An attempt was made to determine more accurately the density of molluskan populations in the Pampulha reservoir, using the quadrate method, intending to detect the fluctuation of the populations density, the habitat conditions and the possible competitive interactions among Biomphalaria tenagophila, Melanoides tuberculata, Pomacea haustrum and Biomphalaria glabrata, through the analysis of populational parameters. Among the most significative facts observed in the reservoir it has to be mentioned: the almost disappearance of B. glabrata; the invasion, colonization, fixation and fast growing of M. tuberculata population until reaching about 11,000 individuals/m2; the density fluctuations of B. tenagophila, P. haustrum and M. tuberculata alives and deads; differences on the habitat preference of these three molluskan species at the edge (at the limit earth-water, at 0.70m and 1.40m from the shore line); monthly mortality rates and reproduction seasons of the species.

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Field work research on population dynamic of snails from the regions of Belo Horizonte and Lagoa Santa give much information about interactions among two or more species of mollusks: Pomacea haustrum, Biomphalaria glabrata, B. tenagophila, B. straminea and Melanoides tuberculata. Data ranging from two years to several decades ago suggest that the Pampulha reservoir is like a cemetery of B. glabrata and B. straminea, species that coexist for more than 14 years in a small part of a stream, whereas only B. glabrata lives in all the streams of the basin. In the last ten to twenty years B. tenagophila has coexisted with P. haustrum and M. tuberculata in the Serra Verde ponds and in the Pampulha dam. However these species have not settled in any of the brooks, except temporarily. The data suggest that the kind of biotope and the habitat conditions are decisive factors for the permanence of each species in its preferencial biotope. B. glabrata, natural from streams and riverheads, quickly disappears from the reservoirs and ponds where it coexists with other species for a short time, independently of the competitive process. Competition needs to be better studied, since in Central America and Caribean islands this kind of study has favored the biological control of planorbid species.

This paper deals with variations in biomass and in the production of Eichhornia crassipes (water hyacinth) in a eutrophic tropical reservoir. Net production was assessed by both a traditional method and a demographic approach. The initial biomass ranges from 96 g DW.m -2 to a maximum of 2027 g DW.m -2 . Net production estimates varied greatly according to which method was used to calculate them (traditional: 20.6 ton DW.hectare -1 .year -1 , demographic: 226.9 ton DW.year -1 ).Key words: Eichhornia crassipes (water hyacinth), biomass production, modular demography, macrophytes. RESUMO Sobre dois métodos para estimar a produção de Eichhornia crassipes no reservatório eutrófico Pampulha (MG, Brasil)Este estudo mostra as variações da biomassa e da produção de Eichhornia crassipes em um reservatório eutrófico tropical. A produção líquida foi estimada por intermédio do método tradicional e do demográfico. A biomassa inicial de 96 g PS.m -2 chegou ao máximo de 2.027 g PS.m -2 A produção líquida variou de acordo com o método utilizado para sua estimativa (tradicional: 20,6 ton PS. ha -1 .ano -1 , demográfico: 226,9 ton PS.ano -1 ).Palavras-chave: Eichhornia crassipes (aguapé), produção de biomassa, macrófitas, método demográfico.

"Híbridos" de Biomphalaria tenagophila provenientes dos cruzamentos de linhagens albinas de Belo Horizonte (BH) ou de Joinvile (SC), com melânicos de Cabo Frio (CF), do Taim (Ta) ou de Curitiba (PR), submetidos à infecção pela cepa do Schistosoma mansoni de Belo Horizonte (=LE), apresentaram os seguintes resultados: em F1, os "híbridos" TaSC, PRSC e CFTa exibiram taxas de 4,5%, 12,5% e 11,2% de suscetibilidade; em F2, todos os "híbridos" foram negativos e em F3, um exemplar albino, filho de (CFBH) ². TaBH se infectou com a LE. Dentre os controles, a B. glabrata apresentou taxas de 66,7 a 93,6% de suscetibilidade à LE e a B. tenagophila de Joinvile exibiu taxas de infecção de 17,1 e 33,3% pela cepa SJ; e os "híbridos" BHTa e BHCF, taxas de 6,0 a 53,8% também pela cepa SJ. Houve grande influência da linhagem materna nas taxas de suscetibilidade. Devido ao fato de descendentes do cruzamento de linhagens refratárias a LE (CF, Ta e BH), terem se infectado, é recomendado o uso de "híbridos" para a detecção de gens de suscetibilidade em tais linhagens. São ainda discutidas, a necessidade do uso de maior número de miracídios nos testes de infecção e a falta de relação entre a freqüência de contatos parasitas-hospedeiros e as taxas de infectividade. Considerando que estas dependem de características genéticas preexistentes na população, a cepa LE seria uma variedade genética (ou raça) distinta da cepa SJ, dotada de pouca aptidão em infectar as diversas populações de B. tenagophila, exceto a de Joinvile (SC).

Samples of water and snails collected through aleatory scoops in a small dam were done to obtain data concerning the physical and chemical characteristics of the water and their possible influence on biological aspects of the life cycle of snails. Among the 17 analysed parameters, Alkalinity and Conductivity showed a positive correlation with the density of B. tenagophila (r = +0.224 and r = +0.290), while CO2 and Acidity were negatively correlated with this populacional parameter (r = -0.592 and 0.601). Alkalinity and Total Hardness values were slightly higher than 100 mg/l CaCO3. Chlorides and Conductivity showed means of 94.9 +/- 38.7 and 680.1 +/- 643 microS/cm; these values are very high for that region. Other factors like pH and OD are according to the pattern for provision untreated waters. The density of B. tenagophila declined in the 6th month after a long period of torrential rains and in the rainy summers. In the colder months of the following year the density was higher (until 124 individuals/months/90 scoops). The monthly mean of the diameters of the snails was always larger than 13 mm, reaching 21.4 +/- 4.1 mm, but the mode was about 17 mm. There was no correlation between diameter/density (r = 0.037) and density/temperature (r = 0.065).

Moluscos criados no campo em gaiolas transparentes e escurecidas apresentaram o mesmo consumo diário de perifíron; entretanto nas primeiras existia maior proporção de organismos vegetais e nas últimas maior proporção de organismos animais. O consumo diário por molusco foi significamente maior em presença de maiores quantidades de alimento, porém a proporção consumida do total de alimento disponível decresceu.

Observation about cohabitation among B. glabrata and B. tenagophila revealed a greater vulnerability of B. tenagophila population during the process of competition when its density was severely decreased in 12 trials, moderate in 2 trials. It was higher than B. glabrata in only one trial. Some snail water chemical parameters analysed such as pH, alkalinity, conductivity and oxygen dissolved, and the viability rate of batch of eggs didn't give subsidy to explain the competition mechanism. The newly-born survival, in the situation of cohabitation, was low for both species. This reveals the existence of intra and interspecific competitive interaction. The fertility rate reduction of B. tenagophila during the cohabitation was considered as a cause of its exclusion. One of the factors that seems to have influenced the fertility rate was a possible wrong crossing.