Pisidium vile Pilsbry, 1897

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 : 42

Pisidium vile Pilsbry, 1897

Figures 7 , 8

Pisidium vile Pilsbry, 1897:292 , pl. 6, figs. 17–20.

Description: Shell small (maximum observed L= 2.85, average= 2.2 ± 0.33), high (I H/L varying among localities between 85 ± 2 and 89.2 ± 2.3), inflated (Ci varying among localities between 79 ± 4 and 82.3 ± 4.6). Beaks wide at base, inflated and high, well visible over dorsal margin, sub-central or slightly displaced posteriorly (located at about 57–60% of L). Shell outline trapezoid, anterior end slightly protruded, posterior end nearly straight, truncated. Dorsal margin short, markedly curve, ventral margin widely curve. Shell surface brilliant, white or yellowish, with somewhat regularly well marked commarginal striae ( Fig. 7A, B, D ). Hinge plate solid ( Fig. 7E, F ). Hinge line moderately long (Hil/L varying among localities between 56 ± 1.6 and 58 ± 3). Hinge: Left valve ( Fig. 7E ): inner cardinal tooth (C2) robust, with pointed cusp bent upward, outer cardinal (C4) slender, curved, covering C2 at posterior end; anterior (A2) and posterior (P2) lateral teeth robust with markedly displaced cusps. Right valve ( Fig. 7F ): cardinal tooth (C3) minute, straight at anterior half, enlarged at posterior end in a triangular cup; inner anterior (A1) and inner posterior (A1) laterals quite robust, outer lateral teeth (A3, P3) reduced in size but strong. Ligament pit enclosed, ligament slender, moderately long (Li= 20 ± 1.4), not visible from exterior. Escutcheon marked by a delicate line, anterior half slightly elevated, over the anterior half of ligament ( Fig. 7C ).

Anatomy: Mantle: Only the anal mantle opening present, siphon retractor muscles of the same conditions as in Pisidium sterkianum and Pisidium dorbignyi ( Fig. 8A, B ). Inner radial mantle muscles forming six strong, well defined, bundles, evenly distributed along the pedal slit. Muscle scars well defined only in anterior part of the shell where they are separate from the mantle line ( Fig. 8C ).

Gills: Only inner demibranch present. Developing brood pouch occupying the upper part of the inner demibranch, formed by 12 (in a 2.4 mm long specimen) to 17 (in a 2.75 mm long specimen) filaments ( Fig. 8D ). Between five and 11 embryos per pouch were found.

Nephridium: Of the closed type, with broad and rectangular dorsal lobe ( Fig. 8E, F ) and lateral loop visible from dorsal view.

Examined material: Uruguay: Syntypes ( ANSP 70489), Arroyo Miguelete, Montevideo (34°51'29”S, 56°10'15”W); Arroyo Miguelete, Montevideo ( MLP 5665). Argentina: Arroyo Miguelín, Ensenada (34° 52'33”S, 57° 52'56”W), Buenos Aires Province ( MLP 5662).

Remarks: The high trapezoidal shell outline, the quite solid hinge plate and the reduced size, clearly identifies Pisidium vile and separates it from other small South American species such as Pisidium dorbignyi (with a lower shell outline and less prominent beaks) and Pisidium punctiferum Guppy, 1867 (with a somewhat subquadrangular, flatter shell). It should be noted that the concentration of mantle muscles in Pisidium dorbignyi and Pisidium vile may be associated with smaller size, however, Pisidium punctiferun , a species of comparable or even smaller size, has not less than 8 bundles of inner radial mantle muscles (Korniushin, personal observation).

Figures

FIGURES 7 A – F. Pisidium vile. A, B. Syntypes (ANSP 70489). A. Right valve, lateral view. B. Posterior view. C – F. Specimens from Arroyo Miguelín (MLP 5665). C. Posterolateral view, detail of escutcheon. D. Right valve, lateral view. E. Left valve, inner view. F. Right valve, inner view. Scale bars: A, B, D – F = 500 µm; C = 200 µm.

FIGURES 8 A – F. Pisidium vile, anatomy. A, B. Lateral views of left (38) and right (39) inner mantle border showing siphonal and inner radial mantle muscles. C. Distribution of scars of inner radial mantle muscles. D. Inner view of inner demibranch showing position of developing brood sac. E. Left nephridium, dorsolateral view. F. Nephridia, dorsal view. (al: ascending lamellae; bs: brood sac; dl: descending lamellae; dln: dorsal lobe of nephridium; irm: inner radial mantle muscles; irms: scars of inner radial mantle muscles; ll: lateral loop of nephridium; pa: posterior adductor muscle; pl: pallial line; sr 1: anal siphon retractor; sr 2: retractor muscles corresponding to the lacking inhalant opening).