A new species of Minidiscus (Diatomophyceae, Thalassiosiraceae) from the eastern English Channel, France

The taxonomy of the genus Minidiscus, whose generic name refers to its small size and shape, is revised. The various species previously described are almost impossible to differentiate without electron microscopy. And even so, some confusion remains in their delineation. During a Phaeocystis survey in March and April 2002 in the eastern English Channel and Southern Bight of the North Sea, specimens of Minidiscus were collected and examined by scanning electron microscopy. We confirm that Minidiscus trioculatus has different morphotypes, ranging from 2.5 um to 6.8 um in diameter, with a small morphotype showing a valve surface with irregular openings of areolae, and a large morphotype with very small exterior openings of areolae arranged in almost linear rows. These extreme morphotypes, as well as intermediate specimens are illustrated here. We report for the first time the presence of Minidiscus chilensis along the French Atlantic coast and the eastern English Channel, and we describe a new marine species of this genus, Minidiscus decoratus, previously illustrated and identified as M. trioculatus by Aké-Castillo et al. in 2001 (Species of Minidiscus (Bacillariophyceae) in the Mexican Pacific Ocean. Cryptogam. Algol. 22: 101?107). A formal description of the new species is given, and a key for species identification is provided.