(FIGS 2, 3; TABLE 1)

Diagnosis: Body size in vivo 1100–1400 × 25–40 μm; 14–25 and 26–40 somatic kineties on head and trunk, respectively; single nuclear group composed of three or four macronuclei and two micronuclei; glabrous stripe narrow, corresponding to area occupied by two somatic kineties; cortical granules colourless and about 0.5 μm in diameter.

Type locality: A mangrove wetland on Techeng island, Zhanjiang, China (21°09′40″N, 110°25′38″E), where the water temperature was 26 °C and salinity about 25‰ (Fig. 1C).

Type specimens: A protargol-impregnated slide containing the holotype specimen marked with an ink circle is deposited in the Laboratory of Protozoology, Ocean University of China, China (No. YY2012112308). One paratype slide is deposited in the Natural History Museum, London, UK, with registration number NHMUK 2015.9.15.1.

Etymology: The species-group name quadrinucleatus reflects the fact that this organism usually possesses four macronuclei.

Description: Fully extended cells about 1300 × 35 μm in vivo; body flexible and contractile (Figs 2A–C, 3A– D). Cell distinctly tripartite, with neck, tail and trunk regions (Figs 2A–C, 3A–C). Head conspicuously claviform; tail wedge-shaped (Figs 2A, D, 3A, F). Body colour dark brown at low magnification due to multiple refractile inclusions, with several food vacuoles containing ingested algae (Figs 2D, G, 3E, F). Single nuclear group located in centre of trunk, containing three or four macronuclei, 7–10 μm in diameter, and two micronuclei, 2–4 μm in diameter (Figs 2G, 3E, H, I). Colourless cortical granules, c. 0.5 μm in diameter, scattered between ciliary rows, which are not found in glabrous stripe (Figs 2F, 3G). Locomotion by gliding between sand grains and organic debris.

Cell surface densely ciliated with unciliated zone, glabrous stripe, about as wide as two somatic kineties (Figs 2I, J, 3J–M). Entire infraciliature consisting of dikinetids. About 18 and 35 somatic kineties on head and trunk, respectively, with cilia c. 13 μm long. Anterior and posterior secant system formed on left side of glabrous stripe where some kineties abut to bristle kinety (Figs 2H, J, 3K, M). Oral infraciliature consisting of uninterrupted circumoral kinety with cilia about 5 μm long and roundish patch of disordered dikinetids in centre of oral cavity with cilia forming conspicuous tuft (Figs 2E, H, 3K, L).

Comparison: Since the genus Trachelolophos was established by Foissner & Dragesco (1996b), only the following two species have been reported.

Trachelolophos filum (Dragesco & Dragesco-Kernéis, 1986) resembles the new species in the number of somatic kineties on trunk. Although there is no in vivo information available for this species, it clearly differs from our new species in possessing more macronuclei (6–30 forming a strand vs. three or four macronuclei forming a single nuclear group) (Foissner & Dragesco, 1996b).

Trachelolophos gigas Foissner & Dragesco, 1996 has a similar body shape to the new form, but can be distinguished from the latter by having a larger size (2000 μm vs. 1100–1400 μm), a conspicuously higher number of somatic kineties on the trunk (52–71 vs. 26–40) and many more macronuclei (17–33 macronuclei forming a strand vs. three or four macronuclei forming a nuclear group) (Foissner & Dragesco, 1996b).