Species of Spirostomum Ehrenberg, 1838 are widely used as model organisms in ecological studies of environmental impacts and symbioses between ciliates and human pathogenic bacteria. However, the taxonomy of this genus is confused by the superficiality of the morphological descriptions of its included species, and the use of only a few characters for their differentiation. The present study provides details of total infraciliature, nuclear apparatus, morphometric data and 18S rDNA gene sequences of Spirostomum teres Claparède & Lachmann, 1858 and Spirostomum minus Roux, 1901, isolated from a sewage treatment plant and a freshwater lake in the city of Rio de Janeiro, Brazil, respectively. For the morphological descriptions of S. teres and S. minus, living cells were observed using bright-field and differential interference contrast (DIC) microscopy, the total infraciliature and nuclear apparatus were revealed by staining with protargol, and ciliary patterns were observed also with scanning electron microscopy (SEM). The complete sequences of the 18S rDNA of S. teres and S. minus were obtained using eukaryotic universal primers, and then compared with sequences of other species and populations of Spirostomum deposited in the GenBank database. Living S. minus measured 400-800 µm in length and 55-115 µm in width, with the following characteristics: adoral zone of membranelles approximately 112 µm long; inconspicuous paroral kinety; 30-40 kineties in somatic ciliature; moniliform macronucleus with 9-25 nodes, approximately 12 micronuclei; single and posterior contractile vacuole; and yellow-brown cytoplasm. Living and fully extended S. teres measured approximately 250 µm in length and 65 ìm in width, with the following characteristics: adoral zone of membranelles approximately 92 µm long; approximately 30 somatic kineties; compact macronucleus, approximately five micronuclei; macronuclear groove present; single and posterior contractile vacuole; and colorless cytoplasm. Evidence from 18S rDNA sequences confirms the identification of S. teres and suggests the existence of cryptic species closely related to S. minus. The use of silver impregnation technique (protargol) allowed the observation and description of a greater number of characters in S. minus and S. teres, thus assisting the research that require identification of these species.

In this work we investigated the quantitative aspects of the relationship between Rhinella pombali (Baldissera, Caramaschi & Haddad, 2004) tadpoles and their trichodinid ectoparasites. Specimens were collected from a small stream located at an agricultural area between rainforest fragments in southeastern Brazil. A total of 80 tadpoles were investigated. Trichodina heterodentata Duncan, 1977 was the only trichodinid ciliate species recovered. All tadpoles were naturally infested by T. heterodentata with a mean intensity of 695.14 ± 33.33 trichodinid ciliates per tadpole. A positive correlation was observed between the intensity of the infestation and tadpole weight. Intensity of infestation did not significantly influence mean trichodinid diameter in the populations studied. This is the first study investigating density-dependent effects in interspecific interactions between trichodinids and anurans.