Pisidium lebruni Mabille, 1884

Ituarte, Cristián & Korniushin, Alexei Victor, 2006, Anatomical characteristics of two enigmatic and two poorly known Pisidium species (Bivalvia: Sphaeriidae) from Southern South America, Zootaxa 1338, pp. 33-47 : 37-38

publication ID	https://doi.org/ 10.5281/zenodo.174347
DOI	https://doi.org/10.5281/zenodo.6260765
persistent identifier	https://treatment.plazi.org/id/03CF87D4-FF99-0E5C-FE84-4BB36E69E875
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scientific name	Pisidium lebruni Mabille, 1884
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Figures 3 View FIGURES 3 A – C , 4 View FIGURES 4 A – D

Pisidium lebruni Mabille, 1884: 47 . Ituarte, 1995: 34, figs. 8–11, 25.

Description: Shell oval, high (I H/L= 85 ± 2), medium to large size (maximum observed L = 5.35 mm), not inflated (Ci = 59 ± 5). Dorsal and ventral margins markedly curved, the anterior margin slopes evenly to the anterior end, which is slightly projected. Posterior end short, rounded, somewhat oblique in adults, markedly truncated in smaller specimens ( Fig. 3A, C View FIGURES 3 A – C ). Beaks wide at base, very low, slightly projected over dorsal margin, located at 57 ± 1.4 % of L ( Fig. 3A, C View FIGURES 3 A – C ).

Hinge plate moderately strong, hinge line short (Hil/L= 53 ± 5) and markedly curve. Ligament pit enclosed, short, deep; ligament internal, long (Li= 21 ± 2), not visible from exterior except from a quite slender gap. Hinge: left valve: cardinal teeth thin, the outer (C4) overlapping C2 at the posterior third; anterior (A2) and posterior (P2) laterals strong, short, cusps sub-central. Right valve: cardinal tooth (C3) delicate, anterior end slender, curved, shorter than the posterior end which is slightly expanded in a triangular sulcated cup; anterior and posterior lateral teeth well developed, the inners (A1 and P1) solid, long, with slightly displaced cusps; the outer laterals (A3 and P3) strong but reduced in size ( Fig. 3B View FIGURES 3 A – C ).

Anatomy: Mantle: only anal mantle opening present. Inner radial mantle muscles weak; 7–8 bundles of few fibres were observed ( Fig. 4A View FIGURES 4 A – D ). Scars of the inner radial mantle muscles, slightly visible, merged with or immediately over the pallial line.

Nephridium: Lateral loop visible from dorsal view, dorsal lobe of nephridium subquadrangular, proximal part of the nephridium, the pericardial duct, usually not visible (closed type) from dorsal side ( Fig. 4A, B, C View FIGURES 4 A – D right), except in few cases (open type of nephridium) ( Fig. 4C View FIGURES 4 A – D left).

Gill: Only inner demibranch present ( Fig. 4A, D View FIGURES 4 A – D ). Brood sac developing from descending lamellae in upper position ( Fig. 4D View FIGURES 4 A – D ).

Type locality: Punta Arenas (53°9'S, 70°55'0W), Patagonia [ Chile].

Material examined: Lectotype, here selected, and, 73 paralectotypes, loose valves, all from the type locality ( MNHN, Paris); 7 alcohol preserved specimens from Río de La Mano, Punta Arenas [ Chile] ( MNHN, Paris).

Remarks: Pisidium lebruni is easily distinguished from other Patagonian species by its large size, markedly oval shell outline and comparatively flat shell. P. lebruni is similar to Pisidium inacayali Ituarte, 1996 from which it differs in having a more equilateral shell, less projected anterior margin, with a higher shell outline. Both species are similar in soft anatomy. Pisidium llanquihuense Ituarte, 1999 , a species from Chilean Patagonia is also a large Pisidium species that differs from P. lebruni by its decidedly sub-quadrangular shell outline. It is to be noted that smaller specimens of P. lebruni (about 3.5 mm length) are remarkably similar in shell outline to Pisidium magellanicum , a species only known from a single, not well preserved left shell, washed into the sea at Beagle Channel near Punta Arenas, Chile, the type locality of P. lebruni . Since Mabille description ( Mabille, 1884), Pisidium lebruni was not reported or cited until 1995 ( Ituarte, 1995). The MNHN, Paris, houses a alcohol preserved sample whose label reads: “Río de la Mano [Punta Arenas], Mission Lebrun, 1833” that likely corresponds to the same collecting event that the syntypes and was a part of the series upon which P. lebruni was described. Soft anatomy described here was based on these specimens.