Alexandrium monilatum is an armoured, marine, dinoflagellate that forms chains of 16+ individuals. Cells are much wider than they are long and anterior-posteriorly compressed. Thecal plates are delicate with numerous small pores. The Sulcus contains a distinguishing large rhomboid posterior sulcal plate (s.p.). The s.p. is recessed and is marked (see fig. 3.) and contains the attachment pore (pap) for chain formation. It is photosynthetic with dark green/brown chloroplasts. The nucleus is crescent moon shaped and located at the centre of the cell. This species produces resting cysts as part of it?s life cycle (Walker & Steindinger 1979).