Ancistrum acutum, Xu & Song & Warren, 2015

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 : 200-202

Ancistrum acutum n. sp. ( Figs 14–17 ; Table 1)

Nomenclature: Ancistrum acutum Xu, Songand Warren, 2002 in Song et al. (2003, p. 132) is invalid because it is not indicated as a new species (see Article 16.1 of the ICZN 1999).

Diagnosis: Size about 90 × 30 µm in vivo; body triangular with pointed anterior end. One ellipsoidal to globular macronucleus and one micronucleus. On average 23 somatic kineties, most extending to caudal pole. Buccal area extending to caudal pole and occupying about 3/4 of body length. Membranelles 1 and 3 short, each composed of three rows of kinetosomes; M2 composed of two rows and occupying about 2/3 of body length.

Type host and site: Marine surf clam Mactra veneriformis Reeve , gills and mantle cavity.

Type locality: Marine culture beds at Rizhao on the Yellow Sea coast of northern China (35°40′N, 119°57′E) .

Type material: A wet-silver slide with the holotype specimen encircled in ink is deposited in the Laboratory of Protozoology , Ocean University of China, with registration number RZ-950428-32 . A protargol slide with paratype specimens is deposited in the Natural History Museum , London with registration number NHMUK 2014.4 .8.2. A voucher slide with protargol-stained specimens from the host mollusc Mactra chinensis is deposited in the Marine Biological Museum , Chinese Academy of Sciences (Qingdao) with registration number QD-971204 .

Etymology: The Latin adjective acutus (acute) refers to the pointed anterior end of body, a main feature of the species. We have retained the species name originally designated by Xu et al. (2003) so as not to inflate the number of ciliate species names.

Description: Size in vivo 80–110 × 25–35 µm, usually about 90 × 30 µm, while on average 85 × 36 µm in protargol-stained specimens, suggesting a slight inflation of the body during the fixation/staining process ( Table 1). Body more or less triangular in lateral view, with anterior end pointed and posterior end rounded or slightly truncate, widest at posterior third of body; ventral margin slightly convex, dorsal margin sigmoid at anterior two thirds of body ( Figs 14–17 ). Single macronucleus usually located in anterior half of body, broadly ellipsoidal, about 23 × 19 µm in protargol-stained specimens. One globular to ellipsoidal micronucleus positioned anterior to macronucleus ( Fig. 17 ). Contractile vacuole located anterior to cytostome near the ventral side. Cortex flexible, with cortical granules loosely arranged in rows. Cytoplasm hyaline in anterior half of body but often opaque in posterior half due to accumulation of many food vacuoles, up to 5 µm across, and lipid droplets ( Figs 14, 15 ). Locomotion relatively slow, by rotation about main body axis; cells can attach to substrate via the anterior thigmotactic area.

On average 23 (21–25) somatic kineties: 11–13 on the left and 10–12 on the right side; anterior two thirds of kineties composed mainly of dikinetids and posteri- or third composed mainly of monokinetids; cilia about 10 µm long, anterior-dorsal cilia very densely arranged and rigid, forming a thigmotactic area ( Figs 14, 16, 17 ). Left-ventral and dorsal kineties terminate at apical plate of naked area, right-ventral preoral kineties progressively shortened anteriorly forming an imbricate structure; all kineties extend to posterior end of body except for several slightly shortened left-ventral kineties ( Figs 16, 17 ).

Buccal field of typical pattern for the genus, occupying about 3/4 of body length. Oral cilia about 10 µm long anteriorly, increasing to about 25 µm long in the posterior region, where they form a conspicuous brush-like structure ( Fig. 14 ). Membranelle 1 short, composed of 12 kinetosomes arranged in three longitudinal rows forming an acute angle with M2; M2 L-shaped, composed of two longitudinal rows, often with two kinetosomes anteriorly, occupying about two-thirds of body length; M3 positioned posterior to M2, slightly longer than M1, and composed of 18 kinetosomes arranged in three oblique rows. Paroral membrane lies parallel to somatic kinety 1, extending slightly beyond M1 anteriorly and terminating posteriorly near caudal pole; composed of a zigzag file of closely arranged dikinetids, but appearing as a single row in silver-stained specimens ( Fig. 16 ). Cytostome at base of PM and M2, shape drop-like, connected with radially arranged fibers ( Fig. 16 ). Scutica (Sc) patch-like, located close to caudal cilium complex which bears a single caudal cilium about 20 µm long ( Figs 16, 17 ).

Occurrence and prevalence: We first isolated Ancistrum acutum from the gills and mantle cavity of the surf clam Mactra veneriformis Reeve in April 1995, when the water temperature was 10°C and the salinity was 22. The prevalence was 9/20 and the intensity of infestation was moderate. In December 1997 we also isolated this species from the sunray surf clam Mactra chinensis Philippi on the Qingdao coast (water temperature of 10°C, salinity 31), where the prevalence was 12/20 and the intensity of infestation was light.

Figures

Figs 14–25. Ancistrum acutum n. sp. from the surf clam Mactra veneriformis (14–17) and Ancistrum japonicum Uyemura, 1937 from the Japanese dosinia Dosinia japonica (18, 19, 21, 22) and the clam Cyclina sinensis (20, 23–25), from life (14, 15, 18–20) and after protargol (23–25) and silver nitrate impregnation (16, 17, 21, 22). 14 – left lateral view of a representative specimen; 15 – body variant and cortical granules; 16, 17 – ventral and dorsal view of the holotype specimen; 18, 19 – lateral view of living cells; 20 – lateral view of a representative specimen; 21, 22 – lateral view of same specimen; 23 – ventral ciliature; 24, 25 – lateral view of the neotype specimen. CCo – caudal complex; Cs – cytostome; CVP – contractile vacuole pore; M1–3 – membranelles 1–3; MA – macronucleus; MI – micronucleus; PM – paroral membrane; Sc – scutica; SK1, n – somatic kineties 1, n. Scale bars: 30 µm.