Resumo

Observação de pequenos ecossistemas estabelecidos quase naturalmente no laboratório a partir de material coletado em "habitats" específicos do rio, permitiu construir uma rede alimentar da Amoeba (discoides), um predador voraz proveniente da fauna encontrada entre capins submersos nas margens de lagos e rios. O comportamento predador é bastante variado; as presas preferidas são ciliados, e, além de vários outros protozoários, a ameba ataca anelídeos (Aeolosoma sp.). Observou-se a caça cooperativa a espécies presas que chegam a fugir se atacadas por amebas solitárias. Cooperação é definida em termos de aumento de alimento obtido. Todos os organismos predadores na rede mostravam-se específicos na escolha da presa, notando-se também preferências por algumas espécies de presas aceitáveis. O gráfico da rede alimentar apresentado na figura 7, permite uma comparação quantitativa de diversas redes equivalentes.

Summary

This is the first of a series of intended reports on observation of mini-ecosystems as they establish themselves naturally in the laboratory from material collected in specific field habitats. "Black Water" (coloured dark by dissolved humic acids) is often found to be poor in plant and animal life, yet little is known on species abundance, population densities and food-chain relationships in lower organisms in the rio Negro region. The amoeba appeared in material collected in the submerged parts of floating gramineae in the lago Samauma, circa 55 km upstream from Manaus, together with the organisms listed in Lista 1. (annexed are species from submerged leaf litter of different places, introduced experimentally to test their suitability as prey. Names of species and Genera set in brackets indicate uncertain identification). Amoeba (discoides) proved one of the most voracious predators (Lista 2). Accidental contact with prey organisms (and only with these) leads to an intensified plasma stream in the direction of contact and to envelopment and vacuolization of the prey adjacent to the amoeba. The same plasma reaction results in accumulation of amoebae in places with high densities of small, faster moving prey and in cooperative predation. Cooperation is expressed as mean increase of food obtained by a given population as a result of group interaction as compared to food obtained by solitary predation.