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Specimens belonging to the mixotrophic genus Fragilidium were isolated from coastal waters of west Iberia (NE Atlantic). Based on plate morphology and large subunit ribosomal DNA (LSU rDNA) sequences two species were identified: F. subglobosum and F. cf. duplocampanaeforme. The life cycle is similar in both species. The asexual cycle is characterized by an alternation between a thecate motile stage and a reproductively active coccoid stage. The sexual cycle is characterized by heterogamous anisogamy and includes a sexual benthic hypnozygote. The first apical plate in vegetative cells does not touch the apical pore plate while in gametes it does, confirming in Fragilidium the homology of what is frequently referred to as the first pre-cingular with the first apical plate in Gonyaulacales. Both species were observed to prey on Ceratium horridum. Molecular phylogenetic analyses with strains isolated from other geographical areas and Fragilidium gene sequences available in GenBank identified three different clades: one comprising F. subglobosum from the type locality and strains from Iberian waters; a second comprising the morphological species F. cf. duplocampanaeforme from Iberia and other strains from NE Atlantic, SE Atlantic and the south Pacific; and a third with an unassigned species from the SE Atlantic.

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