Falcicyclidium citriforme Fan 2017

1. Falcicyclidium citriforme Fan et al., 2017 Material examined. Marine water (salinity 33.7‰, temperature 27.9°C) collected from Anin Beach, Gangdongmyeon, Gangneung-si, Gangwon-do, Korea (37°44 ʹ 2 ʺ N, 128°59 ʹ 26 ʺ E) on August 03, 2021. Diagnosis. Body size 30-35 × 15-20 μm in vivo, on average 30 × 20 μm after protargol impregnation (n = 5) (Fig. 1A, B). Body shape ordinary to broadly oval (Fig. 1A- C). One globular macronucleus, 6 μm across on average after protargol impregnation, located slightly above the body center; nucleoli globular to ellipsoidal and evenly distributed; micronucleus lacking or indistinguishable (Fig. 1B). Contractile vacuole terminal, 4-5 μm across in vivo at end of diastole (Fig. 1A). Cytoplasm with food vacuoles, crystals, and lipid (Fig. 1A). Somatic and caudal cilia 6-10 μm and 14-18 μm long in vivo, respectively; invariably ten bipolar somatic kineties; kineties 1 and 10 with 24- 27 and 22-26 mono/dikinetids, respectively (Fig. 1A- C). Oral apparatus occupies about 66% of body length after silver nitrate impregnation, composed of three adoral membranelles and paroral membrane; paroral membrane about 16 μm long (Fig. 1A- C). Silverline consists of primary meridians, which connect the somatic kinetids of each kinety, and transverse connectives without secondary meridians (Fig. 1C). Distribution. China (Fan et al., 2017) and Korea (present study). Remarks. The Korean population of Falcicyclidium citriforme is similar to the Chinese type population described by Fan et al. (2017). However, the Korean population has a slightly smaller body length compared to the type population (30-35 μm vs. 40-60 μm in vivo; Fan et al., 2017). In comparison to other species in the genus, F. atractodes Fan et al., 2011 resembles F. citriforme but can be easily distinguished by its body shape, with F. atractodes having spiny anterior and posterior ends, while F. citriforme lacks these spines (Fan et al., 2017). Additionally, Falcicyclidium fangi Fan et al., 2011 differs from F. citriforme in the ratio of the buccal field to body length (0.75 vs. 0.54; Fan et al., 2011; 2017). Compared to other similar genera, Cyclidium plouneouri Dragesco, 1963 resembles F. citriforme but can be easily distinguished by a lower number of somatic kineties (14- 16 vs. 10) and according to Wilbert, 1986 also by a lower number of somatic kineties (11 or 12 vs. 10). Additionally, Cyclidium borrori Borror, 1965 differs from F. citriforme by a lower number of somatic kineties (12 or 13 vs. 10). Voucher slides. Two slide with protargol-impregnated specimens (MABIK PR00045077, MABIK PR00045078) and one slide with silver nitrate-impregnated specimens (MABIK PR00045079) were deposited at the National Marine Biodiversity Institute of Korea.