Guide to Pseudogomphonema

Pseudogomphonema is one of three marine diatom genera created from division of the genus Gomphonema by Medlin and Round (1986), and is one of several genera that they referred to as the “marine gomphonemoids”. These “marine gomphonemoids” are not monophyletic groups. However, they exhibit a superficial resemblance to Gomphonema in their symmetry and attachment to substrates via a stalk.

The following genus description is based on the original description of Medlin and Round (1986), updated with observations of more recently described taxa.

Pseudogomphonema cells are solitary. Most species are epiphytic on marine macroalgae, but a few are endophytic on Rhodophyta (Klochkova et al. 2014, Stonik and Skriptsova 2024). Epiphytic taxa are attached by a short mucilage stalk or pad secreted at the foot pole. Valves are heteropolar and clavate. The striae are uniseriate with lineolae (i.e. apically elongated, slit–like pores within striae that are delimited internally by thickened virgae and occluded by hymenes). A pseudoseptum is present at the footpole in some species. The pseudosepta are variously developed in different taxa. The pseudoseptum is often in the form of a bridge across the foot pole and fused with the helictoglossa, forming an apical chamber that opens to the cell exterior via a row of slit-like openings. Similar chambers showing varying degrees of development may also be present at the head pole. The raphe sternum is central, but more strongly developed on the primary side. The raphe opens laterally along most of its length. External proximal raphe ends are expanded and pore-like. Internal proximal raphe ends differ in their morphology. Distal raphe ends also vary, but are often curved or hooked. The cingulum consists of open, non-porous copulae. The valvocopula is the widest band and several small, ligulate bands are present at the footpole. Observation of cultured live cells of Pseudogomphonema sp. (Matt Ashworth, unpublished; guide point composite image) revealed two girdle-appressed plastids.

Pseudogomphonema lacks an apical pore field at the foot pole that is universally present in Gomphonema as well as a stigma found in many species of the latter genus. The hymenate lineolae of Pseudogomphonema clearly distinguish it from the Gomphonemataceae (with the genera Gomphonemopsis, Gomphoseptatum and Gomphonema). The lineolae with hymenes, along with the nonporous copulae, and two girdle appressed plastids suggest a close relationship with Naviculaceae. 

Pseudogomphonema occurs in polar to temperate marine waters. According to Algaebase (accessed February 16 2025), there are currently 8 accepted taxa within Pseudogomphonema, including one variety. There are also several varieties of Gomphonema kamtschaticum that have not yet been evaluated for potential transfer to Pseudogomphonema.