Pleurosigma decorum W. Smith
There has been substantial confusion of this species with P. formosum, as Hendey (1964) discusses in detail. That species has also been listed as occurring in the IRL system. The valves of P. decorum are lanceolate, somewhat narrow, and gently sigmoid with sub-acute apices (Fig.1, live cell with multilobed chloroplasts and transparent central nucleus; phase contrast; Fig. 2, entire valve, SEM). The raphe is median, with the central raphe ends slightly overlapping and undulate on the valve exterior (Fig. 3, enlarged central area, SEM), but not so on the valve interior (Fig. 4, SEM). The raphe gradually becomes excentric near the apices (Fig. 2). There are two curved siliceous bars enclosing the interior central raphe ends (Fig. 4). The areolae are slotted on the exterior (Fig. 3), and mostly oval and bisected on the interior, except in the small laterally expanded central area (Fig. 4). This species apparently has a global distribution, but confusion with P. formosum makes the distinction difficult. In the IRL, P. decorum is fairly common throughout the year. The cells are variable, 145-270µm in length, 23-29µm wide, with 18-22 transverse and 16-21 oblique striae in 10µm. The striae angle is 73-78 degrees, and the raphe angle (see Sterrenburg 2002) is 8-10 degrees. Living cells have multilobed chloroplasts (Fig. 1). Note that, with few exceptions, Pleurosigma cannot be identified to species from living cells, and frequently, from light microscopy alone.
Hendey NI. 1964. An introductory account of the smaller algae of British coastal waters. Part 5, Bacillariophyceae (Diatoms). In: Ministry of Agriculture, Fisheries and Food. Fisheries Investigations Series IV. 317pp. HMSO, London.
Sterrenburg FAS. 2002. Nulla Vestigia Retrorsum. The case of Pleurosigma aequatoriale Cleve. Constancea 83: 1-10.