Sellaphoraceae

Introduction

This group of benthic raphid diatoms is found world-wide in freshwater and marine epipelon. Until the 1990s, only a few 10s of species had been described and all were classified in Navicula. It is now clear that the family contains many hundreds of species, most of which await names. The genus Sellaphora has become a model system for studying speciation and was the first to be used to explore the possibility of DNA barcoding for diatom identification.

The family Sellaphoraceae was first described in Mereschkowsky in 1902 to contain his new genus Sellaphora, which he separated from Navicula because of its saddle-like plastid. A further genus, Fallacia, was added by Mann (in Round et al. 1990), who also transferred Rossia and Caponea to the Sellaphoraceae. Microcostatus was described by Johansen & Sray (1998), but it is unclear whether this can or should be separated from Fallacia. Some species currently classified in Eolimna may belong in or near Sellaphora, but the type species of Eolimna is a fossil and it is probably impossible to assess whether it belongs to the Sellaphoraceae because of the lack of cytological or molecular data.

Most members of the family have modest dimensions (valve lengths mostly 10–60 µm) and simple shapes.

Characteristics

• small to moderate-sized raphid diatoms (mostly < 80 µm long), usually wider than deep and therefore usually lying in valve view
• usually a single H-shaped chloroplast per cell, lying with its two long lobes against the sides of the girdle and its central isthmus against the epivalve
• one or two pyrenoids per chloroplast, penetrated by finger-like invaginations of cytoplasm
• nucleus oscillatory between cell divisions (2:1 ratio of trans: cis frustules)
• valve outline elliptical, linear or lanceolate, sometimes with rostrate or capitate poles (rarely with a slight central constriction)
• striae usually uniseriate (rarely biseriate), containing simple round poroids
• valve face planar or depressed parallel to the raphe to form simple or curved grooves externally
• a conopeum sometimes present externally, extending out from the raphe-sternum over part or all of the valve face; conopeum plain (e.g. some Sellaphora) or perforated by tiny pores (Fallacia, Rossia)
• internal central raphe endings turned towards the primary side of the valve
• external central raphe endings usually deflected slightly towards the primary side of the valve
• helictoglossa narrow, elongate
• external polar raphe endings curved or hooked towards the secondary side of the valve

Discussion of Phylogenetic Relationships

Relationships between Sellaphora, Fallacia and Rossia are not yet clear. Sellaphora species have been studied extensively (using SSU rDNA, ITS rDNA, rbcL and cox1) but there are few sequences from the larger genus Fallacia and only one species of Rossia is commonly encountered.

The genera Caponea, described by Podzorski, and Microcostatus, described by Johansen & Sray (1998), both need further study.

References

Johansen, J.R. & Sray, J.C. (1998). Microcostatus gen. nov., a new aerophilic diatom genus based on Navicula krasskei Hustedt. Diatom Research 13: 93-101.