Philobrya delli, Urcola & Zelaya, 2024

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 : 168-169

Philobrya delli n. sp.

( Figure 10 )

Philobrya capillata . Dell, 1964: 170 (in part), fig. 2.8, pl. 4, fig. 9 [not text figs. 2.5, 2.9, pl. 4, figs. 4, 5 = P. capillata ].

Type locality: Isla de los Estados , 54°50.96’S 63°52.01’W, 330 m.

Type material: Holotype (MACN-In 44440) and five paratypes from the type locality (4 paratypes MACN-In 44441; 1 paratype MLP-Ma 16118).

Zoobank registration number: 01B0FCD6-0233-493C-A723-9123E433C416.

Other material examined: Burdwood Bank: 53°51.50’S 61°51.50’W, 368‒463 m ( NHMUK 1962868 : 2 v) ; 54°23.82’S 58°30.53’W, 135 m (MACN-In 44442: 2 v).

Distribution: Isla de los Estados and Burdwood Bank; 135‒ 463 m.

Etymology: The species is named after Richard Kenneth (“Dick”) Dell (1920–2002), in recognition for his first input to the knowledge of sub-Antarctic and Antarctic philobryids.

Diagnosis: Shell mytiliform, delicate, with the anterior portion of dorsal margin short. Dissoconch sculptured with 20‒24 strong radial ribs. Inner shell surface and margins strongly crenulated. Resilifer narrow, elongate.

Description: Shell large (maximum L = 9.6 mm), mytiliform, relatively high (H/L = 0.84 ± 0.03, n = 3), inflated (W/H = 0.80 ± 0.08, n = 3), delicate; with a narrow, ill-defined dorsal slope. Umbos broad, well-outstanding from shell outline, located at anterior end. Umbonal angle: 90‒100° (n = 6). Dorsal margin with the anterior portion short, straight, vertical, and the posterior portion arched, forming a continuous curve with the posterior margin. Ventral margin slightly sinuated by byssal notch.

Prodissoconch D-shaped, with posterior end higher than anterior end, widely projected ventrally, of 625‒650 µm Lp (n = 6), separated from the dissoconch by a prominent rim. Antero-dorsal and postero-dorsal areas raised, sculptured with prominent commarginal folds. Central area with radial sculpture (but badly preserved in the available material).

Dissoconch whitish, dull. Sculpture consisting of 20‒24 strong radial ribs and irregular but well-marked growth lines.

Periostracum unknown (absent in the available material).

Inner shell surface with 22‒24 prominent folds, radiating from the umbo to shell margins, where they produce strong crenulations. All radial folds and crenulations similar in strength. Posterior adductor muscle scar located in the posterior third of the valves, dorsally displaced.

Hinge: No G1b teeth were observed in the size range studied (6.0‒ 9.6 mm). Only in one 7.4 mm specimen were small pustules observed at the distal end of the posterior series. Resilifer long and narrow.

Remarks: Dell (1964) pointed out some differences in shell outline and prodissoconch sculpture between one of the specimens from Malvinas / Falkland Islands he recognized as paratype, and the holotype of Philobrya capillata . The finding of additional specimens with identical characteristics to that paratype allows us to describe (herein) this entity as a new species. Philobrya delli n. sp. differs from P. capillata ( Fig. 9 ) by having fewer (20‒24 vs. 32‒40) and wider radial ribs, a larger prodissoconch (625‒650 µm vs. 470‒575 µm L), and stronger radial folds and marginal crenulations on the inner shell surface.

Philobrya delli n. sp. is an easily recognizable species due to the strong radial sculpture of the dissoconch and short anterior portion of dorsal margin. No other species of Philobrya from the studied area shows these characteristics (Appendix 1).

Figures

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 10. Philobrya delli n. sp. A, B: holotype (MACN-In 44440), C‒K: specimens from Burdwood Bank (C, D, G, H: NHMUK 1962868, E, F, I‒K: MACN-In 44442). A, C: outer views right valve (without periostracum), B, D, E: outer views left valve (without periostracum), F, H: inner views left valve, G: inner view right valve, I: detail of prodissoconch, J: hinge plate right valve (specimen of 6.9 mm L), K: hinge plate left valve (specimen of 8.5 mm L). Scale bars A‒H: 1 mm; I: 100 µm; J, K: 500 µm.