Guide to Gomphonella

Valves are wedge-shaped. Striae are biseriate (to triseriate) and composed of small, round areolae. The areolae are unoccluded. A diagnostic feature of the genus is an absence of a feature; valves lack stigmoids or stigmata. The raphe is filiform. Externally, the proximal raphe ends are slightly expanded. Internally, the proximal raphe fissures are slightly unilaterally curved. Internally, the distal raphe ends terminate in helictoglossae that are somewhat removed from the apices. The apical porefield is composed of porelli that are of the same size and shape of the areolae; the apical porefield is undifferentiated. The apical porefield is bisected by the terminal raphe fissure.

The species Gomphonella olivacea is considered to be cosmopolitan, found across the world along with many varieties (Hofmann et al. 2013). A conspicuous feature of this diatom is its habit of extracellar polysaccaride production (EPS). It forms large gobs of mucilage from the apical porefield, and likely, from the raphe as well. Indeed, it was originally described by Hornemann (1810) for its production of mucilage, “Substantia gelatinosa lubrica supellucida".

Although the genus Gomphonella was described in 1859 (Rabenhorst), the species Ulva olivacea was described earlier (Horneman 1810). The genus of the epithet "olivacea" been variously treated by North American and European authors. In North America, it has been treated as a member of Gomphoneis, as Gomphoneis olivacea (Hornem.) P.Dawson ex R.Ross and P.A.Sims 1978. This website also treated it as Gomphoneis olivacea, until 2019. In Europe, it has been treated within Gomphonema, as Gomphonema olivaceum (Hornem.) Bréb. 1838 (Hofmann et al. 2013, Levkov et al. 2016).

Recent work demonstrates that Gomphonella is not closely related to Gomphonema, and that it is not even within the Gomphonemataceae (Jahn et al. 2019). Note that the only difference in morphology between Gomphosinica and Gomphonella is the number of stigmata (Tuji 2020). Molecular data is needed to determine the relationship of taxa in these genera.