 One study found that the area in contact with the highest volume of external medium due to water flow is also a region where there are markedly less barriers to nutrient transfer (two less membranes - including theca).By comparing extant morphologies in dinoflagellate species, it has been suggested that ancestral species were benthic and had streamlined cells.As lifestyles became more planktonic, the large cingular and sulcal lists evolved alongside.