Figure 1. Valve view (upper) and 3-cell chain in girdle view (lower) of P. vanheurckii. Image adapted from Hasle et al. 1983.
Figure 2. Girdle view of P. vanheurckii (SEM). Arrowheads indicate pili. Pseudoseptum is visible as whiter area within the middle of each valve.
Figure 3.Detail of Figure 2 (SEM). Black arrow indicates one areola. Black arrowhead indicates secondary ocellus on elevated apex. White arrows indicate single rows of pores on girdle bands.
Figure 4. Living short chain of P. vanheurckii in girdle view. Pili, pseudoseptum and chloroplast are visible
Plagiogrammopsis vanheurckii (Grunow) Hasle, v. Stosch et Syvertsen
Plagiogramma vanheurckii Grunow ex Van Heurck
Although previously considered to be a monospecific genus, additional species have been recently described:
P. crawfordii Witkowski
P. mediaequatus Gardner et Crawford
P. panduriformis Schulz et Drebes
Because there appears to be substantial morphological variability in Plagiogrammopsis vanheurckii, the validity of these species may be questioned.
In girdle view, the cells are rectangular, slightly domed in the center, and the apices are elevated (Figures 2 & 4). A small, rounded sieve-like ocellus is present at the tips of the apices. Occasionally, secondary ocelli appear on the sides of the elevated apices (Figure 3). Several girdle bands are associated with each valve, and these are each ornamented with a single row of pores (Figure 3). There is a single chloroplast abutting the girdle (Figure 4). Additional detailed morphological descriptions are found in Hasle et al. (1983), Round et al. (1990) and Sato (2010).
Cells of this species occur in short chains, though they may form long ribbons in culture. Colonies are sometimes twisted and easily separable. The valves are lanceolate with rostrate or capitate apices (Figure 1), and each valve of a cell has a different structure (heterovalvate). One valve is equipped with pili (i.e. long hair-like siliceous appendages) (Figures 1, 2 & 4), bending along the apical axis, and lacks a rimoportula. The other valve lacks pili and has a rimoportula near the margin in the median area (Figure 1, upper). Both valves have marginal narrow spines that probably aid in colony formation (Figure 2), and also have rounded multiperforate areolae (Figure 3). There is a relatively large hyaline central area (fascia) lacking areolae (Figures 1 & 2). Here, on the cell interior, is a transverse siliceous thickening called a pseudoseptum (Figures 1 & 2).
HABITAT & REGIONAL OCCURENCE:
Plagiogrammopsis vanhurckii is commonly described as a benthic coastal species, often associated with sand grains. In shallow estuaries, it is often found in the plankton. Although it is found in temperate water, it seems to be more common in tropical and subtropical locations worldwide.
INDIAN RIVER LAGOON DISTRIBUTION:
This species is common in the IRL throughout the year. However, it is never abundant and rare during the colder months.
Valve dimensions are 3-50 µm in the apical axis (length), about 4 µm in the transapical axis (width), with about 12 areolae in 10 µm (Hasle et al. 1983). Poroid areolae on the girdle bands are 12-22 per 10 µm.