Spirostomum minus, Roux, 1901

Chi, Yong, Duan, Lili, Luo, Xiaotian, Cheng, Ting, Warren, Alan, Huang, Jie & Chen, Xiangrui, 2020, A new contribution to the taxonomy and molecular phylogeny of three, well-known freshwater species of the ciliate genus Spirostomum (Protozoa: Ciliophora: Heterotrichea), Zoological Journal of the Linnean Society 189 (1), pp. 158-177 : 171-172

COMMENTS ON SPIROSTOMUM MINUS

Spirostomum minus is ubiquitous in freshwater habitats, especially in eutrophic sites, and is commonly used as a bioindicator for the assessment of water quality ( Lynn & Gilron, 1992; Berger & Foissner, 2003). As a common species, it has been commonly reported (Roux, 1901; Kahl, 1932; Repak & Isquith, 1974; Foissner et al., 1992; Boscaro et al., 2014; Kim et al., 2016). According to previous descriptions, S. minus is characterized by its regular cylindrical body shape, moniliform macronucleus and 12–40 somatic kineties. The Qingdao population closely resembles the previous populations, so its identity is not in doubt ( Table 3).

Five other species of Spirostomum have a moniliform macronucleus and should be compared with S. minus : S. subtilis , S. inflatum , S. loxodes , S. ambiguum and S. semivirescens ( Table 3). Spirostomum subtilis can be clearly separated from S. minus by the arrangement of cortical granules between adjacent somatic kineties (a single row vs. several rows). Spirostomum inflatum can be distinguished from S. minus by its body shape, i.e. its posterior portion is significantly wider than its anterior portion (vs. both portions of similar width) and habitat (brackish water vs. freshwater) ( Kahl, 1932; Repak & Isquith, 1974). Likewise, S. loxodes differs in body shape, having an anterior beak-like projection (vs. absent in S. minus ) ( Repak & Isquith, 1974). Individuals of S. ambiguum are

Abbreviations: BL, body length; BR, brackish water; BW, body width; CG, cortical granules; FW, freshwater; Ma, macronucleus; MW, marine water; NA, not available; OL, oral length; SK, somatic kineties.

References: [1] present work; [2] Boscaro et al. (2014); [3] Repak & Isquith (1974); [4] Foissner et al. (1992); [5] Fernandes & da Silva Neto (2013); [6] Kim et al., 2016; [7] Jang et al. (2012); [8] Chen et al. (2017); [9] Kahl (1932).

* data from living cells (μ m); ** the column unit is %; *** the colour of cortical granules and number of rows between adjacent somatic kineties; adata from drawing.

larger (1000–4000 vs. 300–900 μ m) and have a larger peristome relative to body size (65–70% vs. c. 35–50%) and more somatic kineties (70–90 vs. 12–40) than S. minus ( Foissner et al., 1992; Boscaro et al., 2014). Spirostomum semivirescens can be easily distinguished from S. minus by the presence (vs. absence) of endosymbiotic algae in the cytoplasm ( Esteban et al., 2009; Hines et al., 2018).

Although S. minus is well characterized, different populations were divided into two distinct clusters in the SSU rDNA tree ( Fig. 8 ), despite having similar morphological characteristics ( Table 4). In addition, the SSU rDNA sequences of S. minus populations in clade I differ from those in clade II by 16–28 nucleotides, whereas sequences in clade I and II differ by only 0–9 and 0–7 sites, respectively ( Fig. 9 ). The high sequence divergence between clades I and II suggests that they may represent different species. Our results thus suggest that S. minus might represent a species complex.

Figures

Figure 8. Maximum likelihood (ML) tree based on SSU rDNA sequences of heterotrichous ciliates. Members of the class Karyorelictea are the outgroup taxa. Numbers near branches show the bootstrap values from ML and posterior probabilities of BI. Disagreements between ML and BI are shown by asterisks. The species newly sequenced in this study are shown in red bold font. The scale bar corresponds to 2 substitutions per 100 nucleotide positions.

Figure 9. Nucleotide differences among populations of Spirostomum minus, S. subtilis, S. teres and unidentified Spirostomum species based on SSU rDNA sequences. The numbers in the header indicate the unmatched site positions. The species newly sequenced in this study are shown in red. The red and blue boxes indicate clades I and II in Figure 8, respectively. Sites not available are shown by asterisks.