Ancistrum crassum Fenchel, 1965

Xu, Kuidong, Song, Weibo & Warren, Alan, 2015, Two New and Two Poorly Known Species of Ancistrum (Ciliophora, Scuticociliatia, Thigmotrichida) Parasitizing Marine Molluscs from Chinese Coastal Waters of the Yellow Sea, Acta Protozoologica 54 (3), pp. 195-208 : 199-200

Ancistrum crassum Fenchel, 1965 ( Figs 8–13 ; Table 1)

Host, site and locality. Purple clam Saxidomus purpuratus (Sowerby) from the Yellow Sea coast at Rongcheng, northern China; gills and mantle cavity.

Voucher specimens. Two voucher slides with silver stained specimens, i.e. one wet-silver (RC-980430-01) and the other protargol (RC-980430-02), are deposited in the Laboratory of Protozoology, Ocean University of China.

Redescription. Size in vivo 50–70 × 15–25 µm, usually about 60 × 20 µm; ratio of length to width usually about 3: 1, bilaterally flattened about 2: 1. Body elongate-ellipsoidal with ventral margin convex and dorsal margin concave in the middle portion; anterior end narrowly rounded with a naked area, which is recognizable both in vivo and in protargol-stained specimens; posterior-dorsal portion forms a cone-shaped prolongation ( Fig. 8 ). Single macronucleus ovoid or elongate-ellipsoidal, about 23 × 10 µm in protargol-stained specimens ( Table 1). One micronucleus, globular to ellipsoidal, frequently positioned anterior-dorsal to macronucleus ( Fig. 10 ). Contractile vacuole located anterior to cytostome near ventral side, 5–6 µm across. Cortex thick, with cortical granules both in clusters and loosely arranged along ciliary rows. Cytoplasm usually hyaline in anterior half of body and opaque in posterior half due to accumulation of many food vacuoles, about 6 µm across, containing bacteria-like remnants. Movement by rotation about main body axis; cells may attach to substrate via anterior thigmotactic area.

On average 23 (20–33) somatic kineties evenly distributed on left and right sides, composed primarily of dikinetids in area above cytostome; somatic cilia about 8 µm long; thigmotactic area indistinct because anterior cilia are less densely arranged ( Figs 8–10 ). Somatic kineties mostly extending to anterior end of cell forming an imbricate structure; several right-ventral preoral kineties which are progressively shortened anteriorly; in posterior region of cell, all kineties excluding SK1 are distinctly shortened forming a distinct glabrous zone on both left and right sides ( Figs 9, 10 ).

Buccal field of typical generic pattern, occupying about 63% of body length. Oral cilia about 16 µm long in vivo in anterior region, increasing to about 25–30 µm long in posterior region where they form a conspicuous brush-like structure ( Fig. 8 ). M1 short, composed of two or three rows of kinetosomes; M2 L-shaped, composed of two longitudinal rows often with several single-rowed kinetosomes anteriorly, occupying slightly more than one third of body length; M3 near end of M2, of similar length to M1, composed of 10 kinetosomes arranged in three oblique rows. Paroral membrane (PM) lies parallel to somatic kinety 1, composed of a zigzag file of dikinetids, extending slightly beyond M1 anteriorly and terminating posteriorly near caudal pole of body. Cytostome at base of PM and M2, ellipsoidal in outline, connected with radially arranged fibers ( Figs 9, 11 ). Scutica patch-like, located close to caudal cilium complex which bears a single caudal cilium about 18 µm long ( Figs 10, 11 ).

Silverline system as depicted in Fig. 11 . Transverse silverlines sparsely distributed between somatic kineties, all of which are joined by silverlines at posterior end; longitudinal silverlines present in buccal area. Cytopyge (Cyp) underneath paroral membrane, connected with PM by silverlines. Single contractile vacuole pore (CVP) located near posterior end of somatic kinety 2 ( Fig. 11 ).

Occurrence and prevalence: The population described here was found in 1998 from the purple clam Saxidomus purpuratus on the Rongcheng coast of the Yellow Sea, where the water temperature was about 14.8°C and salinity about 28.The prevalence was 10/10 and the infestation was moderate to heavy. It co- -occurred with an ectoparasitic ciliate, Trichodina ruditapicis , on the same host, the prevalence of which was 8/10 with low intensity.

Ancistrum crassum was originally found by Fenchel (1965) in high numbers in the European aurora venus clam Venerupis aurea (Gmelin) with prevalence of 16/20, and in a specimen of V. pullastra (Montague) from the Øresund coast of the Baltic Sea. This species was not subsequently reported until Xu et al. (1997) isolated it in April 1995 from the short-necked clam Ruditapes philippinarum (Adams) from the Qingdao coast of the Yellow Sea, where the water temperature was about 10°C, salinity about 32 and pH 8.0. The prevalence was 50/50 and the intensity of infestation was moderate to high. Later, Song (2000) redescribed A. crassum based on the Qingdao population parasitizing Ruditapes philippinarum which he mistakenly identified as Haliotis discus hannai Ino. We have repeatedly found Ancistrum crassum parasitizing Ruditapes philippinarum from the China coast of the Yellow Sea (unpublished data). Xu and Song (2003) also recorded it from the venus clam Protothaca jedonensis (Lischke) and the purple clam Saxidomus purpuratus (Sowerby) . However, the host record of Ruditapes variegata was misidentified as Ruditapes philippinarum , and the host record of the small sand clam Caecella chinensis Deshayes needs reconsideration. The population described by Oishi (1978) under the name Ancistrum edajimanum from Ruditapes philippinarum in Japan is likely also to be A. crassum (see Discussion below).

Figures

Figs 1−13. Ancistrum haliotis n. sp. from the abalone Haliotis discus hannai Ino (1–5), Ancistrum mytili (Quennerstedt, 1867) from the blue mussel Mytilus edulis (6, after Kidder 1933) and the horse mussel Modiolus modiolus (7, after Hatzidimitriou and Berger 1977) and Ancistrum crassum Fenchel, 1965 from the purple clam Saxidomus purpuratus (Sowerby) (8–11) and from the short-necked clam Ruditapes philippinarum (12, 13, after Xu et al. 1997), from life (1, 2, 6, 8) and after protargol (3–5, 9, 10, 12, 13) and silver nitrate impregnation (7, 11). 1 – left lateral view of a representative specimen; 2, 3 – ventral view to show the oral structure; 4, 5 – left and right lateral view of the holotype specimen; 6, 7 – lateral and ventral view of A. mytili, which possesses a characteristic reniform macronucleus and a broad buccal field; 8 – left lateral view of body variants; 9–13 – lateral and ventral view of three specimens to show the ciliary pattern. CCo – caudal complex; CyP – cytoproct; M1–3 – membranelles 1–3; MA – macronucleus; MI – micronucleus; PM – paroral membrane; Sc – scutica; SK1, n – somatic kineties 1, n. Scale bars: 30 µm (4, 5 and 7, 9–13 drawn to scale).