Abstract Within each ecosystem, organisms and populations maintain a complex set of relationships. These interactions can determine the distribution area of a species and play an essential role in its evolution. Parasites are ubiquitous components of nature and have a high influence on various aspects of the biology and ecology of organisms, affecting the populations of their hosts and, therefore, their communities and ecosystems. Free-living amoebae are unicellular organisms that can be found in water, soil or air. Some species are of great importance in human health. In Hydra, there are several reports of Hydramoeba hydroxena infections. In this work we present a double parasitosis: two concatenated infectious periods in the host polyp of Hydra vulgaris and Hydra vulgaris pedunculata for three freshwater bodies in the province of Buenos Aires, Argentina. Hydramoeba sp. and Acanthoamoeba sp. unchain a series of anatomical lesions that in all cases cause the death of the polyps due to total disintegration. This finding becomes important at a sanitary level due to the appearance of Acanthoamoeba sp. in waters associated with human recreational activities; For the Hydra genus, the importance lies at an ecological and evolutionary level, considering the possible impact on its natural populations. ecosystem relationships evolution therefore ecosystems Freeliving Free living water air health infections parasitosis Aires Argentina sp disintegration activities genus

This paper deals about the nematocysts like a source of biometric information for comparison between the species Hydra vulgaris Pallas, 1766, Hydra vulgaris pedunculata Deserti et al., 2011 and Hydra pseudoligactis (Hyman, 1931). This biometric tool lets us carry out statistical comparisons and adding these results to the identification of specimens from different classificatory groups. In this particular study, we obtained significant differences between species, individuals of each species and nematocysts type when compared the biometry of its nematocysts. Another result was the variation in of particular nematocysts, like atrichous isorhiza and holotrichous isorhiza for the species H. vulgaris in relation to the column size.

El presente estudio utiliza los nematocistos como fuente de información biométrica para llevar a cabo comparaciones entre las especies Hydra vulgaris Pallas, 1766, Hydra vulgaris pedunculata Deserti et al., 2011 y Hydra pseudoligactis (Hyman, 1931). La utilización de dicha herramienta nos permitió realizar comparaciones estadísticas y sumar los resultados obtenidos a la identificación clásica especímenes pertenecientes a diferentes grupos clasificatorios. Utilizando entonces la biometría de los nematocistos, se hallaron diferencias estadísticamente significativas al comparar especies distintas, individuos de cada especie y tipos de nematocistos. Se observo variación del tamaño de las cápsulas de los nematocistos atrico isorriza y holotrico isorriza para la especie H. vulgaris en relación al tamaño de la columna del ejemplar.