Prorocentrum pseudonux sp. nov. ( Prorocentrales, Dinophyceae ): A new nanoplanktonic dinoflagellate from the open tropical Pacific Ocean

SUMMARY While phytoplankton from the coasts of temperate seas has received considerable attention, phytoplankton from the open tropical ocean, especially the smallest fraction, remains understudied. Here, we describe a new nanoplanktonic dinoflagellate from the tropical Pacific Ocean. Prorocentrum pseudonux sp. nov. is slightly oval in thecal view, measuring 6–9 μm long and 5–8 μm deep. The theca is smooth with a few pores of different size classes. The pore arrangement is similar to P. nux , but the four marginal pores bordering the posterior margin of the thecae are more dispersed. The differences between the two species are most evident in the periflagellar platelets. The edge of platelet 1 in P. pseudonux sp. nov. shows a prominent, wing‐like projection that appears folded inward. In P. nux and other closely related members of Prorocentrum sensu stricto, platelet 7 is barely visible and limited to a thin plaque between the accessory pore and platelet 1, whereas in P. pseudonux sp. nov. platelet 7 is broad and may contact the edge of the periflagellar area. Molecular phylogeny (SSU, LSU rDNA, and ITS region) places P. pseudonux sp. nov. as the second known species in the P. nux lineage, with P. perforatum and the members of the P. micans clade as closest relatives. The strain is available in public culture collections and contributes to the knowledge of the genus Prorocentrum and the understudied diversity of nanoplanktonic species in the open tropical ocean.