Morphology and molecular phylogeny of Pleurosigma pacificum sp. nov. (Pleurosigmataceae), a new tropical pelagic species from the Western Pacific Ocean Author Du, Fei-Chao https://orcid.org/0009-0005-0674-5761 Laboratory of Marine Organism Taxonomy and Phylogeny, Qingdao Key Laboratory of Marine Biodiversity and Conservation, Shandong Province Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China & University of Chinese Academy of Sciences, Beijing 100049, China Author Li, Yu-Hang https://orcid.org/0000-0002-5546-1774 Laboratory of Marine Organism Taxonomy and Phylogeny, Qingdao Key Laboratory of Marine Biodiversity and Conservation, Shandong Province Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Author Xu, Kui-Dong https://orcid.org/0000-0002-5186-519X Laboratory of Marine Organism Taxonomy and Phylogeny, Qingdao Key Laboratory of Marine Biodiversity and Conservation, Shandong Province Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China & University of Chinese Academy of Sciences, Beijing 100049, China & Laboratory for Marine Biology and Biotechnology, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China kxu@qdio.ac.cn text PhytoKeys 2023 2023-06-02 227 99 108 http://dx.doi.org/10.3897/phytokeys.227.103890 journal article http://dx.doi.org/10.3897/phytokeys.227.103890 1314-2003-227-99 03A0DB8123885CDFA16EEA1583274595 Pleurosigma pacificum sp. nov. Fig. 1 Description. Valves lanceolate, gradually tapering towards the subacute ends, 45.0-51.5 µm long and 13.0-15.6 µm wide (Fig. 1A-D ). Raphe filiform, straight, slightly curved near poles (Fig. 1A-D ). Raphe angle +2° to +4°. Valve center roundish (Fig. 1E , arrowhead). Terminal area funnel-shaped (Fig. 1F ). Transverse striae straight, parallel throughout, 21-22 in 10 µm , oblique striae 21-23 in 10 µm , intersecting at an angle between 32° to 35°. Figure 1. A-F LM photographs of P. pacificum A-D cleaned frustules showing parallel transverse striae under LM E valve center showing a roundish central area (arrowhead) F funnel-shaped terminal area in apical position G-L SEM photographs of P. pacificum G internal view of the whole valve H internal valve central area showing straight, slight expanded raphe fissures (arrowhead) and nodule bordered by two arched bars I internal raphe fissures terminate in helictoglossae at the apices (arrowhead) J external view of the whole valve showing the terminal fissures curving into opposite directions K external central area of valve showing straight, simple raphe fissures (arrowhead) L terminal fissures (arrowhead). Scale bars: 10 μm ( A-F ); 5 μm ( G , J ); 1 μm ( H , I , K , L ). In SEM, internally, raphe fissures terminate in helictoglossae at apices, orientated in opposite direction to one another (Fig. 1G, I , arrowhead). Internal central fissures straight, slightly expanded (Fig. 1H , arrowhead). Central nodule is raised and flanked by two central bars (Fig. 1H ). External central raphe fissures straight, simple (Fig. 1K , arrowhead). Terminal fissures bent to opposite side of valve, not extending onto valve mantle (Fig. 1L , arrowhead). Areolae loculate with external opening slits and internal poroids (Fig. 1G, J ). Holotype. MBMCAS286904, an example is illustrated in Fig. 1A, E, F . This slide was deposited in the Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China. Isotype. MBMCAS286905, an example is illustrated in Fig. 1C . This slide was deposited in the Marine Biological Museum, Chinese Academy of Sciences, Qingdao, China. Type locality. 7°0.26'N , 141°59.63'E , the upper 200 m water column in the tropical Western Pacific Ocean. Etymology. Named after the Pacific Ocean where the species was discovered. Distribution and ecology. Pleurosigma pacificum is a planktonic species known only from the type locality. The water temperature was 28.5 °C and salinity about 33.4 during sampling. Gene sequences. These sequences were deposited in the GenBank (SSU rDNA OQ437519, LSU rDNA OQ549922, rbc L OQ473490 and psb C OQ437519). PhycoBank registration. http://phycobank.org/103761. Molecular phylogenetic analyses The BLASTn search showed that the SSU rDNA sequence of P. pacificum shares 97.65% identity with an uncultured marine eukaryote (KC771201). The rbc L gene sequence of P. pacificum shares 92.54% identity with P. intermedium (NC_066077). The ML, BI and MP phylogenetic tree based on the concatenated SSU rDNA and rbc L gene showed that P. pacificum belongs to the clade of the genus Pleurosigma with high support (IQ-TREE ultrafast bootstrap value = 99, Mrbayes posterior probability = 1.00, MP standard boostrap value = 77 and MP jacknife value = 85). The P. pacificum is basal to all other species of Pleurosigma , and branched earlier than the two slightly sigmoid species, P. planctonicum and P. intermedium (Fig. 2 , Suppl. material 2). Figure 2. Maximum likelihood (ML) and Bayesian inference (BI) phylogenetic trees based on the concatenated SSU rDNA and rbc L sequences. The values on each node indicate ML bootstrap and Bayesian posterior probabilities (%), respectively. Only bootstrap values over 50% are shown on the tree. Black dot indicate ML/BI = 100/1.00. The asterisk indicates the topological incongruence between ML and BI trees.