Atopochilodon, KAHL, 1933

GENUS ATOPOCHILODON KAHL, 1933

ATOPOCHILODON DISTICHUM DEROUX, 1976 ( FIG. 3; TABLE 2)

In the original description, Deroux (1976b) mainly focused on the ciliary pattern and did not supply the characteristics of live cells. Therefore, based on his investigation and our new findings, an improved diagnosis is supplied in the current work.

Improved diagnosis: Marine Atopochilodon , size about 30–40 × 20–30 µm in vivo; oval shape with two protrusions in anterior and posterior left margins, respectively; a glabrous region present between right kineties and left kineties; 14–16 somatic kineties comprising of eight to ten right kineties and six left kineties; four frontoventral kineties; terminal fragment loosely arranged; 12 or 13 nematodesmal rods; two diagonally positioned contractile vacuoles.

Deposition of voucher slides: One voucher slide with protargol-stained specimens was deposited in Natural History Museum, London, with the registration number NHMUK 2016.3.21.2.

Description: Cell about 30–40 × 20–30 µm in vivo. Body oval in outline, with two ends broadly rounded ( Fig. 3A, F–H, J, L). Right margin convex; two protrusions located in anterior and posterior portions of left margin ( Fig. 3F, J). Ventral side flat, and dorsal side slightly vaulted with rod-shaped ectosymbiotic bacteria sparsely arranged ( Fig. 3D, I). Cytostome located in anterior quarter of cell, surrounded by 12–13 nematodesmal rods. Cytoplasm colourless, containing several food vacuoles (c. 4 µm in diameter) and granules (2–3 µm in diameter; Fig. 3K). Two contractile vacuoles, up to 2 µm in diameter, diagonally positioned in anterior and posterior third, respectively ( Fig. 3K). Macronucleus, ovoid and heteromerous, centrally located. Cilia 6 µm long; a glabrous region in middle of ventral side ( Fig. 3H), separating right kineties from left kineties. Movement by slow gliding on substrates.

Sixteen somatic kineties, comprising ten right kineties and six left kineties, separated by a crescent-shaped glabrous region ( Fig. 3B, C, O). Rightmost four kineties surpassing cytostome and bending to left, with inner three interrupted by perioral kineties ( Fig. 3M). Equatorial fragment composed of zero to nine kinetosomes. Terminal fragment loosely arranged along anterior margin, comprising of five to seven kinetosomes ( Fig. 3C, N).

Perioral kineties Y-shaped and completely composed of dikinetids ( Fig. 3B, C, M).

SSU rRNA gene sequence: The SSU rRNA gene sequence of A. distichum has been deposited in the GenBank database with the accession number, length, and G + C content as follows: KT461933 View Materials , 1731 bp, and 42.98%.