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.

A new freshwater benthic diatom Amphora baotuensis is described from a karst spring in China, based on morphological and molecular phylogenetic data. Based on SSU rRNA and rbcL sequences the molecular phylogeny indicates that this species is closely related to A. copulata (Kützing) Schoeman & Archibald within the Amphora sensu stricto clade. Amphora baotuensis differs from congeners by a combination of morphological features including smaller cell size, a wide, rectangular to fan-shaped dorsal fascia, a well-developed raphe ledge on the ventral side and uniseriate ventral striae.

A new marine monoraphid diatom species, Planothidium pseudolinkeisp. nov., is described from the coast of Guangxi, China. The detailed morphology of this epipsammic diatom is studied by using both light and scanning electron microscopy. P. pseudolinkei differs from congeners by a combination of morphological features including capitate apices, multiseriate striae, a small central area on the raphe valve and an oblong sinus on the rapheless valve. Ecological preferences of Planothidium are also briefly discussed.

A new species of pelagic diatom, Pleurosigma pacificumsp. nov., is described from the tropical Western Pacific Ocean. It has the typical features of Pleurosigma, including a slightly sigmoid raphe, intersected transverse and oblique striae, and loculate areolae with external opening slits and internal poroids. Morphologically, P. pacificum belongs to a species group of Pleurosigma with lanceolate valves, including P. atlanticum Heiden & Kolbe, P. nubecula W. Smith, P. indicum Simonsen, and P. simonsenii Hasle. However, P. pacificum differs by its smaller lanceolate valve and smaller intersection angle as well as elliptical areolae without a silica bar. The SSU rDNA and rbcL sequence data place P. pacificum in a basal position relative to other species of Pleurosigma. Our molecular phylogenetic analyses did not support the monophyly of lanceolate and slightly sigmoid species. Thus, the sigmoidality of valve outline cannot be considered as a criterion to define the species group.

Specimens of Fallacia tenera were collected from the surface sediment at in a river estuary in Japan. Auxosporulation occurred in a rough culture. Morphological structures of vegetative cells and auxospores were observed in detail. The vegetative cells have one H-shaped chloroplast. The striae were interrupted by two depressed lateral sterna internally and partly covered by a finely porous conopeum on the external surface. The lateral sterna and porous conopea formed two more or less curved longitudinal canals connecting with the exterior via opening pores on both sides of a terminal fissure. This combination of characteristics is unique to the genus Fallacia. The cingulum was composed of three bands, such as an open valvocupula and two comparatively thin pleurae. The two pleurae could be distinguished by the shape of their ligulae. The second band had a triangular ligula, whereas the ligula of the third band is arc-shaped. The auxosporulation was type IA1a in Geitler’s classification. Two paired gametangia formed two anisogametes in each of them. Two auxospores formed in the thecae of the gametangia after a trans physiological anisogamy. The perizonium of the auxospore consisted of a set of transverse bands and five longitudinal bands. The primary transverse band was about twice wider than the secondary ones. The circular incunabular scales were present on the two terminals of the auxospore and on the surface of the primary transverse band. The primary longitudinal band had an acute terminal and was flanked by secondary longitudinal bands. Each side had two secondary longitudinal bands. All longitudinal bands were immediately beneath the transverse bands. Morphological comparison between Fallacia and Pseudofallaica, and the taxonomic position of F. tenera is also discussed.