ABSTRACT This communication describes the first records of byssal hairs on the postlarvae shells of Mytilus edulis platensis d’Orbigny, 1842. The postlarvae used in this study were sampled on artificial collectors used for mussel culture in San Jorge Gulf, Sudoccidental Atlantic Ocean. A general description of the hairs is made as well as an exploratory analysis of their spatial distribution on the shell surface. The presence of these byssal hairs is suggested as a criterion to differentiate M. edulis platensis from other mytilids.

ABSTRACT Although the presence of apertural folds and lamellae is the most recognizable character of the Odontostomidae, some species lack them, mostly in Anctus Martens, 1860, Bahiensis Jousseaume, 1877 and Moricandia Pilsbry & Vanatta, 1898. Eudioptus avellanedae Doering, 1881 - a slender odontostomid species that lacks even the slightest trace of folds or lamellae in its shell aperture - was however transferred to Odontostomus by Pilsbry in 1902 on the basis of its building forward of the aperture-margins. It is currently placed in its own monotypic subgenus, Cyclodontina (Ventania) Parodiz, 1940, on the basis of about the same argument. In this paper we redescribe its shell morphology and, for the first time, describe the internal anatomy of the pallial complex and the reproductive and digestive systems. The presence of a spongy gland in the pallial complex; of a short penis sheath with no retractor muscle; of a bursa copulatrix duct longer than spermoviduct, and of an epiphallic gland strongly support the inclusion of this unusual species in Odontostomidae. The species is diagnosable by the sculpture of the protoconch, which is not smooth as previously described, but has waved axial ribs crossed by spiral lines in young specimens; the distinctive external and internal shape of the bursa copulatrix duct; the internal penis wall divided in three regions of different sculpture; the smooth inner wall of the vagina; the long and cylindrical epiphallus with a distal widening indicating the presence of an epiphallic gland, and the penis retractor muscle inserted in the distal end of a short flagellum. These characters support the validity of Ventania Parodiz, 1940, different from Cyclodontina Beck, 1837.

Quinquelaophonte aestuarii sp. nov. (Copepoda, Harpacticoida) from Bahía Blanca estuary is the first record of Quinquelaophonte Wells, Hicks & Coull, 1982 in Argentina and its southernmost location in the world. The setal formula (P2: exopod 1.2.3-endopod 1.2.0; P3: exopod 1.2.3-endopod 2.2.1; P4: exopod 1.2.3-endopod 1.2.0) of the last segment of P2-P4, in both the exopod and the endopod, distinguishes this species from all known species in Quinquelaophonte, except for Quinquelaophonte varians Bjornberg, 2010. The new species differs from the latter in the setal formula of the antennule, the mouth parts and maxillipeds, the absence of an inner seta on the second exopod segment of P2, the length of the second exopod segment of P4 and in the shape of baseoendopod setae of P5.

Mitotic chromosomes of the freshwater snail Pomacea patula catemacensis (Baker 1922) were analyzed on gill tissue of specimens from the type locality (Lake Catemaco, Mexico). The diploid number of chromosomes is 2n = 26, including nine metacentric and four submetacentric pairs; therefore, the fundamental number is FN = 52. No sex chromosomes could be identified. The same chromosome number and morphology were already reported for P. flagellata, i.e., the other species of the genus living in Mexico. The basic haploid number for family Ampullariidae was reported to be n = 14 in the literature; so, its reduction to n = 13 is probably an apomorphy of the Mexican Pomacea snails. Lanistes bolteni, from Egypt, also shows n = 13, but its karyotype is much more asymmetrical, and seems to have evolved independently from P. flagellata and P. patula catemacensis. The nominotypical subspecies, P. patula patula (Reeve 1856), is a poorly known taxon, whose original locality is unknown. A taxonomical account is presented here, and a Mexican origin postulated as the most parsimonious hypothesis.

O molusco Mesodesma mactroides (Deshayes, 1854) é uma espécie migrante sazonal que na primavera move-se para o nível entremarés superior da praia. Neste estudo, analisamos a distribuição espacial da densidade e o tamanho médio da população do bivalve na praia de Monte Hermoso (Argentina) no inverno de 1995, i. é, três meses antes da mortalidade massiva desses moluscos, acontecida em novembro de 1995. A amostragem cobriu 32 km da praia, utilizando-se 22 transectos distribuídos regularmente. O gradiente ambiental principal na praia foi determinado mediante Análise de Componentes Principais (ACP) a partir da matriz de correlação dos dados ambientais (morfologia e declividade da praia, e granulometria da areia). A análise da correlação foi utilizada para avaliar a relação entre o escore de um local (transecto) no primeiro e segundo componentes principais, a densidade média dos moluscos e tamanho médio da concha. A maior parte da praia parece ser um habitat apropriado para esses moluscos e sua heterogeneidade espacial não foi explicada pelas variáveis medidas, uma vez que, embora o primeiro eixo da PCA tenha demonstrado um gradiente físico no sentido L-O, a densidade dos moluscos não apresentou correlação com o mesmo. A densidade apresentou o máximo valor próximo aos molhes, pontos de grande fluxo turístico, atividade esta que quando não acarreta coleta de moluscos parece não afetar a densidade da população, mas sim o tamanho médio da concha através de uma redução da taxa de crescimento. A abundância da população no inverno, em comparação com a avaliação feita após a mortalidade em massa de novembro, sugere que a maior parte da população passa o inverno na faixa entremarés.

The yellow clam Mesodesma mactroides (Deshayes, 1854) is a seasonal migrant that moves in spring to the sandy upper intertidal level. In this paper we analyze the spatial distribution of density and mean shell size of the yellow clam population in Monte Hermoso beach (Argentina) in winter 1995, i.e., three months before the mass mortality occurred in November 1995. Sampling covered 32 km of beach, with a regular design of 22 transects. The major environmental gradient in the beach was determined using principal component analysis (PCA) on the correlation matrix of the environmental data (beach morphology, slope, and sand granulometry). Correlation analysis was used to assess the relationship between the score of a site (transect) on the first and second principal component, and clam mean density and mean shell size. Most of the beach seems to be habitable for clams, their spatial heterogeneity not having been explained by the measured variables since, although the first axis of the PCA has demonstrated an E-W physical gradient, clam density was not in correlation with it. Density was maximum near the piers, even though these are points with high tourist activity. It seems that non-extractive touristic activities do not affect population density but rather mean shell size, probably due to reduction of growth rates. The abundance of the winter population, as compared with the assessment done after the mass mortality of November, strongly suggests that a great part of the population was overwintering in the intertidal fringe.