Pleurosigma acutum sensu Peragallo
Pleurosigma acutum Norman ex Ralfs is a morphologically highly variable species whose structure has been examined by Sterrenburg (2003), and is closely related to P. angulatum (Queckett) W.Smith. The diatom illustrated here (Fig. 1, entire valve exterior, SEM) corresponds closely to P. “acutum” as shown in Peragallo (1890-1891) but is not the same as Norman’s P. acutum. The two species are rather different in many of their features, and so the IRL specimens are not P. acutum Norman (F. Sterrenburg, personal communication, 9 April 2009). Peragallo’s examples (and mine, PE Hargraves) therefore need new names, which will not be attempted here.
The sigmoid valves are about 150µm long and 22-24µm wide (Fig. 1), with 23 transverse striae in 10µm and 18-20 oblique striae in 10µm (Fig. 2, central portion of valve exterior, SEM). The angle of the oblique striae is 75 degrees, and the raphe angle is about 15 degrees, curving to the opposite margins at either apex (Fig. 3, apex of valve exterior, SEM). The areolae are slits on the exterior side (Fig. 2, 3); interiors were not observed. Because of confusion with Norman’s P. acutum, as well as P. angulatum, the global distribution of this species is unknown. It was very rare in the IRL, and a few specimens were seen near the Fort Pierce Inlet.