This species could be either Cylindrotheca closterium OR Nitzschia longissima (both in the family Bacillariaceae). According to Hasle and Syvertsen (1997), both may have a cosmopolitan distribution and can only be distinguished from each other when frustules are cleaned. In C. closterium, the ends of the weakly silicified frustule (rostra) may be slightly twisted around the apical axis. The striae of N.longissima are barely visable under LM, but many narrow bands should be seen on the frustule on a cleaned C. closterium at high magnification. A comparison of morphological attributes is detailed in the table below:

Species	Apical axis / μm	Transapical axis / μm	Fibulae in 10 μm
C. closterium	30 - 400	2.5 - 8	10 - 12
N. longissima	125 - 450	6 - 7	6 - 14 (?) (sic)

Of the individual cells measured off the Norfolk coast for this project thus far, the range of apical axes for C. closterium / N. longissima has been 39 μm - 138 μm, with a mean average of 75 μm. The transapical range has been transapical axes has been 3 μm - 6 μm, with a mean average of 5 μm. With the small transapical axis, this may suggest that C. closterium is the more likely identification, but without a closer look at the frustule, that cannot be confirmed.

Due to its small size, this species may be under-recorded in the study area.

References

Hasle, G.R., and Syvertsen, E.E. (1997) Identifying Marine Phytoplankton. pp. 294. Edited by C.R. Tomas. San Diego: Academic Press.

Loir, M. (2024) Genera Cylindrothecato Licmophora. Available at: http://www.diatomloir.eu/SiteDiatom/Cretdiatrois.html (Accessed: 25 April 2024).