Plagiogramma staurophorum (Gregory) Heiberg
Valves are broadly lanceolate or elliptical (Fig. 1, line drawing, modified from Hustedt 1930-59) with rounded apices, and ornamented with coarse areolae arranged in both horizontal and vertical striae on both sides of a narrow sternum. A transverse hyaline stauros-like area (fascia) is present, sometimes slightly raised above the valve surface (Fig. 2, SEM exterior), and subtended internally by thickened pseudosepta at both sides of the fascia (Fig. 3, SEM interior). A single row of short peg-like spines is present at the edges of the valve face (Fig.2). At each valve apex is a pore field partly surrounded by a raised rim (Fig. 2). Cells in girdle view are approximately rectangular and are united in short chains (Hoppenrath et al. 2009), occasionally solitary, and benthic – either epipsammic or attached to sand grains. There are four chloroplasts, according to Round et al. (1990)
This species is morphologically quite variable in shape and structural details. From literature records, the apical axis (length) is 12-84µm, 5-20µm in the transapical axis (width), with 8-11 transverse or slightly radiate striae in 10µm. The valve in Figure 2 is 50µm long and 14.5µm wide with 10 striae in 10µm, and came from the Sebastian Inlet area (Brevard County, FL). The shorter valve in Figure 3 is 24.1µm long and 9.3µm wide, from the St. Lucie inlet (Martin County, FL). It is a widespread species in coastal waters, often on sandy substrates from the Arctic through the tropics, though apparently lacking from the Antarctic region. It is also widespread in the IRL, but rarely in any abundance.
Hoppenrath M, Elbrächter M, Drebes G. 2009. Marine Phytoplankton. Kleine Senckenberg-Reihe 49:1-264.
Round FE, Crawford RM, Mann DG. 1990. The Diatoms, Biology & Morphology of the Genera. Cambridge Univ Press, Cambridge. 747 pp.