Spirostomum subtilis

COMMENTS ON SPIROSTOMUM SUBTILIS

Spirostomum subtilis was first described by Boscaro et al. (2014) as follows:

700–1000 μ m long. Length: width ratio about 14–24:1. 9–12 kineties on each side; a single, homogeneous cortical granules row per stripe. Peristome about 1/2 of the body length. Contractile vacuole often conspicuous, up to 1/3 of the body length, contrasting with the dark cytoplasm of the anterior part. Moniliform macronucleus with 15–24 nodules. Several (1–6) micronuclei 2–3 um long. Only found in freshwater.

The Qingdao population closely matches the original description in body shape, general morphology and habitat, except for the number of somatic kineties (18–24 in the former vs. 19–28 in the latter). Because this character was based on observations in vivo in the original description, it is likely that the authors overlooked the shortened somatic kineties on both the ventral and dorsal sides. We, therefore, think that the identification of the Qingdao population is correct.

The Qingdao population has a regular band-like body shape, which, therefore, superficially resembles S. ambiguum , S. minus , S. dharwarensis , S. yagiui and S. teres . However, it can be clearly distinguished by the arrangement of cortical granules between adjacent somatic kineties (a single row in S. subtilis vs. several rows) ( Fernandes & da Silva Neto, 2013; Boscaro et al., 2014; Chen et al., 2017). Although S. semivirescens is similar in body shape to S. subtilis , that species can be easily separated by the presence (vs. absence) of endosymbiotic algae in the cytoplasm ( Esteban et al., 2009; Hines et al., 2018). Spirostomum subtilis can be easily distinguished from S. inflatum by its body length (680–1100 vs. 300–400 μ m), its body length:width ratio (14–35 vs. 10–12) and its preferred habitat (freshwater vs. salt water) ( Kahl, 1932; Repak & Isquith, 1974). Spirostomum caudatum has a long tapered tail, allowing it to be easily distinguished from S. subtilis ( Jang et al., 2012) . Spirostomum loxodes differs from S. subtilis by its anterior beak-like projection ( Repak & Isquith, 1974). Molecular data also support the identity of the Qingdao population as S. subtilis , because its SSU rDNA sequence is identical to those of two other populations of S. subtilis ( HG939549 View Materials , HG939550 View Materials ) that together form a fully supported cluster in the phylogenetic tree ( Figs 8 View Figure 8 , 9 View Figure 9 ).