A composição das assembléias de Chironomidae e os possíveis fatores ambientais que influenciam a distribuição temporal deste grupo no alto rio Paraguai, em Cáceres, foram abordados no presente estudo durante o período de novembro de 2004 a outubro de 2005. Para considerar a diversidade de habitats na região, selecionamos nove pontos amostrais, onde coletamos 2988 indivíduos, distribuídos em 34 espécies pertencentes a oito tribos de três subfamílias. Os gêneros Labrundinia sp.1, Polypedilum (Asheum) e Polypedilum (Tripodura) estiveram presentes em quase todos os meses amostrados. A maior riqueza e abundância foram registradas no período de seca, revelando Polypedilum como o gênero mais expressivo. Ficou evidente o efeito das vaiáveis ambientais sobre os atributos da diversidade do alto rio Paraguai.

The aim of this study is to determine the influence of environmental variables on the time distribution of assemblages of Chironomidae in the Upper Paraguay River. The samples were collected from November 2004 to October of 2005. To consider the diversity of habitats in the region, nine different sampling points were sampled and 2988 individuals were collected, distributed in 34 species, eight tribes and three subfamilies. The genus Labrundinia sp.1, Polypedilum (Asheum) and Polypedilum (Tripodura) occurred almost every month during the study. Richness and abundance were higher during the dry period of the year, and the genus Polypedilum was the most abundant. There was evident the effect of the environmental variables on the attributes of diversity in the Upper Paraguay River.

Projeções da superfície externa do perióstraco e micro estruturas da concha foram redefinidas com base na microscopia óptica e eletrônica de varredura. A superfície do perióstraco é praticamente lisa em ambas espécies. Sob vista mesoscópica, A. trapesialis (Lamarck, 1819) apresenta corrugações regulares formando seqüências radiais de arcos na região do disco, raios isolados ou seqüências horizontais na região antero inferior. A. elongatus (Swainson, 1823) apresenta séries de arcos organizados radialmente no disco e arcos oblíquos na carena. Sob MEV, as micro estrias são mais evidentes em A. elongatus; A. trapesialis apresenta micro franjas muito diversas quanto a forma e padrão, principalmente em jovens. Considerando as camadas da concha, o perióstraco é mais fino em A. elongatus. A camada prismatica é espessa nas duas espécies, com uma série única de prismas alongados e em forma de cunha próximo à superfície externa. A camada nacarada compõe-se de lamelas muito finas, sem padrão ou do tipo escadaria; em A. elongatus esta camada é dividida por uma inclusão laminar de conchiolina. As franjas são abundantes e diversificadas em A. trapesialis, espécie menos resistente à dessecação devido à presença de ampla fresta intervalvar. A ocorrência de uma grande densidade de micro franjas e espinhos nos juvenis, estaria relacionada à orientação do animal na busca por local ideal ao seu desenvolvimento ou para escapar das regiões sujeitas às secas sazonais, como o Pantanal.

Based on optical and SEM microscopic observations, the projections of the outer surface of the periostracum and inner micro-structures of the shell are described and redefined. The outer surface of the periostracum is practically smooth in both species. Considering a mesoscopic view of the periostracum, A. trapesialis (Lamarck, 1819) presents regular corrugations in the form of radial sequences of arches on the disk region, isolated rays or horizontal sequences of rays on the anterior lower region. A. elongatus presents corrugations formed by series of oblique arches on the disc and oblique rays on the carina. Under SEM, micro ridges were more evident in A. elongatus, but a wide diversity of shapes and patterns of micro fringes were observed in A. trapesialis, especially in young individuals. Considering the profile of the shell layers, the periostracum is relatively thin and apparently simple in A. trapesialis and thinner in A. elongatus. The prismatic layer is thick in both species, consisting of a single series of elongated prisms and wedge-shaped prisms close to the outer surface. The nacreous layer consists of very fine lamellae without pattern or with a slight staircase-like; in A. elongatus this layer is divided by a laminar inclusion of conchiolin. The fringes are abundant and diversified in A. trapesialis, a species less resistant to desiccation due to the presence of a wide intervalvar gap. The existence of a greater density of micro fringes and spikes in juveniles may be related to the orientation of the animal in order to search for an ideal site for development or for escape from regions subject to seasonal droughts, like Pantanal.

At the end of 1998 and the beginning of 1999, the golden mussel Limnoperna fortunei (Dunker, 1857), Mytilidae, originating from Southeast Asia, was registered for the first time in the Guaíba Lake Basin, Rio Grande do Sul State, Brazil. Quantitative samples were taken since the first register and for two years at various localities in the Jacuí Delta, northern limit of Guaíba Lake, and in the south limit of the later where it empties into Patos Lagoon. Samplings in Patos Lagoon revealed the presence of L. fortunei in October 2000 at Arambaré (30º54'S, 51º30'W) and in January 2001 at São Lourenço do Sul (31º20'S, 51º58'W). Quantitative samplings on roots of water hyacinths Eichhornia azurea (Sw.) Kunth and E. crassipes (Mart.) Solms-Laubach, on rhizomes of rushes Scirpus californicus (C.A. Mey.) Steud. and on trunks of Cephalanthus glabratus (Spreng.) K. Schum, were made in Jacuí Delta and on the beaches of Vila de Itapuã and Pombas camping area, in the Municipality of Viamão. Limnoperna fortunei encreased in number to a maximum density of 27,275 individuals/m² one year and five months after the first register and 62,100 individuals/m² two years later. In November 2000, i.e., two years after the appearance of the species in Guaíba Lake, it was recorded the first macrofouling in the pipes catching water for the city of Porto Alegre and in filters and pipelines of the cellulose industry Riocell-Klabin, Municipality of Guaíba (30º06'S, 51º20'W). Limnoperna fortunei preferentially occur on rhizomes of rushes, initially forming flat clusters that grow to large mass. It also fix on the shells and soft parts of native bivalves and on shells and operculum of gastropods, prevening full closure of these mollusks. The recent decrease of rushy areas on the shores of Guaíba Lake is attributed to invasion of L. fortunei.