The Ceratium genus is most distinguished by the characteristic arms or horns. These help the Ceratium float; but also prevent them from moving too quickly. The horns tend to be shorter and thicker in cold, salty water, and longer and thinner in less salty, warmer water.
Ceratium are covered by an armour-like cell wall or theca, composed of many textured plates, this can easily been seen on CPR samples.
In living cells Ceratium have two flagella, a transverse flagellum beats in a spiral motion, and a longitudinal flagellum pulses in waves. The grooves in which the flagella operate are prominently seen in this species.
Ceratium are relatively harmless organisms. They are non-toxic and are necessary for the food web. However, they can cause a red tide if conditions allow for excessive blooming. While this red tide is not toxic, it can deplete resources in its environment, and causes a strain on the ecosystem. In general though, Ceratium are necessary components of their habitats. They serve not just as nutrients for larger organisms, but they keep smaller organisms in check through predation. Ceratium are mixotrophs; obtaining food both through photosynthesis and phagocytosis. (Information from Tomas 1996, Microbewiki and Britannica online encyclopedia).
A long thin shaped Ceratium. From a swollen body the apical horn
is long, slightly tapers and has a slight curve. The hypothecal
horns are asymmetric, one is rudimentary, the other is long,
slightly tapers and has a slight curve.
This species is very similar to C. extensum the difference lie in the length (C. fusus <500µm) and the distribution with C. fusus occuring in warmer waters.