Pisidium sterkianum Pilsbry 1897
Creators: name: Ituarte, Cristián
name: Korniushin, Alexei Victor
Publisher: name: Zenodo
Year: 2006
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Veneroida
Sphaeriidae
Pisidium
Pisidium sterkianum

description: Pisidium sterkianum Pilsbry, 1897
Figures 5, 6
Pisidium sterkianum Pilsbry, 1897:291, pl. 6, figs. 1–4.
Description: Shell medium to large (maximum L: 6,1 mm), somewhat high (I H/L= 81.9 ± 1.8), somewhat inflated (Ci= 69.3 ± 5,4) (Fig. 5B). Shell outline ovate, dorsal margin shorter than ventral margin; ventral margin evenly and widely curved. Anterior end produced, posterior end somewhat truncated. Beaks full, posteriorly displaced (located at about 58% of shell length) well visible above dorsal margin. Shell surface fine and regularly striated, straw-yellowish (Fig. 5A, C). Hinge plate not solid, stretching markedly at sides of cardinal teeth. Hinge line widely curve, moderately long (Hil/L= 58 ± 2.7) (Fig. 5D). Hinge: Right valve: cardinal tooth (C3) enlarged at posterior end in a slightly sulcated cup; right lateral teeth well developed, the inner anterior (A1) a long broadly curved lamella, cusp low, sub-central or slightly displaced forward, the outer anterior (A3) shorter with cusp distal. Posterior lateral teeth (P1 and P3) nearly straight. Left valve (Fig. 5D): inner cardinal tooth (C2) short, bent upward; outer cardinal (C4) slightly longer than C2, slender, evenly arcuate, overlapping C2 at posterior half. Anterior left lateral strong, high, cusp triangular shifted distally; posterior lateral (P2) reduced in size, with distal cusp. Ligament external, protruded, representing about 19.7 ± 2.7% of shell length.
Anatomy: Mantle: Only anal mantle opening present. Pedal slit very long, its posterior end correspond to the branchial opening. Two well developed pairs of siphon retractors are noticeable; the upper retractors belonging to the anal siphon (Fig. 6A, B: sr1), the lower ones correspond to the region of the branchial opening (lacking in this species) (Fig. 6A, B: sr2), the lower retractor fibers are connected with the pre-siphonal suture (Fig. 6A, B: pss), between the siphon and the pedal slit. Inner radial mantle muscles long but rather thin, weak; posterior fibers converge to the lower siphon retractors, the other fibres form 6 to 7 bundles evenly arranged along the pedal slit. Bundles of the anterior and middle portion of pedal slit are equally developed. Scars of the inner radial muscles are small and usually not clear; when defined, they are separate from the pallial line (Fig. 6C).
Nephridium: Of closed type, the pericardial duct is not visible as it is covered by the dorsal lobe (Fig. 6E); when viewed from dorsal side the organ is S-shaped because a part of the lateral loop is always visible, dorsal lobe rectangular; in some specimens, a small portion of the pericardial duct is visible from dorsal side (open type of nephridium) (Fig. 6D).
Gill: Only inner demibranch present. Brood pouch developing from thickenings in the dorsal portions of descending filaments (Fig. 6F). On later stages of development, brood pouch occupies the greatest part of the gill (Fig. 6G). The number of filaments involved in the formation of the brood pouch depends on the size of the maternal individual, varying from 13 (in a 4.9 mm long specimen) to 20 (in a 5.2 mm length specimen). The number of embryos per brood pouch varied from 7 to 15.
Material examined: Uruguay: Syntypes (ANSP 70490) Arroyo Miguelete, Montevideo (34°51'29”S, 56°10'15”W); Parque Arequita, Minas (34°22'12”S, 55°13'30”W), Lavalleja Department (MLP 5343); unnamed brook in the neighbors of Canelones City, Canelones Department (MHNM 5602). Argentina: Miguelín Rivulet, Ensenada, Buenos Aires Province (MLP 5058), 4 specimens dissected.
Remarks: Pisidium sterkianum is easily identified by having an external ligament, clearly protruded, a unique feature among South American species. This only feature differentiate P. sterkianum from similar species such as Pisidium forense Meier Brook, 1968, Pisidium pipoense Ituarte, 2000 and Pisidium taraguyense Ituarte, 2000.
description: Published as part of Ituarte, Cristián & Korniushin, Alexei Victor, 2006, Anatomical characteristics of two enigmatic and two poorly known Pisidium species (Bivalvia: Sphaeriidae) from Southern South America, pp. 33-47 in Zootaxa 1338 on pages 39-41, DOI: 10.5281/zenodo.174347