Stenostomum paraguayense

Stenostomum paraguayense (Martin, 1938) Fig. 4 C, D Sampling site. Display case with vegetation – Tank 3. Description. A Stenostomum reaching the length of 500 μm in a single zooid stage. The body is cylindrical with a distinguishable tail and a rounded anterior part (Fig. 4 C). The sensory pits are elongated and connected to the anterior, metameric brain lobes (Fig. 4 D). There are three pairs of refracting bodies. The first pair occurs at the middle of the length of the pits, the second at the end of the pits, and both consist of a single sphere. The third pair is connected to the end of the posterior brain lobes and is made of two connected spheres (Fig. 4 D). The mouth opening is small and slender, not easily recognizable on the photos; however, it is accompanied by a very distinguishable ring of darker cells (Fig. 4 C). The short pharynx passes into another ring of darker cells at the beginning of the intestine. The presence of these two dark rings is characteristic of the species. The body is yellowish with a uniform layer of cilia. The gut does not reach the caudal end of the body, which ends in a long tail (Fig. 4 C). This species is highly similar to S. arevaloi; however, according to Marcus (1945), it differs in width, mouth shape, gland cells at the beginning of the intestine, and dorsocaudal appendix. We did not observe any easily distinguishable appendix, but according to Marcus, the structure is not always present. The animal can survive for a few days in Chalkley’s Medium, fed on rotifers Lecane inermis, but the worms progressively decreased in size and died, likely due to starvation. We did not succeed in attempts of culturing them. Distribution. Brazil, São Paulo tanks and aquariums, Pinheiros River meadow, Canindé lake (Marcus 1945); Argentina, Córdoba (Adami and Damborenea 2021) – described as S. arevaloi but very likely S. paraguayense.