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Morphology, infraciliature, morphogenetic features, and some ecological data for Pelagostrobilidium wilberti n. sp. are described. This new species was collected from a temporary pond in Magdalena, Buenos Aires province, Argentina, which was sampled monthly from August 2003 to July 2005. The species was found in autumn and winter. Observations were made in vivo and after staining with protargol. Pelagostrobilidium wilberti n. sp. measures 63-84 x 42-49 microm in vivo and is conical in shape, with a posterior spine-like cytoplasmic process. It possesses 6 somatic kineties, with kinety 2 sinistrally curved and shorter than the others. The oral apparatus is composed of 25-32 external and two internal membranelles. The macronucleus is horseshoe-shaped and located beneath the oral apparatus; two or three spherical micronuclei lie dorsally. There is a posterior contractile vacuole. Morphogenesis is hypo-apokinetal and begins dorsally between the curved kinety 2 and kinety 3. After the discovery of this new species, the diagnosis of the genus Pelagostrobilidium was amended.

This report includes a detailed description of the reproductive behaviour of the freshwater crab Zilchiopsis collastinensis in the family Trichodactylidae and some observations on two other genera. It presents observations on the courtship, copulation, spawning, incubation and maternal care in Z. collastinensis. No discernible courtship behaviour occurred. We found instead that forced copulation is practised by males. Z. collastinensis copulations were observed in between April and October, for Dilocarcinus pagei between January and August, and throughout the year for Trichodactylus borellianus. The spawning behaviour of all of the species observed took place out of the water. Incubation lasted between 46 and 62 days in D. pagei, 37 and 41 days in Z. collastinensis and 8 and 12 days in T. borellianus. The number of juveniles carried by each ovigerous female ranged from 651 ± 172 in Z. collastinensis, through 582 ± 8 in D. pagei to 63 ± 33 in T. borellianus. The results support the proposition that members of the family Trichodactylidae, comprising exclusively endemic freshwater crabs, have evolved unusual adaptive reproductive behaviours to optimize their reproductive fitness in unstable environments.

The spatial evolution of South American Syncarida (Anaspidacea and Bathynellacea) and related taxa is evaluated applying a panbiogeographic approach, where Ocean basins are used to identify major patterns of intercontinental distribution. The Pacific basin, corresponding to a southern temperate track, is identified as the major evolutionary centre for Stygocaris(Stygocarididae), Bathynella (Bathynellidae), Atopobathynellu, and Chilibathynella (Parabathynellidae), whereas distribution of Nannobathynella (Bathynellidae) and Cteniobathynella (Parabathynellidae) is centred on the Atlantic Ocean, belonging to a northern tropical track. It is concluded that the biotic origin of the South American Syncarida is complex, implying the existence of at least two ancestral biotas.

Soil samples were taken from a temporary pond located in Buenos Aires province, Argentina, during the dry phase in the summer of 2005. The ciliates were studied alive and after staining with protargol. Deviata rositae n. sp. measures 112-154 microm in length and 21-28 microm in width in vivo and has a vermiform body. The contractile vacuole is located in the mid-body on the left. The macronucleus is moniliform and there are 1-3 micronuclei. The oral apparatus is composed of 14-18 adoral membranelles and straight paroral and endoral membranes that never intersect each other. The somatic ciliature is arranged in four frontal cirri, one buccal cirrus, six long and slightly spiraled rows of cirri with the first right row extending up to the equatorial or sub-equatorial region, and two dorsal rows of dikinetids. This new species of Deviata primarily differs from its congeners by the number of macronuclear nodules and the number and disposition of the dorsal rows of dikinetids.

The genus Chlamydotheca Saussure, 1858 is a typical representative of the Neotropical ostracod fauna that occupies lotic and lentic environments including temporary and permanent ponds. Up to the present, four species have been recorded in Argentina: C. iheringi (Sars), C. incisa (Claus) and C. leuckarti (Claus), and C. symmetrica (Vávra). Temporary and permanent ponds of this region were sampled for ostracods, using a fine mesh net. Three species belonging to the genus Chlamydotheca were collected of which C. arcuata (Sars) is recorded for first time for the Chacoan region. Limb morphology was studied under light microscope and line drawings were made using camera lucida. Valves were photographed under scanning electron microscope and redescriptions of C. arcuata and C. iheringi are provided and distributional aspects of the species sampled are discussed.

ResumenSe realizó el estudio microanatómico del testículo y la espermatogénesis del anfípodo dulceacuícola Hyalella curvispina Shoemaker 1942. Los crustáceos fueron recolectados en el Arroyo Rivera (Saldungaray, provincia de Buenos Aires, Argentina). Las células foliculares experimentan cambios durante la espermatogénesis debidos a su función secretora. En la etapa inicial presentan un núcleo oval y el citoplasma es fuertemente basófilo. A medida que progresa la espermatogénesis, los núcleos se hipertrofian, y el citoplasma adquiere aspecto espumoso debido a la presencia de vacuolas. La pared de la vesícula está formada por dos estratos: un epitelio constituido por células glandulares cúbicas y una capa externa de tejido conectivo fibroso. Las células presentan una secreción granular eosinófila y vuelcan su contenido en la luz de la vesícula. El líquido seminal está constituido por la secreción que aportan tanto las células del epitelio testicular como las células glandulares de la vesícula seminal.Palabras clave: histología, testículo, Amphipoda, agua dulce. AbstractA microanatomical study of the testis and spermatogenesis of the freshwater amphipod Hyalella curvispina Shoemaker 1942 has been performed. The crustaceans were collected in the Arroyo Rivera (Saldungaray, Buenos Aires province, Argentina). Follicular cells undergo changes during spermatogenesis due to its secretory function. In the initial stage they present an oval nucleus and a strongly basophilic cytoplasm. As spermatogenesis progresses, nuclei hypertrophied, and the cytoplasm took on a foamy appearance due to the presence of vacuoles. The wall of the vesicle consists of two layers: an epithelium consisting of cubic glandular cells and an outer layer of fibrous connective tissue. The cells have eosinophilic granular secretion and turn their content into the lumen. Seminal fluid is constituted by secretions from cells of the testicular epithelium as well as glandular cells of the seminal vesicle.

Temporary ponds represent special environments that are inhabited by organisms adapted to changing environmental conditions. Ciliates are able to survive complete loss of water in these transient habitats through cyst formation. However, ciliates from the Neotropical region in general have been poorly studied with modern techniques. The main goal of this study is to describe the ciliates in dried sediments of a temporary pond from Buenos Aires Province, Argentina, through sampling efforts that were performed 2003-2005. Soil samples were obtained during drought phases and rewetted in laboratory to establish raw and enriched cultures. Ciliates were then studied both in vivo and after impregnation with protargol. In this study, we present 4 new records for Argentina (Gonostomum affi ne (Stein, 1859