Yessotoxin is one of the shellfish toxins leading to mussel farm closure in the Adriatic Sea of Italy. Two putative Gonyaulax spinifera strains are known as yessotoxins producers but their identities have remained elusive since 2005. To address this gap, we established five Gonyaulax strains by incubating sediments from the Adriatic Sea and subsequent sing-cell isolation. Both cyst and theca morphology were examined by light and scanning electron microscopy. In addition, LSU and/or SSU rRNA gene sequences were obtained for all strains. Two strains produce cysts resembling Spiniferites mirabilis and one strain was related with S. scabratus. Both other strains are described as Gonyaulax montresoriae sp. nov, characterized by a high cingular displacement and overhang, and two unequal antapical spines. Cysts of G. montresoriae are pear-shaped, showing a smooth surface and exclusively gonal processes with perforations at the base, similar to S. lazus. LSU rRNA gene sequence comparison suggests that the G. spinifera strain GSA0501 isolated from the Adriatic Sea in 2005 should also be identified as G. montresoriae. Maximum likelihood and Bayesian inference analyses based on LSU and SSU rRNA gene sequences reveal that G. montresoriae is monophyletic, and close to several toxic strains of presumable Gonyaulax spinifera from the Adriatic Sea and New Zealand, whose taxonomic positions are uncertain. One strain of G. montresoriae was examined for yessotoxin production using LC-MS/MS, and found to produce predominantly yessotoxin of 3.0 pg cell^-1. Our results highlight the rich diversity and risks associated with Gonyaulax species in the Mediterranean Sea.

cysts, dinoflagellate, harmful algal bloom, Mediterranean Sea, Spiniferites