Biremis ridicula (Giffen) D.G. Mann

Lobban, Christopher S. & Witkowski, Andrzej, 2023, Marine benthic diatoms of Guam: new records, Dictyoneis apapae sp. nov., and updates to the checklist, Micronesica 2023 (2), pp. 1-75 : 6-7

Biremis ridicula (Giffen) D.G. Mann Figs 32–37

Ref. illus.: Giffen 1976, figs 10–13 (as Amphora ridicula ); Round et al. 1990 pp. 548–549 (figs c, d, but not identified to species); Witkowski et al. 2000, pl. 154, figs 14–16

Samples: GU52O-4

Dimensions: Length 46–74 µm, width of valve 9 µm, width of frustule in girdle view 24 µm; striae 7 in 10 µm

Diagnostics: Amphora -like frustule, asymmetrical ( Figs 32, 35). Valve asymmetrical ( Figs 34, 36), lanceolate, with short, broad striae along each margin and the raphe strongly displaced to the “ventral” striae, leaving strongly asymmetrical axial area. Striae alveolate ( Fig. 37).

Comments: This species is similar in size and stria density to Biremis ambigua (see above), but that species is symmetrical. The relatively large frustules are shaped like Amphora arenaria Donkin , with which it co-occurred in this sample from calcareous sand, but the striae are very distinctive. Round et al. (1990) described the areolae in this genus as consisting of two incompletely separated chambers, closed internally by a cribrum, and opening externally by slit-like foramina, one at each end of the areola, resulting in two longitudinal lines that are the most evident feature in external SEM views. Witkowski et al. (2000) mention this species being reported only from South Africa and the North Sea (Europe), populations for which there is no SEM yet; a comparison needs to be made to confirm the identity of the Guam specimens.