They are about 0.03 mm long. Dinoflagellates that live in ponds usually have a brown, spiny shell covering their soft insides. The shell is made of lime. The flagella stick out from grooves in the shell. Dinoflagellates are more common in ponds with hard water where there is more lime dissolved in the water.
Dinoflagellates swim in open water.
Dinoflagellates have chlorophyll and so can make their own food. To reproduce, they split into two new plants. One of the new plants keeps the original shell covering. The other plant leaves and must make a new shell by secreting lime. Certain ocean dinoflagellates are more red than brown in colour.