Cyst-theca relationship of a new dinoflagellate with a spiny round brown cyst, Protoperidinium lewisiae sp. nov., and its comparison to the cyst of Oblea acanthocysta Kenneth Neil Mertens, 1 * Yoshihito Takano, 2 Haifeng Gu, 3 Aika Yamaguchi, 4 Vera Pospelova, 5 Marianne Ellegaard 6 and Kazumi Matsuoka 2 1 Research Unit for Palaeontology, Gent University, Gent, Belgium, 2 Institute for East China Sea Research (ECSER), Nagasaki University, Nagasaki, 4 Research Center for Inland Seas, Kobe University, Kobe, Japan, 3 Third Institute of Oceanography, SOA, Xiamen, China, 5 School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada, and 6 Department of Plant and Environmental Sciences, University of Copenhagen, Frederiksberg, Denmark SUMMARY Round spiny brown cysts with apiculocavate processes were isolated from sediments of Lake Saroma, Japan, Changle Harbor, East China Sea, China, Jinzhou Harbor, Bohai Sea, China, and San Pedro Harbor, California, USA. Superficially similar round spiny brown cysts of the species, Oblea acanthocysta were, for comparison, restudied through light microscopy and scanning electron microscopy (SEM) and by sequencing of small subunit (SSU) and large subunit (LSU) rDNA obtained through a single cyst from Lake Saroma. These morphological measurements and SEM observations showed that the new cysts can be discriminated from O. acanthocysta by the archeopyle, number of processes, shape of process bases and its apiculocavate processes. Based on LSU sequences, the most closely related species was Proto- peridinium monovelum, for which no cyst stage has been described so far. However, the thecal morphology of the speci- mens found in this study differed from P. monovelum in details of the sulcal plates and shape of apical pore and 2a plate. We therefore describe Protoperidinium lewisiae sp. nov., which can be found in estuarine subtropical to temperate waters of the Pacific Ocean. Key words: Bohai Sea, Changle Harbor, East China Sea, Jinzhou Harbor, Lake Saroma, large subunit rDNA, San Pedro Harbor, small subunit rDNA. INTRODUCTION Free-living marine dinoflagellates form a very large and diverse group of planktonic organisms, currently encompass- ing 1555 species (Gómez 2005). About 13 to 16% of living dinoflagellates form so-called resting cysts as part of their sexual cycle (Head 1996). These cysts can be linked to their respective motile stage by incubation experiments (Wall & Dale 1968) and conversely, motile stages may be induced to form cysts in culture. Molecular phylogenetic analyses based on either cultured strains or single-cell polymerase chain reaction (PCR) have helped to elucidate taxonomic relation- ships within this group (e.g. Bolch 2001; Matsuoka et al. 2006; Takano & Horiguchi 2006; Matsuoka & Head 2013). Many new cyst species are still being discovered (e.g. Verleye et al. 2011; Mertens et al. 2014), which is impor- tant as dinoflagellate cysts are frequently used for paleoclimate reconstructions (e.g. Mertens et al. 2009; Price et al. 2013). Some dinoflagellate cysts have a distinct round, spine- bearing shape and are brown in color, and are therefore informally designated as ‘round brown spiny cysts’ (see review in Radi et al. 2013). The paleontological cyst-based taxonomy has erected two genera to classify these species. Cysts that belong to the genus Echinidinium (Zonneveld 1997) open with a theropylic archeopyle (Head et al. 2001), i.e. an angular slit that follows paraplate boundaries but without complete release of plates (Matsuoka 1988). The second genus, Islandinium was erected by Head et al. (2001) and opens with a saphopylic archeopyle, i.e. having a free operculum (Matsuoka 1988). At species level, char- acteristics of processes and wall texture are useful distin- guishing features of round brown spiny cysts (e.g. Zonneveld 1997; Head et al. 2001; Mertens et al. 2012b; Radi et al. 2013). Cyst-motile relationships have been established only for a few species of ‘round brown spiny cysts’: Protoperidinium monospinum (Paulsen) Zonneveld et Dale (Zonneveld and Dale 1994), Oblea acanthacysta Kawami, Iwataki et Matsuoka (Kawami et al. 2006; =cyst of Diplopelta parva (Abé) Matsuoka sensu Matsuoka 1988), P. tricingulatum Kawami, van Wezel, Koeman et Matsuoka (Kawami et al. 2009), dif- ferent types of Archaeperidinium minutum (Kofoid) Jørgensen – previously called P. minutum (Kofoid) Loeblich III; the genus Archaeperidinium was reinstated by Yamaguchi et al. (2011) – and the closely related species A. saanichi Mertens, *To whom correspondence should be addressed. Email: kenneth.mertens@ugent.be Communicating editor: Mona Hoppenrath Received 6 June 2014; accepted 4 October 2014. Phycological Research 2015 doi: 10.1111/pre.12083 © 2015 Japanese Society of Phycology