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 2а – 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.