Javier H. Signorelli

Shell phenotypic variation in the south-western Atlantic gastropod Olivancillaria carcellesi (Mollusca: Olividae)

Phenotypic variation on shell size and shape of Olivancillaria carcellesi from four representative localities is confirmed using geometric morphometric techniques. This species lives along the entire range of the genus in subtidal soft bottoms from Rio de Janeiro (Brazil) to Punta Pardelas (Chubut, Argentina). Thickness of columellar callus, length and width are the main shell differences from specimens collected at several intermediate localities: Cassino (Brazil), La Paloma (Uruguay) and Mar del Plata (Argentina). All forms showed significant differences in shell size except O. carcellesi from La Paloma compared with those from Mar del Plata. The group with larger specimens was from Cassino. Olivancillaria carcellesi from Punta Pardelas, the southernmost population, was the smaller sized group and showed allometry between size and shape and this is responsible for their relatively higher spires. The possible ecological implications of the observed pattern are discussed. It is confirmed that O. carcellesi although variable, is a clearly defined species.

Phenotypic variation of south-western Atlantic clam Mactra isabelleana (Bivalvia: Mactridae)

The phenotypic shell shape variation of Mactra isabelleana was tested using the geometric morphometric method. Four localities were sampled along the Rio de la Plata estuary and the coast of Buenos Aires province. Principal component analysis and canonical variates analysis of the first principal components were performed to reveal the shell variation and differences among localities, respectively. The specimens from different microhabitats mostly overlapped, although differences in shape were observed in the development of the umbo, the enlargement of the dorsoventral axes and the elongation of the posterior end. The ecological and physical parameters that could influence shell shape variation are discussed.

A New Species of Laubiericoncha (Bivalvia: Vesicomyidae) from Deep Waters Off Argentina

ABSTRACT A new species of bivalve belonging to the family Vesicomyidae, Laubiericoncha puertodeseadoi, n. sp., is described. The material was collected during several cruises to the continental margin of Argentina on board of the R/V “Puerto Deseado”. The distribution of the genus Laubiericoncha is expanded to include the southwestern Atlantic Ocean. The description was done by using morphological characters of the shell and mantle cavity organs. All type material of congeneric species was examined and illustrated. Laubiericoncha puertodeseadoi, n. sp., is characterized by a compressed, subrectangular, elongated, inequilateral shell, with slightly prosogyrous umbo and a well-defined triangular pallial sinus, strong external ligament and ctenidia with two robust demibranchs, foot anteriorly projecting and fused siphons. This new species constitutes the first record of the genus Laubiericoncha living on the continental margin off Argentina.

Occurrence of Pteria colymbus (Bivalvia: Pteriidae) in Argentine waters

La familia Pteriidae se encuentra bien representada en areas tropicales y subtropicales. A lo largo del oceano Atlantico sudoccidental, dos nombres pertenecientes al genero Pteria han sido citados en la bibliografia. Estos son: P. colymbus (Roding) y P. hirundo (Linnaeus). El estudio de nuevos ejemplares vivos recolectados frente a las costas de Buenos Aires a bordo del BO Puerto Deseado permitio actualizar el rango de distribucion geografica de la especie Pteria colymbus . Adicionalmente se ilustro el material tipo de las especies nominales relacionadas. Se relevaron los requisitos ecologicos y se describio la morfologia de la concha de P. colymbus . Este estudio constituye la primera revision de esta especie en aguas argentinas

The Anatinellidae and Kymatoxinae: A Reassessment of Their Affinities within the Superfamily Mactroidea (Mollusca, Bivalvia)

Abstract: Morphological phylogenetic analysis of the mactrid subfamilies Mactrinae and Kymatoxinae, plus the mactroidean family, Anatinellidae, indicate that the Kymatoxinae shares closer common ancestry with the Anatinellidae than with the Mactridae. Kymatoxinae is, therefore, reassigned from Mactridae to subfamily rank within Anatinellidae. The lack of a pallial sinus in Anatinella G. B. Sowerby I, 1833 suggests that its family was derived from anatomically plesiomophic members of Mactroidea with at best only weakly developed siphons.

Figure 6 In Taxonomic Revision Of Brazilian Mactridae Lamarck, 1809 (Bivalvia: Cardiida)

FIGURE 6. Mactrotoma fragilis (Gmelin, 1791). A: original illustration of Chemnitz; B – C: Lutraria candida Lamarck, 1818, syntype (MNHN unnumbered), RV; D – F: Mactra braziliana, Lamarck, 1818, syntype (MNHN unnumbered), D: RV, E: LV; G – H: Mactra oblonga Say, 1822 syntype (ANSP 51418) LV and RV respectively; I – J: Mactra bilineata Deshayes in Reeve, 1854, Syntype, NHMUK unnumbered; K: Mactra anserina, original illustration of Guppy, 1875, scale bar = 1 cm.