Pyxicola pusilla

Lu, Borong, Li, Lifang, Hu, Xiaozhong, Ji, Daode, Al-Rasheid, Khaled A S & Song, Weibo, 2019, Novel contributions to the peritrich family Vaginicolidae (Protista: Ciliophora), with morphological and phylogenetic analyses of poorly known species of Pyxicola, Cothurnia and Vaginicola, Zoological Journal of the Linnean Society 187 (1), pp. 1-30 : 21-23

COMMENTS ON PYXICOLA PUSILLA

( FIG. 12 ; TABLES 2, 3)

This species was first described as Cothurnia pusilla ( Wrześniowski, 1866) . Kent (1882) established the genus Pyxicola for the operculated loricate peritrichs and reassigned this species as P. pusilla . This species has been described multiple times under different names and the descriptions for this form vary, especially in respect to the annular ridges of the lorica (inconspicuous to conspicuous) and the length of the stalk (from 3–60 μm; Kent, 1882; Stokes, 1895; Finley & Bacon, 1965; Nusch, 1970; Stiller, 1971; Trueba, 1978; Shen & Gu, 2016). Even so, there are some common features in these reports. For example, the lorica shape quotient (i.e. the length divided by the width) is about 2, the aperture is obliquely truncated, the colour of the lorica changes with age, but is often brown, and the cell protrudes just beyond the aperture. Our population corresponds with all these common features. Accordingly, we believe that the Ningbo isolate is a population of P. pusilla .

Before the genus Pyxicola was created, these operculated loricate peritrichs were treated as cothurnids. Thus, some populations were placed in the genus Cothurnia at one time or other. The isolate reported by Hutton (1878), namely Cothurnia furcifer , matches perfectly with Wrześniowski’s form ( Fig. 12B ) and thus can be considered as P. pusilla . Pyxicola affinis was first described by Kent (1882) and subsequently cited by Blochmann (1886) and Hickson (1903) as Cothurnia affinis and shows a larger lorica (80 × 40 μm vs. 46 × 22 μm). It has a longer stalk than Wrześniowski’s form (27–40 μm vs. 3 μm) ( Fig. 12C ). Pachytrocha cothurnoides , reported by Kent (1882) and also as Cothurnia cothurnoides by Blockmann (1886), corresponds in all points with Pyxicola pusilla except that the indurate operculum of Pyxicola is replaced by a fleshy pad for this isolate ( Blochmann, 1886) ( Fig. 12D ). We agree with Kahl (1935) and Trueba (1978) who suggested that this isolate was very likely a mutilated Pyxicola pusilla with a fallen operculum. Stokes (1895) reported a new genus and species, Caulicola valvata , largely because the operculum adheres to the aperture rather than the zooid, but this was probably an optical illusion (which in fact also occurred in our observations; Fig. 12E ). So, we agree with Trueba’s view in treating C. valvata as a junior synonym of P. pusilla . Trueba (1978) considered that P. carteri sensu Sommer, 1951 is a population of P. pusilla . We follow the proposal, because the shape quotient of its lorica is 2 and the contractile vacuole is situated at the same position as in our population (from the drawing; Fig. 12F ). Pyxicola eforiana reported by Tucolesco (1962) from brackish water is also a synonym of P. pusilla . The lorica of this form is larger than in the original population (75–80 vs. 46) and our population (75–80 vs. 60–67), and a larger proportion of the body projects out of the lorica (one-third vs. significantly less than one-third in the original and our population). Furthermore, it corresponds well with both the original description and present population ( Fig. 12G ). Nusch (1970) described a form under the name of Pyxicola carteri forma constricta and considered that Pyxicola nolandi Finley & Bacon, 1965 is a synonym. However, the lorica of this form is plumper than in the original population with a shape quotient about 2. Thus, we agree with Trueba (1978) in regarding this as a population of P. pusilla . In addition to the above forms, three other populations of P. pusilla were described ( Kent, 1882; Trueba, 1978; Shen & Gu, 2016). These are similar to each other and correspond with the original description. Unfortunately, some previous descriptions are cursory and the photomicrographs of P. pusilla are only available from Trueba’s paper. It is possible, therefore, that there are some mistakes in the above synonymy list. Pyxicola annulata described by Leidy (1882) was divided into two forms by Trueba (1978) and considered as synonyms of P. pusilla and P. carteri , respectively. However, in this case Trueba’s opinion is contestable: although the form with comparatively conspicuous annular ridges was treated as P. pusilla , one-fifth of the body protrudes outside the lorica, with a small contractile vacuole just below the peristomial lip (from Fig. 8 from pl. 2 in Leidy, 1882) and its lorica shape quotient is 2.5–3.0; all these characters are divergent with P. pusilla . Thus, we consider P. annulata to be a synonym of P. carteri .

The other three species in the P. pusilla complex, namely Pyxicola carteri Kent, 1882, Pyxicola operculigera (Kent, 1869) Kent, 1882 and Pyxicola psammata HadŽi, 1940 , should be compared with P. pusilla , considering, in particular, that P. pusilla possesses an urceolate lorica smaller than 100 μm with an annulated wall and a habitat in freshwater or brackish water. Pyxicola carteri differs from P. pusilla in having a slenderer lorica (shape quotient about 2.5 vs. 2.0) with a longer neck and a body that protrudes up to one-third of its length outside the lorica (vs. just the peristomial lip beyond aperture; Fig. 12I ) ( Kent, 1882; Trueba, 1978). Pyxicola operculigera possesses a long stalk, usually longer than the lorica and even up to 150 μm, which is much longer than the 2–60 μm long stalk of P. pusilla . Moreover, its lorica is never clearly annulated, whereas it is commonly and clearly annulated in P. pusilla ( Fig. 12J ) ( Kent, 1882; Trueba, 1978). The lorica of P. psammata is much slenderer than that of P. pusilla (shape quotient from 2.4–3.3, average 2.9 vs. about 2.0) and the operculum of the former is smaller than that of the latter (7 μm across vs. 12–18 μm across; Fig. 12K ) ( HadŽi, 1940).

Figures

Figure 8. Phylogenetic tree inferred from concatenated genes (SSU rDNA, ITS1-5.8S rDNA-ITS2, LSU rDNA), revealing the phylogenetic relationship within the family Vaginicolidae. Numbers near nodes denote maximum likelihood (ML) bootstrap values and Bayesian inference (BI) posterior probability, respectively. Asterisks indicate disagreements between ML and BI. The scale bar indicates five substitutions per 100 nucleotide positions. The out group includes Glaucoma chattoni, Ichthyophthirius multifiliis, Tetrahymena corlissi and Tetrahymena pyriformis.

Figure 12. Different isolates of Pyxicola pusilla (A–H) and its similar species (I–K), different isolates under the names of Cothurnia ceramicola (L–R) and its similar species (S–U), different isolates under the names of Vaginicola tincta (V–Z) and its similar species (Z1–Z4). A, Cothurnia pusilla sensu Wrześniowski, 1866. B, Cothurnia furcifer sensu Hutton, 1878. C, Pyxicola affinis sensu Kent, 1882. D, Pachytrocha cothurnoides sensu Kent, 1882. E, Caulicola valvata sensu Stokes, 1895. F, Pyxicola carteri sensu Sommer, 1951. G, Pyxicola eforiana sensu Tucolesco, 1962. H, after Shen & Gu, 2016. I, Pyxicola carteri (from Kent, 1882). J, Pyxicola operculigera (from Kent, 1869). K, Pyxicola psammata (from HadŽi, 1940). L, after Kahl, 1933. M, after Precht, 1935. N, after Felinska, 1965. O, after Küsters, 1974. P, after Song, 1992. Q, after Sun et al., 2009. R, after Shen & Gu, 2016. S, Cothurnia curvula (from Entz, 1884). T, Cothurnia harpactici (from Kahl, 1933). U, Cothurnia fibripes (from Kahl, 1933). V, Vaginicola tincta after Ehrenberg, 1838. W, after Kahl, 1935. X, after Stiller, 1971. Y, after Vucetich & Escalante, 1979. Z, after Shen & Gu, 2016. Z1, Vaginicola ceratophylli (from Penard, 1922). Z2, Vaginicola lagena (from Vucetich & Escalante, 1979). Z3, Vaginicola plicata (from Shen, 1980). Z4, Vaginicola festivus (from Shen & Gu, 2016). Scale bars = 30 μm.