Note on the morphology of two planktonic diatoms: Chaetoceros bacteriastroides and C.seychellarus, with comments on their taxonomy and distribution

Two planktonic species of diatoms, Chaetoceros bacteriastroides and C.seychellarus, have been studied by light and electron microscopy. Important features that may be used as taxonomic characters are described. A new subgenus, Bacteriastroidea, is proposed to include C. Bacteriastroides, because of its unique characteristics within the genus. The taxon is considered to be a link between the genera Bacteristrum and Chaetoceros. Morphological and taxonomic affinities between C.seychellarus and other related species of Chaetoceros subgenus Chaetoceros section Borealia, art noted, and a phylogenetic sequence is suggested. Some general evolutionary tendencies are discussed. The known distribution of C. Bacteriastroides and C.seychellarus is revised: both species are restricted to warm waters, but while C. Bacteriastroides shows an Indo-Pacific distribution, C.seychellarus is found in all three oceans.