 The cyanobacterial symbionts known as phaeosomes are located between the upper and lower lists of the horizontal groove of the cells.One study of these cyanobacteria symbiotes found that the cell size ranged from 3.5-10 µm (thus approximately an order of magnitude larger than prevalent planktonic "Synechococcus" forms).A 2010 study found species to house both the described extracellular cyanobacteria as well as larger rod-shaped non-photosynthetic bacteria on their sulcal lists.