237 Nova Hedwigia 82 1—2 237—246 Stuttgart, February 2006 DOI: 10.1127/0029-5035/2006/0082-0237 0029-5035/06/0082-0237 $ 2.50 © 2006 J. Cramer in der Gebrüder Borntraeger Verlagsbuchhandlung, D-14129 Berlin · D-70176 Stuttgart Observations on Decussata hexagona (Torka) Lange-Bertalot (Bacillariophyta) from Holocene sediments in Bulgaria by Rosalina Stancheva 1 and Dobrina Temniskova Department of Botany, Faculty of Biology, St. Kliment Ohridski University of Sofia, D. Tsankov Blvd 8, BG-1164 Sofia, Bulgaria With 21 figures and 1 table Stancheva, R. & D. Temniskova (2006): Observations on Decussata hexagona (Torka) Lange- Bertalot (Bacillariophyta) from Holocene sediments in Bulgaria. - Nova Hedwigia 82: 237-246. Abstract: The diatom species Decussata hexagona (basionym Navicula hexagona) was investigated by light microscopy (LM) and scanning electron microscopy (SEM). The studied specimens were collected in sediment cores from two peat bogs, located in the Central Sredna Gora Mountains (Bulgaria). These are the first and only locations of this species for Bulgaria so far. Comparison of Decussata hexagona and Decussata placenta shows important ultrastructural differences. Distribution and autecology of Decussata hexagona are discussed. Keywords: Bacillariophyta, Decussata hexagona, ultrastructure, Holocene sediments, Bulgaria. Introduction The genus Decussata (Patrick) Lange-Bertalot & Metzeltin comprises three taxa, Decussata placenta (Ehrenberg) Lange-Bertalot & Metzeltin var. placenta, Decussata placenta var. obtusa (Meister) Lange-Bertalot and Decussata hexagona (Torka) Lange- Bertalot. Decussata placenta var. placenta (originally described as Navicula placenta Ehrenberg) has probably a cosmopolitan distribution and grows in populations with small individual numbers (Cleve-Euler 1953, Hustedt 1962, Patrick & Reimer 1966, Mölder & Tynni 1967, Germain 1981, Krammer & Lange-Bertalot 1986, Lange- Bertalot 2000, 2001). Decussata placenta var. obtusa is only known from the type locality in Asia (Meister 1932, as Navicula placenta var. obtusa). Decussata hexagona 1 Present address: Environmental Science and Resources, Portland State University, Portland, Oregon 97207, U.S.A.. e-mail: rosalinastancheva@abv.bg, temn@biofac.uni-sofia.bg