Chaetoceros is probably the largest genus of marine planktonic diatoms with approximately 400 species described. Although a large number of these descriptions are no longer valid. It is often very difficult to distinguish between different Chaetocerosspecies. Several attempts have been made to restructure this large genus into subgenera and this work is still in progressHowever, most of the effort to describe species have been focused in boreal areas, and the genus is cosmopolitan, so there are probably a large number of tropical species still undescribed.
(From Wikipedia, May 31st, 2012)