engleski

Morphology and ultrastructure of marine monoraphid diatoms related to Achnanthes brockmanii Simonsen

Achnanthes brockmannii Simonsen described from the western Baltic Sea has never been studied in electron microscope to reveal its ultrastructure. Until our study it has been practically known only from the type habitat. Unpublished reports of this species have also been known from the Aegean Sea. Similar in terms of light microscopic appearance is Achnanthes lyrata Proshkina-Lavrenko described from the Black Sea and the Azov Sea. The holotype of Achnanthes brockmannii has only been studied by means of light microscopy (LM), however, recently fairly abundant populations of both species mentioned have been identified in the sediment and on the rock substrate (Adriatic Sea and Black Sea) and amongst fouling flora on a seaweed Caulerpa racemosa (Adriatic Sea). The latter material was accessible for scanning electron microscope (SEM) analysis. As it is already evident from LM images, in terms of ultrastructure A. brockmannii significantly differs from A. adnata, generitype of Achnanthes s. str. None of the two species mentioned here conform to any other genus established as a result of splitting of Achnanthes sensus lato. The new genus differs from Achnanthes s. str. in all aspects. The frustules are flat, plate-like not arched and bent about apical axis. However, the most conspicuous difference is the raphe sternum system. The transapical striae are occluded by simple depressed hymenes, not by elaborate vela. Finally the striae forming areolae are differentiated into two types. Those surrounding the Raphe Valve (RV) margin are elongate and fairly large, whereas the middle part of the valve bears small oblong areolae. A very characteristic feature of the RV is a sort of hyaline area which in LM looks like sigmoid stauros (A. brockmannii) or a double (crossing each other) sigmoid stauros (A. lyrata). Sternum Valve (SV) has less distinct ornamentation and reminds a little bit those of species like: Achnanthes danica and A. fimbriata. In our opininon the above differences justify a description of a new genus but this requires a formal transfer which will be published later.

Benthic diatoms; new genus; Adriatic Sea; Black Sea

nije evidentirano