Metacystis similis

Zhang, Xiumei, Ji, Daode, Zhang, Qianqian & Li, Chenghua, 2015, Description and phylogeny of a new prostomatid, Metacystis similis nov. spec. (Protista, Ciliophora) from the East China Sea, Zootaxa 4033 (4), pp. 584-592 : 585-586

Morphology of Metacystis similis nov. spec.

( Figs. 1 , 2 ; Table 1 )

Diagnosis. Cylindrical marine Metacystis with a slightly blunt anterior end, in vivo 50- 70 x 18-23 Μm; cell not loricate, uniformly cinctured by 16–18 transverse rings of cilia, which are arranged into 32–36 longitudinal somatic kineties; the oral apparatus consisting of one circumoral kinety of dikinetids and six girdles of monokinetids that pack together; one spherical macronucleus located in the center of the body; the caudal cilium possessed one hemicycle transparent and protruding terminal vacuole.

Type of material. A protargol slide with the holotype specimen was deposited in the collection of Laboratory of Protozoology, Ocean University of China, with registration number 20141107Z. The holotype specimen is marked by an ink circle on the coverslip. A paratype slide with protargol-impregnated specimens was deposited in the Laboratory of Protozoology, Yantai University, China with registration number 20141108Z.

Etymology. The Latin adjective " similis " denotes general similarity between the new species and its congeners.

Description. Cells are 50– 70 x 18–23 µm in vivo, and the body shape is slightly changeable with mostly barrel-shaped or cylindrically-shaped ( Figs. 2 A–C). Body is rarely contractile, and the ratio of length: width is ca. 2.5–3:1 ( Table 1 ). From the blunt posterior end, the body slightly tapers toward the truncated anterior aperture ( Figs. 2 A–C). Lorica is absent.

The cytoplasm is colorless to slightly grayish and contains many large granules (5–8 Μm in diameter) but is transparent posteriorly, which is due to the existence of a large caudal vacuole (15–18 Μm in diameter) ( Figs. 2 B– E). The pellicle is rigid, and its surface consists of transverse ridges ( Figs. 1 A, 2G, arrowheads) and longitudinal somatic kineties, thus clear rectangular meshes can be observed above 100× magnification ( Figs. 1 J, 2F). The macronucleus is large and round to oval, mostly centrally located, in vivo always exhibits a transparent area. After impregnation, several nucleoli were scattered in the macronucleus. A large hemicycle transparent vacuole protrudes at the terminal end of the cell ( Figs. 2 A–D).

Cilia of Metacystis similis nov. spec. are densely arranged with a length of ca. 9 µm at the oral part and a length of 6 µm in somatic kineties. Caudal cilia are absent. Metacystis similis nov. spec. always utilizes a single mode of locomotion: when swimming, the body moves forward at moderate speed and rotates along its longitudinal axis. The cell is not sensitive to disturbance or stimulation. When observed under microscope, the cell constantly moves back and forth without pause at the bottom of the water.

The circumoral ciliature is composed of one circumoral dikinety and six densely packed girdles of monokinetids ( Fig. 2 I), which has clear circumscription with somatic ciliature. Additionally, the oral infraciliature is considerably denser than the somatic counterparts. The somatic infraciliature is composed of 32–36 longitudinal kineties extending over the entire length of body and patterned as 16–18 transverse rows of monokinetids ( Fig. 2 H).

Abbreviations: CV, coefficient of variation (%); Max, maximum; Mean, arithmetic mean; Min, minimum; n, number of individuals investigated; SD, standard deviation; SE, standard error of arithmetic mean.

Figures

FIGURE 1. Metacystis similis nov. spec. (A, I) and similar congeners (B-H). A. Morphology of a living cell. B. Metacystis striata (Pop. I), from Stokes, 1893. C. Metacystis hyperhalina, from Dietz, 1964. D. Metacystis parastriata, from Chen, 2009. E. Metacystis elongata, from Kahl, 1928. F. Metacystis striata (Pop. II), from Borror, 1972. G. Metacystis striata (Pop. III), from Foissner, 1984. H. the infraciliature of Metacystis striata (Pop. III), from Foissner, 1984. I. Metacystis sp., from Dragesco & Dragesco-Kernéis, 1986. J. Infraciliature of Metacystis similis nov. spec. Cy, cytostome; DK, circumoral dikinetids; FV, food vacuole; Ma, Macronucleus; MK, circumoral monokinetids; arrowhead shows circumoral dikinetids; arrow indicates circumoral monokinetids. Scale bars: 50 µm (Figs. A, B-F, I); 40 µm (Fig. E).

FIGURE 2. Photomicrographs of Metacystis similis nov. spec. from living specimens (A – G) and after protargol staining (H – I). A. Living individuals under low magnification. B-D. Lateral view of two typical individuals, arrows show the caudal vacuole. E. Food vacuoles and granules. F. Detail of the body surface; rectangular pellicle meshes. G. Detail of the pellicle structure; ridges on the cell surface (arrowheads). H. General infraciliature. I. Buccal area, double arrowhead refers to the circumoral ciliature. Scale bars: 50 µm (Fig. A); 30 µm (Figs. B, C, H).

Tables

TABLE 1. Morphometric characteristics on Metacystis similis nov. spec.

Characters	Min	Max	Mean	SD	SE	CV	n
Body length (µm)	50	70	60.9	5.4	1.2	8.9	20
Body width (µm)	18	23	19.9	1.4	0.3	7	20
No. of circumoral kineties	7	7	7.0	0	0	0	25
No. of transverse rings	16	18	17	0.8	0.2	4.7	25
No. of longitudinal kineties	32	36	34	1.4	0.3	8	25
No. of macronucleus	1	1	1.0	0	0	0	25
Macronucleus length (µm)	8	15	12	2.1	0.5	17.5	15
Macronucleus width (µm)	7	15	12	2.5	0.6	20.8	15