RESUMO
Details on Condylostoma arenarium infraciliature have not been described; therefore, it is considered a poorly known species. The lack of detailed description on C. arenarium morphology caused several misidentifications that have accumulated in the literature. In this study, we present the first complete description of C. arenarium infraciliature based on protargol-impregnated organisms and scanning electron microscopy. We also have inferred the phylogenetic position of this species based on 18S rRNA sequences. The main characteristics of C. arenarium population from Guanabara Bay are as follows: in vivo elongated body shape with 350-600 µm length × 70-220 µm width, they are highly contractile when subjected to disturbances, green-yellowish cortical granules are present, contractile vacuoles absent, V-shaped peristome comprises approximately 1/5 of the total length, adoral zone with 83-145 membranelles, 1-2 small frontal cirrus observed only in impregnated specimens, 10-15 fiber-like stripes arranged transversely on the inner wall of the oral cavity, 30-45 somatic kineties, moniliform macronucleus with 15-20 nodules. Some observations on morphogenesis of C. arenarium were also included. In phylogenetic analyses, C. arenarium clustered with Condylostoma sp. within a clade composed of three C. curva sequences with high support values.