1. Chaetognatha.
Arrow worms

3. Contains 125 species
Most chaetognaths are planktonic, but some species attach to algae or other substrates.
They are found in marine ecosystems all over the planet, including the open ocean, tide pools, polar waters, marine caves, coastal lagoons, and the deep sea.

5. Often, they can be the second most dominant group of zooplankton
Often, they can be the second most dominant group of zooplankton. They have been found to be the most abundant type of animal present in some deep-sea environments. They are predators on copepods and other small crustaceans, larval fish and other chaetognaths. They are food for fishes, squids and some sea birds.