Spirostomum minus, ROUX, 1901

SPIROSTOMUM MINUS ROUX, 1901 View in CoL

( FIGS 2A–F View Figure 2 , 3A–O; TABLE View Figure 3 2)

Spirostomum minus View in CoL was first recorded as a variety of S. ambiguum (Roux, 1901) View in CoL . During the past century, numerous populations of this taxon have been reported, although most descriptions lack molecular and detailed morphological data. It is, therefore, challenging to evaluate these isolates ( Repak & Isquith, 1974; Foissner et al., 1992; Fernandes & da Silva Neto, 2013; Boscaro et al., 2014). An improved diagnosis is supplied here based on both previous and current data.

Improved diagnosis: Body about 300–900 × 30–50 μ m in vivo when extended, length to width ratio about 7–20:1, lanceolate in outline when contracted; peristome length variable, 35–65% of body length; adoral zone with 166–216 membranelles; 12–40 somatic kineties; cortical granules arranged in 2–4 rows between adjacent somatic kineties; macronucleus moniliform with 5–50 nodules; contractile vacuole terminally positioned, less than 20% of body length, with a long canal that extends anteriorly.

Voucher specimens: Four protargol slides (registration no. CY 2018043001- 01, 02, 03, 04) with voucher specimens are deposited in the Laboratory of Protozoology, Ocean University of China (OUC).

Description of Qingdao population: Body size about 425–580 × 40–50 μ m in vivo when extended, slender body with length to width ratio about 7–13:1 ( Figs 2A View Figure 2 , 3A–C, F View Figure 3 ); lanceolate when contracted ( Fig. 3D View Figure 3 ). Pellicle thin and soft, with greenish spherical cortical granules (0.7–1.0 μ m in diameter) sparsely distributed in 2–4 rows between adjacent somatic kineties ( Figs 2D View Figure 2 , 3I, J View Figure 3 ). Peristome about 50–65% of body length ( Figs 2A, C View Figure 2 , 3A–C View Figure 3 ). Cilia of adoral membranelles about 15 μ m long. Somatic cilia about 10 μ m long. Cytoplasm colourless or brownish, usually packed with numerous dark-grey granules and inclusions, which render the main part of the body somewhat opaque ( Fig. 3C View Figure 3 ). Macronucleus moniliform with 9–19 interconnected nodules arranged in a line on the dorsal side of the body ( Figs 2A, E View Figure 2 , 3G, H, K View Figure 3 ); micronuclei inconspicuous, invariably closely associated with macronuclear nodules ( Fig. 3M View Figure 3 ). Contractile vacuole up to 20% of body length, terminally located, with a long collecting canal on the dorsal side that extends almost to the anterior end of the body ( Figs 2A View Figure 2 , 3A, C, F, G View Figure 3 ). Locomotion by gliding over substratum, occasionally swims slowly while rotating about the main body axis.

Ciliary pattern as shown in Figures 2B, E, F View Figure 2 , 3K– O View Figure 3 . Adoral zone of membranelles makes one turn of the body (when observed in spirally contracted individuals), beginning at the apical end and terminating in the mid-body region ( Figs 2E, F View Figure 2 , 3K View Figure 3 ); adoral zone composed of 166–216 membranelles, each of which consists of one short and two long rows of basal bodies ( Figs 2B View Figure 2 , 3N View Figure 3 ). Paroral visible in vivo ( Fig. 3E View Figure 3 ) and located to the right side of the proximal portion of the adoral zone of membranelles ( Figs 2B View Figure 2 , 3L View Figure 3 ). Circumoral kinety comprises a single row of kinetids and lies parallel to the right margin of the distal portion of the adoral zone of membranelles, connects with the anterior end of the paroral and continues a short distance beyond the posterior end of the paroral ( Figs 2B, E, F View Figure 2 , 3L View Figure 3 ).

Twenty-five to 32 spiral somatic kineties composed of dikinetids; most kineties extend the entire cell length but there are several shortened rows on both ventral and dorsal sides ( Figs 2E, F View Figure 2 , 3O View Figure 3 ; Table 2).