First described in 1844, Chaetoceros is one of the largest and most diverse genera of marine diatoms, containing well over 100 species. Chaetoceros danicus is a colonial form, and groups of seven cells are not uncommon (as shown here). It is easily recognized because it has highly distinctive long, stiff hairs, called setae, which project perpendicularly from the margins of its test. and have prominent secondary spines along their length. Chloroplasts, which contain pigments used in photosynthesis, are numerous and found inside both the cell and the setae of Chaetoceros danicus. The setae are easily broken, and if large quantities lodge in the gills of a fish, they may kill it. The secondary spines anchor the setae to the sensitive gill tissue, causing irritation, and the fish reacts by producing mucus. Eventually, it dies from suffocation.