Petalomonas poosilla Larsen and Patterson 1990

(fi gures 14g, h, 16e -g)

Description. We describe two populations of cells.

Population 1: small cells; ovate, 5-9 Mm long, 3-5 Mm wide, dorso-ventrally fl attened. This species may or may not have up to three dorsal ridges and the ridges may or may not be well developed. The organism may also have two fi ne ventral ridges which are di ffi cult to observe. The right ventral ridge runs beneath the reservoir along the cell. With one fl agellum as long as the cell. The reservoir is in the right side of the cell and the nucleus is in the left side. Moves by gliding. Common.

Population 2: Larger cells; cell outline oblong-oval, 10-12 Mm long and 5-6 Mm wide, dorso-ventrally fl attened. It has four ridges extending along the cell: two indistinct dorsal ridges located at the lateral margins of the cell; two fi ne ventral ridges, the right one of which runs beneath the reservoir. One fl agellum is about as long as the cell or slightly longer. The ovate reservoir is anteriorly situated in the right-hand side of the cell and the nucleus in the left-hand side of the cell. Often co-occurs with the smaller cell.

Remarks. Larsen and Patterson (1990) provided a new name for P. pusilla Skuja, 1948 (P. poosilla) because under the principle of co-ordination of the ICZN, P. pusilla Skuja, 1948 is a homonym of P. mediocanellata var. pusilla Klebs, 1893 (Lemmermann, 1913) —which we refer to as P. minuta (see P. minuta above). Petalomonas poosilla and P. minuta resemble each other in shape and appearance, but can be distinguised by the presence of what we regard as a longitudinal groove in P. minuta.

Petalomonas poosilla was originally described under the name P. pusilla (5-12 Mm long) from freshwater sites (Skuja, 1948) and has since been found in marine sites in Australia, Brazil, Danish Wadden Sea, Fiji, Gulf of Finland; Hawaii and NE Canada (Lackey and Lackey, 1970; Larsen, 1987; Larsen and Patterson, 1990; Vørs, 1992a; Patterson and Simpson, 1996; Tong et al., 1998). We observed cells (in the small population) which are similar in size and outline to P. poosilla as described by Larsen and Patterson (1990). This species has been described without ridges or grooves (Skuja, 1948; Larsen and Patterson, 1990). Cells assigned to this species have been described more recently as having ridges (Patterson and Simpson, 1996; Schroeckh and Patterson, unpubl.). We suspect that this species has previously been under-described and regard the latter descriptions as more accurate than the former. The ventral face of this species is very similar to that of P. minor, described here, in having two fi ne ventral ridges and in the position of the ridges, but the two species can be distinguished by cell shape and the dorsal fl ange in P. minor. Petalomonas poosilla and P. minuta resemble each other in shape and appearance, but can be distinguised by the presence of what we regard as a longitudinal groove in P. minuta. This species resembles P. cantuscygni (see Cann and Pennick, 1986) from which it can be distinguished by never having fewer than six dorsal ridges.

We have observed larger cells about 10-12 Mm long, but they consistently had four discrete ridges on both sides of the cell. The two populations often co-occur. Although these may represent a separate species, we have been unable to establish morphological discontinuities by which single cells could be easily identi fi ed when comparisons are made. Further studies are required to establish the status of the two populations.