Species Description: This distinctive diatom differs from many Gyrosigma species in that it is not particularly sigmoid (Fig. 1, entire valve, SEM). The valves are narrow, somewhat lanceolate, tapering to elongate ‘spoon-shaped’ apices (Sterrenburg 2000), making the cells quite unmistakable. The cells are 175-230µm long and 8.5-12µm wide, with 24-30 transverse striae in 10µm and approximately the same for longitudinal striae (Fig. 2, external valve apex, SEM). Each stria is composed of areolae with externally opening slits. The raphe is approximately median, shifting to the convex margin near the valve apices (Fig. 2). The convoluted taxonomy of this and related species is discussed by Sterrenburg (2000). The IRL examples differ somewhat in their measurements from the examples described by Sterrenburg from Indonesia. Although one of the characteristics of G. reversa, as originally described by Gregory (1857), is the dilation of valve tips in opposite directions (Fig. 3, from Gregory 1857), and this example does not show that (Fig. 1) because the cell is twisted, all other characteristics place it in G. reversum rather than the similar G. naja. The apparent delicate nature of the narrowing probably permits this twisting during drying (Sterrenburg FAS, personal communication, 17 May 2013).
G. reversum is primarily a tropical benthic species, found worldwide. It is quite rare in the IRL, and was found near the Sebastian Inlet, in the St. Lucie estuary, and near the Jupiter Inlet.
Gregory W. 1857. On new forms of marine diatomaceae found in the Firth of Clyde and in Loch Fine. Trans Royal Soc Edinburgh 21: 473-542.
Sterrenburg FAS. 2000. Studies on the genera Gyrosigma and Pleurosigma (Bacillariophyceae). Gyrosigma reversum (Gregory) Hendey and G. naja (Meister) Sterrenburg. Proc Acad Nat Sci Philadelphia 150: 301-304.