A description of the frustular morphology of Frustulia creuzburgensis (Krasske) Hustedt, with comments on its systematic position

Abstract

Frustulia creuzburgensis was originally assigned to the genus Navicula and later transferred to Frustulia due to its unilaterally deflected external proximal raphe ends resembling those of F. weinholdii. Unlike most members of the genus, F. creuzburgensis is found in saline waters, and has been reported from Europe, North America, South Africa and Hawaii. Light and scanning electron microscopy of F. creuzburgensis from coastal streams in California indicate the presence of a ‘porte-crayon’ arrangement around the external distal raphe ends, a feature used to diagnose Frustulia. However, there is also a lack of ribs bordering the raphe system, which is inconsistent with other members of Frustulia. A notch in the girdle bands at about mid-valve, accompanied by girdle band protuberances, is described for the first time in this species. Populations of this taxon produce both cis and trans frustules in a 1:2.5 proportion, respectively. Enlarged external areolae openings near the external proximal raphe ends are also described for the first time. The possible relationships of this enigmatic taxon amongst close relatives and within the Naviculales is discussed.