Discussion
Protoperidinium vietnamicum closely resembles P. oceanicum(Vanhöffen) Balech (sensu Vanhöffen 1897: p. 268, 298, 312, 382, Tafel V, 2; Paulsen 1908: p. 54, Fig. 69a–c; Graham 1942: p. 24, Fig. 30; Dodge 1985: p. 60) by cells outline, especially from the ventral side, location and structure of apical and antapical horns, cingulum and sulcus morphologies, and the first apical plate 1’ (ortho–type). At the same time, the new species is clearly distinct from P. oceanicumby the morphology of the intercalary plates 1а–3а, and the plate 2a in particular, which in case of P. vietnamicum is penta, whereas inP. oceanicum quadra (sensu Jörgensen 1912: p. 5; Leboure 1925: p. 120, Fig. p. 36, b; Graham 1942: 24, Fig. 30; Balech 1974: 57; Hermosilla 1973: p. 26, Lám. 9, Figs 1–15; Dodge 1982: p. 180; Balech 1988: p. 85, Lám. 23, Figs 7–10; Okolodkov 2008: p. 119, Pl. 6, Figs 4–7; Al-Yamani and Saburova 2019: p. 316, Pl. 175). Abé (1981: p 324, Fig. 46(300–302) considered that the plate 2a in P. oceanicum could be quadra or penta. However, it should be noted that the illustrations of P. oceanicum in the mentioned study are more reminiscent of P. murrayi (sensu Kofoid 1907: p. 176, Pl. 5, Fig. 29), which is reported to exhibit the 2a plate of penta–type (Phan-Tan et al. 2017: p. 136, Figs 6a–e). Protoperidinium vietnamicumshows similarity with P. murrayi (Kofoid) Hernandez-Becerril (1991) in some morphological characteristics, including developed apical and antapical horns, shape, and ortho–penta plate arrangement (Figs 4a-f). However, P. vietnamicum is distinguished from P. murrayi by the following features: (1) the epitheca swiftly transitions into the shorter apical horn; (2) the fourth precingular plate (4”) is trapezoidal, with the up-side being about equal to the two lateral sides; (3) the antapical horns are slightly divergent (the angle between the horns is about 24–26ᵒ); (4) the wide cingulum slightly descending by 0.5–0.75 cingular width. In contrast, P. murrayi possesses a long apical horn arising abruptly from the epitheca; strongly diverged antapical horns (Figs 4e-f), with the angle between the horns being around 38–42ᵒ (Phan-Tan et al. 2017); the plate 4” approximately two times wider than long (Fig. 4h); the narrow cingulum descending about 2.0–2.5 times its width (Fig. 4e). The length of examined P. vietnamicum specimens ranges from 140 to 145 µm, and the width – from 60 to 63 µm. In comparison, the registered length of P. oceanicumvaries from 220 to 300 µm (Leboure 1925), 110–200 µm (Wood 1968), 156–167 (Hermosilla 1973), 160 µm (Dodge 1985), 127–160×87–95 µm (Al-Yamani and Saburova 2019); and the registered length of P. murrayi varies from 250 µm (Kofoid 1907), 205 µm (Matzenauer 1933), 190–208 µm (Hernández-Becerril 1991), and 170–185 µm (Phan–Tan et al. 2017). Over thirty cells of P. vietnamicum from net samples were examined by means of light microscopy and SEM, showing that it has distinct morphological features from other members of the section Oceanica .
Böhm (1931) presented a new form – Peridinium oceanicum f.curvicorne from the Persian Gulf and the Gulf of Oman.Protoperidinium curvicorne , which is proposed here as a new combination, possesses a unique set of morphological characteristics and is easily distinguished from other members of the genusProtoperidinium by its round, broad, strongly dorsoventrally compressed body with strongly inclined cingulum relative to the equatorial plane of the cell, and a pronounced tilt of a slender apical horn towards the ventral side. The taxon is insufficiently studied. Prior to the current work, there have been no data in the literature regarding the structure of the dorsal part of its epitheca, and the plates 1a–3a, in particular. The SEM micrographs provided in the monograph of Taylor (1976: Plate 45, Figs 522 b, 523), which was devoted to the dinoflagellates of the Indian Ocean, probably presented the best illustration of Protoperidinium curvicorne . We believe that the author erroneously determined the examined specimens as ”Peridinium murrayi Kofoid”. However, the line drawings ofP. murrayi , which were also provided by Taylor (1976: Pl. 34, Figs 379 and 380), do resemble the latter species.
Furthermore, in order to prove the identity of Protoperidinium curvicorne specimens (= P. oceanicum f. curvicorne ) found in the coastal waters of Viet Nam with the specimens collected from the type locality of this taxon (Böhm 1931), we conducted a comparative morphological study between P. curvicorne from the plankton of the Persian Gulf (Strait of Hormuz, coastal waters of Hormuz Island) and material from Vietnamese waters. We established the complete morphological identity of P. curvicorne specimens from these two populations (Fig. 3), despite the difference in environmental conditions and their remoteness from each other (about 6000 km). Dimensions ofP. curvicorne cells from the Persian Gulf showed a wider range of values compared to the specimens from the coastal waters of Viet Nam: length 178.9–224.6 µm, width 112.8–140.7 µm, depth 46.5–53.7 µm; and length 210–220 µm, width 110–120 µm, depth from 38–42 µm, respectively.