Petalomonas abscissa (Dujardin 1841) Stein 1859

Lee, Won Je & Patterson, David J., 2000, Heterotrophic flagellates (Protista) from marine sediments of Botany Bay, Australia, Journal of Natural History 34, pp. 483-562 : 517-518

Petalomonas abscissa (Dujardin 1841) Stein 1859

(fi gures 14a, 15c -e)

Description. Cell 12-22 Mm long, 8-19 Mm wide, narrow anteriorly, widest slightly posterior to midline. With lateral hyaline fl anges. This species has two dorsal keels which run along the entire cell; the right one is well developed. One narrow ventral groove with marginal ridges forming a double keel extends from the collar around the canal. One fl agellum emerges from the canal and is about 1.0-1.2 times the cell length. The reservoir is situated in the right anterior side of the cell and the nucleus lies near the midline and adjacent to the left margin of the cell. Moves by gliding. Commonly observed, but not abundant.

Remarks. This species has been recently reported with lengths from 10 to 25 Mm from marine sites in subtropical and tropical Australia, Brazil and in the temperate Danish Wadden Sea ( Larsen, 1987; Larsen and Patterson, 1990; Ekebom et al., 1996; Patterson and Simpson, 1996). Generally, our observations are in accord with those of Larsen and Patterson (1990) although in contrast to most authors ( Klebs, 1893; Lemmermann, 1914; Shawhan and Jahn, 1947; Huber-Pestalozzi, 1955; Larsen, 1987; Ekebom et al., 1996; Patterson and Simpson, 1996), Larsen and Patterson (1990) described P. abscissa as having one dorsal keel. We found one cell with one dorsal keel and included the cell here, and surmise that the number of dorsal keels may not be a good diagnostic characteristic. This organism resembles a few species of the genus Petalomonas , such as P. spinifera Lee and Patterson , n. comb. (see below), P. mira var. bicarinata Skuja, 1939 and P. raiula Larsen and Patterson, 1990 in having a ventral ridged groove. Petalomonas abscissa can be distinguished by its elliptical, regular cell shape and the indented posterior end ( P. spinifera , P. mira var. bicarinata and P. raiula have posterior protrusions). It resembles P. applanata Skuja, 1939 in general appearance, but it has a posterior indentation and is smaller than P. applanat a (30-36 Mm).