Morphology and ecology of Achnanthidium caravelense (Bacillariophyceae), a new species from Portuguese rivers

A new benthic freshwater diatom species belonging to the genus Achnanthidium Kutz. has been recorded from several watercourses in the North of Portugal. Achnanthid- ium caravelense Novais et Ector is described as a new species based on light and scanning electron microscopic observations, as well as on its ecological preferences as reconstructed from field observations. The most characteristic morphological features of this species are the different outline of the raphe valve (narrowly elliptic with linear margins) and the ra- pheless valve (narrowly elliptic to narrowly rhombic with moderately convex margins), the non-protracted broadly rounded apices and the length/width ratio. Furthermore, the areolae of the single row along the mantle are elongated and are more or less widely open, which is a characteristic discernible in girdle view under light microscopy. The species that A. caravelense resembles most is A. eutrophilum (LaNgE-BErt.) LaNgE-BErt.; nevertheless it can be distinguished from the latter by the different raphe valve outline, its higher valve length/width ratio and autecology. A. caravelense is common and abundant in soft waters with low to moderate nutrient content in the North of Portugal.