Gogorevia exilis

Valves are linear-elliptic to elliptic-lanceolate with narrowly capitate, subcapitate, rostrate, or subrostrate apices. Larger valves are sometimes slightly constricted in the middle. Both raphe and rapheless valves have a narrow slightly sigmoid axial area.

The raphe valve has a distinct fascia that is often slightly wider on one side. The rapheless valve has a small, transapically rectangular, often asymmetric, central area. The raphe is straight, but deflected to opposite sides near the apices, with terminal raphe fissures strongly curved to opposite sides. The external proximal raphe ends are simple, located in slight “pin-hole” depressions, giving the appearance that they are expanded. Internally, the central raphe ends curve toward opposite sides. The striae are radiate on both valves, but almost parallel at the apices. The raphe valve has 24-34 striae in 10 µm in mid-valve, up to 40-45 in 10 µm at the apices.

The raphless valve has 20-25 striae in 10 µm at mid-valve and up to 35-40 in 10 µm at the apices. A few very small areolae may be present on the mantle of both valves. Areolae are round or transapically elongated externally, apically elongated internally, ca. 55-70 in 10 µm within the striae.

The variety Achnanthes exiguum var. heterovalvata was described as having a stria density of 34 in 10 µm. Our observations of material from the US, however, show that the nominate variety includes a gradient of stria density from 24 to 34 in 10 µm. We see that the variation of A. exiguum is quite broad, and therefore, consider Achnanthes exiguum var. heterovalvata as a subjective synonym.

The taxon was originally described as Stauroneis exilis Kützing, from the island of Trinidad (1844). It was later renamed as Achnanthes exigua Grunow 1880. Note that Grunow’s epithet for this taxon was a new name, since when he transferred it to Achnanthes, Kützing’s original epithet was already occupied by Achnanthes exilis Kützing 1833.

Czarnecki transferred Achnanthes exigua Grunow to Achnanthidium exiguum (Grunow) Czarnecki 1994 but did not justify the new combination. Later, the taxon was transferred to Lemnicola exigua (Grunow) Kulikovskiy, Witkowski and Plinski 2011. In 2020, Kulikovsky et al. recognized Gogorevia based on molecular and morphological features.