Anatomical characteristics of two enigmatic and two poorly known Pisidium species (Bivalvia: Sphaeriidae) from Southern South America Author Ituarte, Cristián Author Korniushin, Alexei Victor text Zootaxa 2006 1338 33 47 journal article 10.5281/zenodo.174347 3d71c6df-0cab-440a-82fc-13fc024f855c 1175-5326 174347 Pisidium dorbignyi Clessin, 1877 Figures 1 , 2 Cyclas pulchella d´Orbigny, 1835: 44; 1846:568, pl.83 figs. 8–10. Pisidium dorbignyi Clessin, 1877 : 62 ( nomen novum pro Cyclas pulchella d´Orbigny, 1835). Sphaerium pulchellum (d´Orbigny), Formica Corsi, 1901: 442. Pisidium dorbignyi . Pilsbry, 1911 : 603 . Description: Shell thin, small (maximum observed size= 3.3 mm ), ovate, moderately high (I H/L= 85±2, n= 10; inflated (Ci = 78±4, n= 10), inequilateral, anterior end protruded in a sharp curve, posterior end short, rounded, truncate. Beaks moderately full, posteriorly displaced, located at 58±2% of shell length ( Fig. 1A, B, E ). Posterior half of dorsal margin slightly arcuate, nearly straight at anterior half. The points of connection of dorsal margin with anterior and posterior margins slightly marked by an angle. Hinge plate narrow, hinge line relatively long (Hil represents about 58% of L). Hinge: left valve: inner left cardinal tooth (C2) short, strong; outer left cardinal (C4) extremely thin, long, faintly curved, overlapping C2 at least at the posterior half. Left lateral teeth (A2, P2) well developed, strong but low ( Fig. 1D ). Right cardinal tooth (C3) weak, anterior half straight, posterior end enlarged and slightly sulcated. Anterior right laterals just below the angle between anterior and dorsal margins, outer anterior (A3) and outer posterior (P3) lateral teeth strongly reduced in size, with distal cusps ( Fig. 1F ). Ligament pit enclosed, slender. Ligament internal, long, representing about 23% of L, visible from the exterior through an elongate and quite slender gap, but never protruding. Escutcheon obscure, marked by a delicate line, slightly elevated directly over the ligament ( Fig. 1C ). Shell surface sculptured with low, regularly spaced commarginal striae. Anatomy: Mantle : only the anal siphon present. Pedal slit shortened, its posterior region corresponds to the branchial opening. Two well developed pairs of siphonal retractor muscles, the lower ones, relatively long, correspond to the retractor muscles of the branchial opening (present in species of Pisidium s. s ). Inner radial mantle muscles well developed, rather strong and concentrated, bundles reduced in number (5 to 6) and evenly arranged ( Fig. 2A, B ). Muscle scars obscure, separate from the mantle line, particularly the anterior scars ( Fig. 2C ). Gill : Outer demibranch absent. Developing brood pouch in upper position, including 12 to 18 filaments. Number of embryos ranged from 8 to 22 ( Fig. 2D ). Nephridium: Of the closed type (pericardial duct completely covered by the dorsal lobe of nephridium). Dorsal lobe triangular in smaller specimens, rectangular in larger ones. Lateral loop easily visible ( Fig. 2E–F ). Remarks: Pisidium dorbignyi resembles Pisidium vile Pilsbry, 1897 , consistently differing in having a lower and more ovate shell outline (not necessary reflected in the morphometric indices which are similar). Pisidium dorbignyi has a flatter shell with less projecting beaks, which are comparatively more displaced posteriorly. Compared with Pisidium sterkianum Pilsbry, 1897 , P. dorbignyi is much smaller and comparatively higher, differing also in having an internal ligament and more concentrated bundles of the inner radial mantle muscles (possibly associated with reduced size, in particular, of the shell length). Note on the types : As it has been previously reported by Pilsbry (1897) , the type of Cyclas pulchella (= Pisidium dorbignyi ) was not among d' Orbigny's shells acquired by the Natural History Museum, London. F. Naggs (personal communication, April 1997 ) confirmed this. In fact, C. pulchella was included in the Gray’s list ( Gray, 1854 ) of d´Orbigny's South American shells acquired by the British Museum of Natural History, London, but the record is not marked with the acronym “BM”, as other specimens received from d´Orbigny. This is also in agreement with the registration book to the BMNH mollusks collection, where no entry for C. pulchella can be found. Neither has the type of C. pulchella been found at the MNHN, Paris (P. Bouchet, personal communication), so it should to be considered lost. In order to define the taxonomic status of Pisidium dorbignyi , a neotype is here selected and housed in the mollusk collection of MHNM 15544, Montevideo, Uruguay . FIGURES 1A–F. Pisidium dorbignyi . A. Neotype (MHNM 15544), right valve, outer view. B–F. P. dorbignyi (MLP 10497). B. Posterior view. C. Detail of the escutcheon. D. Left valve, inner view. E. Dorsal view. F . Right valve, inner view. Scale bars: A, B, D–F. = 500 μm; C = 200 μm. Type locality: The neotype was selected from a sample taken from an unnamed water course on the road from Maldonado to San Carlos City ( 34°48'S , 54°55'W ), Maldonado Department, Uruguay (a site in the vicinity of the type locality of Cyclas pulchella ). Other examined material: Uruguay : same locality of neotype ( MLP 10497); artificial rice-fields irrigation channels at San Gregorio, Artigas Department, Uruguay ( 3 specimens dissected) ( MLP 5296). Distribution: Uruguay : water courses connected with the Uruguay River at Artigas Department and coastal lagoons and small streams at Maldonado Department. The records of Pisidium dorbignyi reported from the Río de La Plata and related water courses at Buenos Aires Province ( Landoni, 1992 ) are doubtful ( Ituarte, 1996 ). The species was also reported from northern Brazil (Lange de Morretes, 1954 fide Figueiras, 1965 ), but this record needs to be confirmed.