Taxonomy and molecular phylogeny of three marine benthic species of Haslea (Bacillariophyceae), with transfer of two species to Navicula

Using light, scanning and transmission electron microscopy, and SSU rDNA and rbcL gene sequencing, we investigated three marine benthic diatoms: Haslea avium Tiffany, Herwig et Sterrenburg; H. feriarum Tiffany et Sterrenburg and H. tsukamotoi Sterrenburg et Hinz collected from intertidal sediments in China. These are the first records of these three species from China. Our observations showed that the typical valve structure of Haslea, with an inner basal layer and an outer tegumental layer, and walls (saepes sensu Sterrenburg et al.) between them is not present in either H. tsukamotoi or H. avium. Moreover, the two species possess small helictoglossae, an internal apical thickened area and hooked terminal fissures reaching to the valve mantle, suggesting a close relationship with Navicula tripunctata, the type species of Navicula s.s. In molecular phylogenetic analyses, Haslea was polyphyletic, being divided into two separate clades: H. feriarum belonged to a clade that comprised only Haslea species, including the type H. ostrearia, while H. tsukamotoi and H. avium were within the Navicula clade. Therefore, we propose transferring H. tsukamotoi and H. avium to Navicula and suggest that the presence of continuous longitudinal strips on the external valve surface is not a synapomorphy, but rather a homoplastic character for Haslea.