Delphinapterus leucas (Pallas, 1776)

Jin, Didi, Qu, Zhishuai, Wei, Bojue, Montagnes, David J. S., Fan, Xinpeng & Chen, Xiangrui, 2021, Two parasitic ciliates (Protozoa: Ciliophora: Phyllopharyngea) isolated from respiratory-mucus of an unhealthy beluga whale: characterization, phylogeny and an assessment of morphological adaptations, Zoological Journal of the Linnean Society 191, pp. 941-960 : 944-945

Host: Delphinapterus leucas (beluga whale).

Etymology: The species name paracetarius is a composite of the Greek prefix παρα -, beside, and κήτος, a whale or sea monster, referring to the speciesgroup name cetarius , indicating that this new species is morphologically similar to Kyaroikeus cetarius Sniezek, 1995 .

Type material: One slide with a protargol-stained holotype specimen (indicated with a black circle of ink on the coverslip) and several paratype specimens has been deposited in the Laboratory of Protistology, Ocean University of China ( OUC) with registration number LJ-I-20170502-01. Two slides with protargolstained paratype specimens were deposited in the collection of Ningbo University (registration numbers: LJ-I-20170502-02, LJ-I-20170502-03) .

General morphology and ciliary pattern: The body size is 150–400 × 20–40 μm in vivo and 150–365 × 35–80 μm after protargol staining. It has a long, spindle-shaped body with a length–width ratio of ~10:1, neither bilaterally nor dorsoventrally compressed ( Figs 1A, B, 2A, 3A). Pellicular ridges are distributed on the surface of the cell, between adjacent somatic kineties; densely packed kinetosomes are located at the base of these ridges ( Fig. 3H, I). A conspicuous bipolar, nonciliated stripe is located on the left ventral surface, 10–30 μm wide ( Figs 1C, D, H, 2F, R, 3B–F). Eleven to 16 pellicular folds are situated on the ‘naked’ stripe and are associated with same number of fibrous tracts that extended along the cell almost from end to end; the leftmost four or five folds and their associated fibres are bent to the right anteriorly; these form a hook-like cap and a subapical depression below ( Figs 1J, 3B, C). Remaining folds extended from this depression to the posterior end of the body. The oral cavity is located in the anterior quarter of the cell ( Figs 1J, 2O, P). A liplike structure on the right side of the oral cavity and 13–29 right kineties end here. Consequently, the liplike region is covered with dense cilia ( Figs 1A–F, H, 2A, B, O, R, green region). The cytostome is elliptical in outline and positioned at about the anterior onethird of the cell, beneath the depression of the oral cavity ( Figs 1J, 2B, O). Cytoplasm is colourless, filled with a few lipid droplets and numerous food vacuoles containing unidentified amorphous inclusions in the posterior end of the cell. A contractile vacuole is not detected.

The single macronucleus is heteromerous and ovoid, about 35 × 20 μm after protargol impregnation, located mid-body ( Figs 1I, 2M). A micronucleus is not detected. A prominent podite is broadly cone-shaped, located at the posterior end of the cell ( Figs 1A, H, I, 3A, J), about 10 μm in length in vivo ( Fig. 2A, F). Podite contains a bifurcated secretory organelle, which has an opening (0.5–1.0 μm across) at its tip; an obvious attachment thread (> 100 μm long) is secreted from the podite opening, forming a strong connection with the substrate ( Figs 1A, 2E). Ciliates are usually attached to the surface of flocs of exfoliated epithelial tissue by the podite and attachment thread. Cells tend to rotate through flocs of epithelial tissue. The ciliary pattern ( Figs 1H–J, 2F–R) comprises of 49–79 somatic kineties, including 37–69 right and seven to 13 left kineties. According to their starting position, right kineties include three parts: (1) the right part of right kineties originates from the right of circumoral kineties and these kineties on the ventral side, with posterior ends gradually shortened from right to left ( Fig. 1J); (2) the middle part of right kineties originates from the right of the cell apex on the ventral side and extends on to the dorsal surface, and then posteriorly terminates at a level near the podite ( Fig. 1I, J); and (3) the left part of right kineties is located at the cell apex on the dorsal side, and posteriorly extends to the podite, with some extending on to the left field on the ventral side ( Fig. 1H, I). Left kineties are densely arranged, located on the left margin of the oral cavity. These kineties are similar in length, and posteriorly ended in the anterior quarter of the cell ( Figs 1J, 2O, Q, R). Four kinetofragments are located slightly below the mid-body on the left margin of right kineties ( Figs 1H, 2F). The equatorial fragment is composed of five to 28 kinetosomes, positioned next to the middle part of the rightmost right kinety ( Fig. 2K, L).

The oral ciliature is composed of a tiny pre-oral kinety (~2 μm long) and two parallel circumoral kineties (~20 μm long) ( Figs 1G, J, 2O, P, R, 3D, G); the former is located on the anterior-right of the cavity and the latter at mid-cavity. Anterior ends of circumoral kineties are close to the pre-oral kinety ( Figs 1G, J, 2R). The cytopharynx is reinforced by argentophilic fibres and is extended to mid-body with the posterior end obviously curved ( Fig. 2P); no nematodesmal rods are found.

Fine structures: Each of the circumoral kineties is composedofmonokinetids ( Fig.4E).Thecytopharyngeal tube consists of ~180 cytostomal lamellae ( Fig.4A, B,D), which are bar-like and oriented obliquely to the centre of the tube, forming an enclosed circle when viewed in cross-section. These lamellae are possibly heterogenous, as speculated from the morphology shown in cross-section: about two thirds of them are thinner in their distal ends, while the others are of uniform thickness ( Fig. 4B, D).

The pellicle can be recognized as a ciliated area and a non-ciliated stripe in both SEM-prepared specimens and TEM sections. In the ciliated area, the pellicular ridges are arranged intrakinetally ( Fig. 3H, I). Each ridge contains a row of postciliary microtubules ( Fig. 4C). Parasomal sacs occur right of kinetosomes at the base of the pellicular ridge ( Fig. 4C). In the non-ciliated area, the pellicular folds are ~2 μm high, containing four or five (mostly five) strata of microtubules beneath the pellicle ( Figs 4H, I, 5), which correspond with the fibrous tracts revealed after protargol staining ( Figs 1J, 2R). The microtubules are highly organized: each stratum is arranged in an arch shape, extending with the same curve of the margin of the pellicular fold; each stratum contains several unequal-sized fragments, which are always two layered but with different numbers of microtubules ( Fig. 5). Pellicular pores occur in the bases of grooves between the pellicular folds and are evenly separated, as revealed by SEM ( Fig. 3E, F). In TEM, sacs (<1 μm at their widest) regularly occur beneath the pellicle of each groove, some of which contain materials and even open toward the outside ( Fig. 4F–I). There are often rich cytoplasmic vesicles containing granular material beneath the folds and near the secretory sacs ( Figs 4I, 5). These results suggest a secretory system composed of the cytoplasmic vesicles, sacs and pellicular pores ( Fig. 5). Mitochondria occur mainly in the cytoplasm of the cortex area ( Fig. 4C, G). Cytoplasm also contains food vacuoles encasing various food granules of different electron-density ( Fig. 4A).