Guide to Seminavis

Valves are semi-lanceolate with a curved dorsal margin and a more or less straight ventral margin. The dorsal mantle is well developed, whereas the ventral mantle development is minimal. Areolae are lineolate, slit-like, and apically elongated, that is, they are lineolae. Internal hymenes are present. The dorsal and ventral striae are weakly to moderately chambered. The raphe is strongly displaced toward the ventral margin and enclosed within an accessory rib on the ventral side of the sternum. The raphe fissure opens laterally toward the ventral margin for most of its length, except at the slightly elongated central nodule and at the apical helictoglossae where it faces in a pervalvar direction. The proximal raphe ends are slightly deflected toward the ventral margin. The terminal raphe ends are strongly hooked toward the dorsal margin. Apical chambers are present at the valve ends, usually with 3 simple pores.

The cingulum is unornamented and wider on the dorsal side of the frustule than on the ventral side. The valvocopula is wide and open at one end where it is closed by an apical ligula. A third narrow copula may be present.

Living cells have 2 girdle-appressed chloroplasts. The plastid facing the dorsal margin is larger than the one facing the ventral margin.

Seminavis is currently a small genus of mostly marine taxa. A total of 22 taxa are currently listed for the genus (DiatomBase) and at least one new taxon has been described in 2022. The principal habitats for Seminavis taxa are marine sediments and on seagrass, although records have been found of cells in epilithic and epizoic habitats.

For at least a century, species of Seminavis have resided in the paraphyletic genus Amphora Ehrenberg ex Kützing (Danielidis and Mann 2002). Cleve (1895) placed taxa in the Subgenus Cymbamphora. It has been shown that Seminavis is not closely related to Amphora (Round et al. 1990). Instead, Seminavis shares many other derived features of Navicula. However, Seminavis and Navicula are distinct genera as supported by molecular phylogeny (Bruder and Medlin 2008), cytology (Danielidis and Mann 2002), and sexual reproduction (Chepurnov et al 2002). In addition, Seminavis species generally show chambers within the valve (Danielidis and Mann 2002).