Answer:
Because this segment is important for their life cycle.
Explanation:
The main reason of many marine organism is due to their place in the food cycle which able them to spend their segment of their life cycle as plankton. There are two types of plankton i. e. phytoplankton which refers to the plants while the other is zooplankton that refers to animals which feed on these phytoplankton. these zooplankton are food of fishes so that's why they are important for the food cycle in marine ecosystem.
