Hydra is a genus of common, sessile, solitary freshwater cnidarians, which are defined as carnivorous and efficient predators. The purpose of this study was to obtain information on the feeding habits and diet of Hydra vulgaris collected from its natural habitat in Nahuel Rucá Lake (Buenos Aires Province, Argentina). We found three categories of food items in the coelenteron: algae, fungi, and small invertebrates. Algae dominated the diet in terms of abundance and frequency of occurrence, but their volumetric contribution was almost negligible, as was their possible nutritional value. Invertebrate prey captured, using active predation, represented the major volumetric contribution, with four different taxa found. The detection of phytoplankton in the gastral cavities reveals the input of some organisms present in the surrounding waters in addition to the invertebrates. This information is novel, since studies on the natural diet of Hydra are very scarce.

RESUMEN. Investigaciones biométricas y estadísticas en el cnidoma del género Hydra (Cnidaria, Hydrozoa). 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 de 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. PALABRAS-CLAVE. Nematocistos, biometría, análisis estadísticos.ABSTRACT. 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.

Cordylophora caspia is a colonial, athecate hydroid inhabiting both in freshwater and brackish habitats. Its global distribution is in part due to its ability to tolerate a wide range of salinity. It is considered an invasive species and its control is widely studied because of the industrial and ecological problems it causes in many environments. We report for the first time the occurrence of this hydrozoan in Nahuel Rucá Lake (Buenos Aires province, Argentina) with some notes on its internal and external morphology.

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.

In the systematics of cnidarians, the different types of cnidocysts are considered an important taxonomic character. In Hydra, the four types of cnidocysts found in the ectoderm, concentrated in tentacles and their measurements, together with other morphological and reproductive characteristics, are very important for the taxonomy of the species. In this study, we explore in detail the biometric and statistical characteristics of the cnidome of three species of Hydra collected in three different environments for each climate season. A total of 17,378 capsules were measured. We used ANOVA test and Generalized Linear Model to analyze the distribution and differences reflected in each cnidome, considering the factors “individuals”, “season”, “lagoon” and “species”. The results were clear: the cnidome keep specific information that, together with other taxonomic characteristics, allows us to discern between species of different groups. The same happens with cnidome of the same species but from different lagoons or climatic seasons: we observed a variation of parameters for each type of cnidocyst that could differentiate “ecological races”, since these differences are not enough to declare different species.