olstadi.
Hain (1990) identified as Philobrya cf. quadrata specimens from the Weddell Sea. However, he described the dissoconch sculptured with radial ribs overlapping the commarginal sculpture. Furthermore, the specimens he figured (Hain 1990: 84, pl. 12, figs. 3a, b) have a wider umbo, straighter ventral margin, uniformly arched dorsal margin, different from the South Georgia and Burdwood Bank specimens studied herein. These differences prevent us from considering these specimens conspecific with the neotype of P. quadrata. Alternatively, Engl (2012) suggested that Hain’s record “can be referred to Philobrya tumida ”.
Dell 1964 and Branch et al. (1991) also mentioned Philobrya quadrata from Marion Island and Moreau et al. (2013) from the Amundsen Sea, although the identity of this material deserves further study.
description: Published as part of Urcola, Matias Ricardo & Zelaya, Diego Gabriel, 2024, The Genus Philobrya J. G. Cooper, 1867 (Bivalvia: Philobryidae) In Patagonia And Adjacent Antarctic Waters, pp. 151-192 in Zootaxa 5437 (2) on pages 178-180, DOI: 10.11646/zootaxa.5437.2.1, http://zenodo.org/record/10984619