Plagiogrammopsis minima (Salah) Sabbe & Witkowski,, 2010

Kim, Bongho & Park, Jong Seong Khim and Jinsoon, 2021, Taxonomy and distribution of two small negligible diatoms of Plagiogrammopsis minima and Diploneis aestuarii from Northeast Asian tidal flats, Journal of Species Research 10 (3), pp. 237-245 : 238-241

Plagiogrammopsis minima (Salah) Sabbe & Witkowski,

2010 ( Figs. 2 , 3 )

Reference. Salah, 1955, Hydrobiologia. 7, p. 91, pl. I, fig. 15; Sabbe et al., 2010, Vie et Milieu, 60(3), 243-256.

Taxonomic remarks. The generic features of Plagiogrammopsis comprise a pseudoseptum, laterally opening ocelluli, long spines, and an external cribrum with spinules ( Sabbe et al., 2010). Plagiogrammopsis minima has a somewhat complex taxonomic history. The species was first described as new species to science by Salah (1955) under the name of Plagiogramma minimum . In the meantime, certain specimens of the very species were also newly described and reported as Plagiogrammopsis crawfordii by Witkowski et al. (2000), yet the taxonomic characteristics of P. crawfordii were not properly defined. The description of the species did not match with photographic information provided in the literature. Sabbe et al. (2010) found that part of the ‘ P. crawfordii ’ specimens were conspecific with P. minimum ; since P. crawfordii represents more than one species, it is taxonomically invalid. Sabbe et al. (2010) also found that Plagiogramma minima should be a member of the genus Plagiogrammopsis judging from its ultrastructure under SEM observation, thus the generic transfer of the species has been made under the nomenclature of ‘ Plagiogramopsis minima ’. In the meantime, the authors have also described a new species viz. Cymatosira minutissima based on certain other specimens of ‘ P. crawfordii ’.

The size of P. minima specimens from the present study is slightly larger compared to the original description of the species in Salah (1955), where the frustules were 6-14 μm in length and 2.5-3 μm in width. Plagiogrammopsis minima is similar to C. minutissima in terms of length (3-9 μm) and width (1.5-3 μm) with striae of 12-15 in 10 μm ( Garcia, 2016). The central area of P. minima is distinctly constricted to pseudoseptum in valve view, and two ocelluli are laterally twisted in opposite directions at valve apices. Cymatosira minutissima is easily differentiated from P. minima in terms of the circular central area and centrally located two ocelluli.

LM observation. Outline of the valves is narrowly lan- ceolate with rounded apices, 8.3-16.0 μm in length, 2.0- 3.7 μm in width ( Fig. 2A- D, G- J ). The frustules are rectangular with slightly produced apices in girdle view ( Fig. 2E, F, K, L ). Central area of the valve is distinctly constricted in LM due to the presence of pseudoseptum. No areolae are present at the apices or in the center of the valve. The density of striae is 12-15 in 10 μm, and the striae are rather irregular ( Fig. 2A- D, G- J ).

SEM observation. External view of the valve face is slightly curved along the edge, merging almost indiscernible with a rather deep mantle ornamented by a row of poroids ( Fig. 3A, B ). Two ocelluli are laterally twisted in opposite directions at valve apices ( Fig. 3C, D ). The central area presents a hyaline area and areolae are occluded by an external cribrum ( Fig. 3C ). Internal and external view of the pseudoseptum extends from margin to margin in the central fascia and has one rimoportula on one side of the pseudoseptum ( Fig. 3D- F ). The striae are developed irregularly one on the valve face and one along the valve margin ( Fig. 3C, D ).

Distribution. Plagiogrammopsis minima was firstly reported from salt marshes at Blakeney Point, Norfolk, England by Salah (1955). Sabbe et al. (2010) reported the species from sandy sediments at Westerschelde Estuary, North Sea, and the Portugal Atlantic Coast. The species was also observed from Bombas, State of Santa Catarina, Brazil by Garcia (2016). The present study is the very first report of P. minima from Korean waters. In the present study, the species was observed from four locations from Korea, two locations from China, and one location from Japan ( Fig. 1 ).

Figures

Fig. 2. Light microscope (LM) photographs of Plagiogrammopsis minima: A-D and G-J. Size range in valve view; E, F and K, L. Size range in girdle view. Scale bar=10μm.

Fig. 3. Scanning electron microscope (SEM) photographs of Plagiogrammopsis minima from A-D (Minami-arao, Japan), E, F (Nakdong, Busan, Korea): A. Entire view of the girdle. Arrow indicates row of the short spines on the valve face. Large arrowhead indicates girdle bands ornamented by a row of poroids; B. External view of the valve face with girdle bands. Right arrow indicates pseudoseptum extends from margin to margin in the central fascia. Left arrow indicates the cribrum occlusion; C. External view of the whole valve. Large arrow- heads indicate the two ocelluli which are twisted to opposite directions along the valve axis. Arrow indicates the cribrum occlusion; D. Internal view of the whole valve. Large arrowheads indicate the two ocelluli which are twisted to opposite directions at valve apex. Arrow indicates internal view of the cribrum occlusion located outside of the areolae; E. Right arrow indicates the pseudosptum while left arrow indicates external aperture of the rimoportula; F. Right arrow indicates the pseudosptum while left arrow indicates a rimoportula.

Fig. 1. The distribution of Plagiogrammopsis minima and Diploneis aestuarii in the Northeast Asia.