Most protozoa are helpful in that they are
important in lower levels of the food chain. They provide food for
living things such as snails, clams, and sponges. Not all protozoa live in
plankton, as well as some protozoa are capable of causing diseases in
humans and other animals. Some diseases caused by protozoa in humans are
malaria, black fever, sleeping sickness, and some types of diarrhea. Many of
these are found in polluted water.
There are representatives in the Animalia that are holoplankton including annelids, arthropods, gastrotrichs, nematodes, flatworms, and rotifers. All have their entire life cycle as plankton.
Annelids have external segments that correspond with repeated digestive and reproductive organs. They live on land, in ocean and fresh water. Fresh water annelids include leeches and bristle worms. Most annelids are predators or scavengers. Swimming annelids actively feed on fish eggs and insect larvae.
Arthropods compose the largest group of organisms on Earth. They are characterized by having an external skeleton with jointed limbs. They include both marine and fresh water groups. Some arthropods are total aquatic while others may only live in the water in their larval or nymph stages. Crustacean refers to arthropods that live in water.
Water fleas are crustacea that have large antennae which they use for locomotion. They have large dual compound eyes above the antennae. Five-six feet are used to filter food (small algae) before it goes through its digestive system. You can also observe its very small heart.