Two new brackish-water species of centrohelid heliozoans (Haptista: Centroplasthelida), Choanocystis mylnikovi sp. n. and C. punctata sp. n., from Russia

The morphology of two new species of centrohelid heliozoans, Choanocystis mylnikovi and C. punctata , from the brackish-water Tuzlukkol’ River, South-Eastern Russia, were studied using scanning electron microscopy. The new species are covered with two types of scales that are plate scales and spine scales with heart-shaped basal plates and proximally curved shafts. C. mylnikovi has spine scales that are highly variable in length, which differs up to six times (2.0–12.9 µm). Spine scales of C. mylnikovi are slightly tapering, with two small teeth of the same length on the apexes. This species also has oviform or oval plate scales (1.6–2.9 × 1.0–1.8 µm) with an axial ridge and thin marginal rim. C. punctata has both short (3.6–9.9 µm) and long (10.0–18.5 µm) spine scales with pointed apexes. Sometimes short spine scales of C. punctata end with a bent apex or a small bent hook. Plate scales (2.9–4.7 × 1.5–3.2 µm) and basal plates of spine scales are ornamented with numerous granules. C. punctata sporadically demonstrates the presence of spine scales with the shaft in a nearly central position and membranous extensions of the basal plate along the shaft, which are the characteristics for the other morphologically defined genera of centrohelids – Acanthocystis, Pterocystis or Raineriophrys . Similarities and differences between the new species and other members of the genus Choanocystis are discussed.