Eunotia zasuminensis (Cabejsz.) Körner 1970 is a rather rare diatom species with star-like colonies. It resemlbles the familiar Asterionella and is probably often identified under this name.
E. zasuminensis differs from Asterionella in the form of the cells. Where Asterionella formosa particularly has cells where one end is much smaller than the other, in E. zasuminensis both ends are quite the same size. But in the middle there is a little bubble. Actually it looks very much like the humerus, the long bow in one’s arm. Of course you have to see the Eunotia cell from the valve face.
If you can’t see the humerus shape and are hesitating wether to call it E. zasuminensis or Asterionella, take a look at the chloroplasts. In Asterionella there are many little chloroplasts after each other, in E. zasuminensis there are fewer, bigger chloroplasts and they are closer to each other:
Asternionella formosa.
Eunotia zasuminensis.
This species has been moving a bit in the house of Taxonomy, as Eloranta writes in his article (see below). It was originally placed in the genus Fragilaria by Cabejszekowna (1937). Lundh-Almestrand (1954) again moved it to the genus Asterionella, while Körner in 1970 finally gave it a place in the genus Eunotia, because it really does have a raphe, albeit a tiny one. The first two genera are members of the class Fragilariophyceae – the ones without a raphe. Eunotia is a member of tha class Bacillariophyceae.
You want to know more? Take a look at Eloranta’s article: Eloranta, P. (1986) Melosira distans var. tenella and Eunotia zasuminensis, two poorly known planktonic diatoms in Finnish lakes. Nordic Journal of Botany
Volume 6, Issue 1, pages 99–103.
Or if you are as fortunate as being able to understand Finnish, you can visit the Finnish plankton guide on the net: http://www.jyu.fi/bio/kasviplankton/uusin/index.php
Literature:
Cabejszekowna, I. (1937) Fragilaria zasuminensis n. sp. w jeziorze Zasuminskim na Pollesiu. Arch. Hydrobiolö Rybactwa 10: 423-425.
Lundh-Almestrand, A. (1954) Some remarks on Fragilaria zasuminensis. Bot. Notiser 1954 (2): 179-182.
Körner, H. (1970) Morphologie und Taxonomie der Diatomeegattung Asterionella. Nova Hedwigia 20 (3-4): 557-724.