Species identification of small bloom-forming Iraqi Peridiniopsis was based on observations of its morphological characters using light, epifluorescence, and scanning electron microscopy. Both alive and preserved cells from field samples were examined. Based on the main diagnostic features of the genus Peridiniopsis (Bourrelly 1968; Popovský and Pfiester 1990) including cell shape and dimensions, thecal surface ornamentation, and the size, shape, and position of the anterior intercalary plate, the small Peridiniopsis taxon from the Shatt Al-Arab River was assigned to P. minima Zhang, Liu & Hu, a species originally described recently from the Jiulongjiang River, China (Zhang et al. 2014).
Systematics
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Class DINOPHYCEAE West & Fritch, 1927
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Order PERIDINIALES Haeckel, 1894
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Family PERIDINIACEAE Ehrenberg, 1830
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Genus Peridiniopsis Lemmermann, 1904
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Peridiniopsis minima Zhang, Liu & Hu, 2014
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Figs. 2 and 3
Description
Cells were oval to almost rhombic in shape and slightly dorso-ventrally compressed, with nearly equal in size conical epitheca and smoothly round hypotheca (Figs. 2a; 3a, c, e). Cells measured 13.5–19.04 μm in length (mean 16.27 ± 1.66, n = 14), 12.8-17.9 μm in width (mean 15.05 ± 1.65, n = 11), and 12.3–15.3 μm in depth (mean 13.71 ± 1.04, n = 7), with length to width ratio of 1.01–1.1. The cingulum was nearly equatorial, excavated, and descending with a displacement of about half of its own width. The sulcus was excavated and extended from the cingulum to the antapex (Figs. 2c, e, f and 3a-c, h). Numerous small, golden-brown disc-shaped chloroplasts were peripherally arranged (Fig. 2a-c). Relatively large round dinokaryon was centrally located. Endosymbiont nucleus was present (Fig. 2b-d). Distinct red eyespot was located in the sulcal area (Fig. 2a, b). No pyrenoid was observed.
The plate tabulation was Po, X, 3′, 1a, 6″, 5c, 5 s, 5′′′, 2′′′′. The epitheca consisted of twelve symmetrically arranged plates. The apical pore plate (Po) was elongate and possessed slit-like and slightly curved apical pore. The canal plate (X) was small, rectangular, and situated between Po, 1′, 2′ and 3′ on the ventral side of the cell (Figs. 2i, j and 3f). The Po plate was encircled by three apical plates (1′–3′), of which plate 1′ was in contact with the sulcus and connected to the apical pore plate by the canal plate (Figs. 2h-j and 3a, e). The first apical plate (1′) was large, rhombic; the plates 2′ and 3′ were large, irregularly six-sided, and mainly located dorsally. The single anterior intercalary plate (1a) was relatively small, rhombic, and located symmetricaly on dorsal cell side between 2′, 3′, 3′′and 4′′ epithecal plates (Figs. 2g-i and 3b, e). There were six precingular plates, among them the plates 1′′, 2′′ and 5′′ were four-sided, whereas the plates 3′′, 4′′ and 6′′ were pentagonal.
The hypotheca possessed rather symmetrically arranged five postcingular (1′′′-5′′′) and two antapical (1′′′′ and 2′′′′) plates. The first (1′′′) and fifth (5′′′) postcingular plates were large and covered most of the ventral side of the hypotheca, whereas the 2′′′-4′′′ plates together with the two antapical plates cover the dorsal side of the hypotheca (Figs. 2f, k and 3a, b).
The cingulum consisted of five unequal plates (c1–c5), among them the first cingular plate was smallest in the cingular series (Figs. 2l and 3a). In the sulcus, which consisted of five plates, the small anterior sulcal plate (Sa) is contiguous with the first cingular plate at the left end of the cingulum and touched the plates 1′, 6′′ and c6. The large right sulcal plate (Sd) was relatively narrow, elongate, and had a thin internal sulcal list (sl) on its left margin. The left sulcal plate (Ss) was elongate and extended along the right side of the first cingular plate c1 and the first postcingular plate 1′′′. The median sulcal plate (Sm) was small and located between the lower parts of Sd and Ss. The posterior sulcal plate (Sp) was the largest of the sulcal series, wider toward the posterior, and elongated to the antapex between the first and fifth postcingular plates (Figs. 2e, f and 3h, i).
The thecal surface was smooth with small rimmed pores (0.15–0.16 μm in diameter) scattered sparsely over the plates (Figs. 2h, j and 3e-g). Densely lying pores formed distinct marginal rows along the upper and lower edges of the pre- and postcingular plates bordering the cingulum (Fig. 3g). The cingular and sulcal plates were lacking pores, except for the Sp plate (Fig. 3h, i).
Known distribution and occurrence
Peridiniopsis minima has been originally described from the freshwater phytoplankton assemblage in the Jiulongjiang River, Fujian Province of China as bloom-forming species (Zhang et al. 2014). Similarly, in the Shatt Al-Arab River system this species represented a major proportion of the overall phytoplankton assemblage at sites 1 and 2 during November and December 2009, and to a lesser extent at site 3 in April 2010. The occurrence of P. minima was recorded along the Shatt Al-Arab River at temperatures ranging from 19.3 °C to 23.9 °C and salinity range from 2 to 7.2 (Table 1).
