Acanthocystis nichollsi Siemensma et Roijackers, 1988

Prokina, Kristina I., Zagumyonnyi, Dmitry G. & Tikhonenkov, Denis V., 2018, Centrohelid Heliozoans (Centroplasthelida Febvre-Chevalier et Febvre, 1984) from Different Types of Freshwater Bodies in the Middle Russian Forest-steppe, Acta Protozoologica 57 (4), pp. 243-266 : 249-253

Acanthocystis nichollsi Siemensma et Roijackers, 1988 [syn.: A. pectinata Penard, 1889 emend. Nicholls, 1983 ] ( Figs 1A , 3A–D)

Description: Diameter of protoplast of living cells is 28–30 µm. Spine scales of two types (long and short), both consist of a hollow cylindrical shaft and circular base. Shaft is 0.20–0.30 µm in diameter, tip divided on 4–6 furcae. Long spine scales are 4.13–10.95 µm in length, base is 0.89–1.22 µm in diameter, furcae diverge on 0.42–0.67 µm. Short spine scales are 1.97–3.52 µm in length, base is 0.62–0.93 µm in diameter, furcae diverge on 0.60–1.35 µm and connected by a membrane, forming a cup-shaped structure. Oval plate scales are 2.28–3.08×1.41–2.04 µm, with concave lateral sides, medial thickening (0.50–0.86 µm in length), radial slits (0.17–0.26 µm in length), and dense margins.

Remarks: Morphology of observed scales corresponds with previous descriptions, however, some authors noted larger plate scales (3.8×2.1 µm) ( Leonov 2010 a, Leonov and Mylnikov 2012, Kosolapova and Mylnikov 2015) and larger spine scales (up to 15 µm) ( Leonov 2010a). Also, up to 8 furcae on short spine scales were described previously ( Dürrschmidt 1987a, Leonov 2010a). This species is similar to A. pectinata Penard, 1889 , A. polymorpha Dürrschmidt, 1985 , A. siemensmae Gerasimova et Plotnikov, 2016 , and A. valdiviense Dürrschmidt, 1987 , but differs by the morphology of cup-shaped tips of short spine scales and presence of radial slits on plate scales ( Mikrjukov 2002).

Distribution: Freshwaters of Europe ( Cavalier-Smith and von der Heyden 2007), Asia ( Kosolapova and Mylnikov 2015), N. America ([as A. pectinata ] Nicholls 1983), S. America ( Dürrschmidt 1987a, Prokina and Mylnikov 2019), Australia ([as A. pectinata ] Croome 1986), New Zealand, Malaysia, Sri Lanka ([as A. pectinata ] Dürrschmidt 1987a). It was found in Russia in freshwaters of European part (Leonov 2009, 2010 a, Leonov and Mylnikov 2012), in saline inland waters of Orenburg Region ( Gerasimova and Plotnikov 2015).

Acanthocystis pectinata Penard, 1889 emend. Siemensma et Roijackers, 1988 [syn.: Acanthocystis pectinata Penard, 1889 emend. Nicholls, 1983 ssp. ceylanica Dürrschmidt, 1987 ] ( Figs 1B , 3E–I)

Description: Diameter of protoplast of living cells is 27–29 µm. Hollow cylindrical shaft of spine scales is 0.20–0.25 µm in diameter. Long spine scales are 3.8– 10.3 µm in length, base is 0.76–1.15 µm in diameter, furcae diverge on 0.30–0.35 µm. Short spine scales are 1.3–3.2 µm in length, base is 0.48–0.82 µm in diameter, furcae diverge on 0.77–1.20 µm. Plate scales are 1.92–3.38×1.26–1.92 µm.

Remarks: Spine scales are similar to A. nichollsi . Plate scales differ from A. nichollsi by lacking the radial slits and presence of numerous granules, medial thickening (0.49–0.58 µm in length) and dense margins. This species differs from A. polymorpha , A. siemensmae , A. valdiviense by the morphology of cup-shaped tips of short spine scales ( Mikrjukov 2002). Morphology of observed scales corresponds to most known descriptions, except a different number of furcae in some papers: 2–3 in Siemensma and Roijackers (1988a), Leonov and Plotnikov (2009), Plotnikov and Ermolenko (2015), Prokina and Mylnikov (2019); 7–8 in Leonov (2010a), Prokina et al. (2017c). Many authors noted a larger length of long spine scales – up to 15 µm ( Leonov and Plotnikov 2009, Leonov 2010 a, Leonov and Mylnikov 2012, Plotnikov and Gerasimova 2017).

Distribution: Freshwaters of Europe ( Siemensma and Roijackers 1988 a, Mikrjukov 1993b), Africa ( Prokina et al. 2017b), S. America ( Prokina and Mylnikov 2019), Sri Lanka ([as A. pectinata ssp. ceylanica ] Dürrschmidt 1987a). It was found in Russia in freshwaters of European part ( Mikrjukov 1993a, Leonov and Plotnikov 2009, Leonov 2010 a, Leonov and Mylnikov 2012, Plotnikov and Ermolenko 2015, Prokina et al. 2017c), in saline inland waters of Orenburg Region ( Plotnikov and Ermolenko 2015, Plotnikov and Gerasimova 2017).

Acanthocystis penardi Wailes, 1925 [syn.: Acanthocystis spinifera Greef, 1869 sensu Penard, 1904 ; Acanthocystis heterospina Nicholls, 1983 ; Acanthocystis dresscheri Siemensma et Roijackers, 1988 ] ( Fig. 3J–M)

Description: Spine scales consist of a hollow cylindrical shaft and circular base. Shaft is 3.84–17.29 µm in length, 0.25–0.41 µm in diameter, base is 0.89–1.89 µm in diameter. Tip of shaft divided on 4–7 furcae, diameter of the widened part is 0.33–0.62 µm. Plate scales are 2.71–4.75×1.88–2.78 µm, with concave lateral sides, numerous granules and dense margins.

Remarks: Morphology of observed scales is similar to previous descriptions, but many authors noted up to 12 furcae on spine scales ( Nicholls 1983; Mikrjukov 1993a, b; Leonov 2009; Leonov and Plotnikov 2009; Prokina et al. 2017b, c). Some authors have described the larger size of spine scales: length of the shaft up to 33 µm ( Mikrjukov 1993b, Leonov 2009), the diameter of the shaft up to 0.8 µm ( Mikrjukov 1993b), the diameter of base up to 3.3 µm ( Mikrjukov 1993b, Leonov 2009, Leonov and Plotnikov 2009). This species is similar to A. mylnikovi Leonov, 2010 , A. taurica Mikrjukov, 1997 , and A. dresscheri . A. mylnikovi differs by size and number of furcae as well as the presence of two type of spine scales. All spine scales of A. taurica are almost equal in length, 4 µm ( Mikrjukov 2002). A. dresscheri differs by the presence of medial thickening on plate scales ( Siemensma and Roijackers 1988a), however, K.A. Mikrjukov con- siders A. dresscheri a junior synonym of A. penardi ( Mikrjukov 2002) . Morphology of scales in A. penardi is also similar to A. spinosa (see below).

Distribution: Freshwaters of Europe ( Mikrjukov 1993b), Asia ( Kosolapova and Mylnikov 2015), Africa ( Prokina et al. 2017b), N. America ( Nicholls 1983), S. America ( Dürrschmidt 1987a, Prokina and Mylnikov 2019), Malaysia ( Dürrschmidt 1987a). It was found in Russia in freshwaters of European part ( Mikrjukov 1993a, Leonov 2009, Leonov and Plotnikov 2009, Leonov and Mylnikov 2012, Prokina et al. 2017c).

Acanthocystis spinosa Cavalier-Smith et von der Heyden, 2007 ( Fig. 3N–P)

Description: Spine scales consist of a hollow cylindrical shaft and circular base. Shaft is 2.88–8.14 µm in length, 0.16–0.24 µm in diameter, base is 0.59–0.68 µm in diameter. Distal part of shaft slightly tapers up to 0.12–0.18 µm in diameter and divided on 2–3 short pointed furcae. Shafts of some spine scales are slightly curved in basal part, flattened and twisted. Oval plate scales are 2.19–3.17×1.65–2.14 µm, with short medial thickening (0.62–0.87 µm) and dense margins.

Remarks: Morphology of observed scales almost completely corresponds with the original description of this species from freshwaters of the UK ( Cavalier-Smith and von der Heyden 2007), except the larger size of scales in the original description. Those authors no- ticed very large plate scales (11–13 µm in length – not typical for scales of Acanthocystis ) as well as spine scales up to 17 µm in the text. But on figures ( Fig. 1a, p . 1187, and Fig. 4i–4k, p. 1193) they show different dimensions. Fig. 4k by Cavalier-Smith and von der Heyden (2007) shows length of plate scales about 12 µm (according to the scale bar), which corresponds to the description in the text. But in Fig. 4i we can cal- culate the length of plate scales as about 3.0–3.5 µm. Also, as we see in the photograph of a living cell ( Fig. 1a ), the length of plate scales cannot be 1/2 of the diameter of the cell (25 µm), otherwise they would be very clearly visible. Sizes of spine scales in Fig. 4j and 4k are four times different. The diameter of shaft of spine scales is about 0.8 µm, and diameter of base of spine scales is about 2.5–3.8 µm in Fig. 4k. But in Fig. 4j diameter of shaft of spine scales is about 0.2 µm, and diameter of base of spine scales is about 0.7 µm. Thus, we can assume that length of the scale bar on the fig. 4k is indicated incorrectly and there is a misprint in the text description. If so, sizes of observed scales completely correspond with the original description.

Spine scales with 2–3 furcae and slightly tapered distal end of the shaft occur in Acanthocystis spinifera sensu Siemensma et Roijackers, 1988 . Plate scales of A. spinosa differ by larger oval plate scales without concave lateral sides and with medial thickening. Also, the shaft of spine scales in Acanthocystis spinifera sensu Siemensma et Roijackers, 1988 is located on the base eccentrically.

Figures

Fig. 1. Morphology of living cells of some observed centrohelid species (DIC):A – Acanthocystis nichollsi; B – A. pectinata (from plant debris and silt of Usman’ River); C – A. turfacea (from plant debris and silt of Lake Chistoye); D – Raphidocystis symmetrica (from plant debris and silt of Lake Chistoye); E – Choanocystis aculeata (from sand of Lake Maklok); F – Raphidiophrys capitata (from sand of Lake Maklok). Abbreviations: ax – axopodia; pr – protoplast; sc – scales. Scale bar: 20 µm.