Current advances on the study of snail-snail interactions, with special emphasis on competition process

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.

Estratégia competitiva entre Biomphalaria glabrata e Biomphalaria tenagophila: estudos de laboratório / Competitive strategy between Biomphalaria glabrata and Biomphalaria tenagophila: laboratory studies

Observation about conhabitation among B. glabrata and B. tenagophila revealed a greater vulnerability of B. tenagophila population during the process of competition when its density was severaly 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 dissolve, an 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 interacition. The fertility rate reduction of B. tenagophila during the cohabitation was considered as a cause of its exlusion. One of the factors that seems to have influenced the fertility rate was a possible wrong crossing.

Preference of Biomphalaria tenagophila among macrophytes and their periphytons determined through the degree of attractiveness

Em presença de extratos de seis macrófitas a Biomphalaria tenagophila apresentou maior atraçäo por quatro delas, na seguinte ordem: Nasturtium pumilum > Polygonum acre > Commelina sp = Echinocloa crusgalli. Para o perifíton destas macrófitas a B. tenagophila foi atraída, sem mostrar preferência entre eles. Na discussäo, tal comportamento é avaliado sob o ponto de vista da co-evoluçäo entre moluscos e plantas

Suscetibilidade de "hibridos" de Biomphalaria tenagophila a cepa LE (BH) do Schistosoma mansoni / Susceptibility of Biomphalaria tenagophila hibrids to the strain LE (BH) of Schistosoma mansoni

"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 infecçäo 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 infecçäo 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 detecçäo de gens de suscetibilidade em tais linhagens. Säo ainda discutidas, a necessidade do uso de maior número de miracídios nos testes de infecçäo e a falta de relaçäo entre a frequência de contatos parasitas-hospedeiros e as taxas de infectividade. Considerando que estas dependem de características genéticas preexistentes na populaçäo, a cepa LE seria uma variedade genética (ou raça) distinta da cepa SJ, dotada de pouca aptidäo em infectar as diversas populaçöes de B. tenagophila, exceto a de Joinvile (SC)