New observations on the morphology of the marine planktonic diatom Chaetoceros seychellarus and taxonomic implications

The diatom Chaetoceros seychellarus was originally described by G. Karsten in 1907 from the marine plankton of the Indian Ocean. It is considered a rare tropical/subtropical species. A previous study showed its general morphology by light and electron microscopy, and a close relationship of this species with some others of the subgenus Chaetoceros was suggested. C. seychellarus has recently been found in samples from the coasts of Baja California, the Gulf of Tehuantepec (Pacific Ocean), the Gulf of Mexico and the Mexican Caribbean Sea (Atlantic Ocean). The characteristic shape of the aperture, the presence of inseparable intercalary valves, and the union of the anterior terminal setae are recognised, and additionally the presence of two opposite rimoportulae in the valve face of each intercalary valve is annotated. This fact is relevant, as no other species of the genus has been reported to have two rimoportulae in each valve, although multiple rimoportulae have been observed in terminal valves or each valve of the chain in other Chaetoceros species. We propose a new section of the subgenus Chaetoceros to accommodate C. seychellarus. A discussion on the formation and possible function of the rimoportulae in the genus Chaetoceros is included.