Categories Of Organisms 

Cnidarians are relatively simple aquatic animals. Main groups of organisms in Cnidaria include corals, anemones, jellyfish, sea fans, sea whips and zooplankton.

Marine Fish
Marine fish are aquatic vertebrates. Marine fish include cartilaginous fish, bony fish and sea horses.

Marine Mammals 
Marine mammals are warm blooded, vertebrate animals. Groups of organisms in marine mammals include dugongs, purpoises, whales and dolphins.
Seagrasses are flowering plants that grow in marine environment. They are the main diet for dugongs.
Marine Algae
Marine algae are simple photosynthetic aquatic organisms. Algae include phytoplankton, dinoflagellates, diatoms, microalgae and seaweeds.

Marine Reptiles 
Marine reptiles are tetrapods and cold-blooded animals. Marine reptiles include turtles and sea snakes.
Sponges are sessile filter feeders. Sponges are the simplest animals.
Marine Worms 
Marine worms are elongated, slender and soft-bodied invertebrate animals. Marine worms include turbellarians, flukes, tapeworms, nematodes, roundworms, peanut worms, echiurians, arrow worms, acorn worms and phoronida.
Marine Arthropods
Marine arthropods include barnacles, crabs, horseshoe crabs, prawns, crayfish, lobsters, shrimps and zooplankton.

Marine Molluscs
Marine molluscs include various animals such as snails, clams, cowries, abalone, squids, cuttlefish, octopuses, chiton, lamp shells, nautilus, nudibranch and zooplankton.

Ascidians are sac-like marine filter feeders which are commonly known as sea squirts or tunicates.
Echinoderms are marine animals which encompass sea cucumbers, starfish, brittlestars, basketstars, sand dollar, sea urchins and featherstars.
Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate. They are also known as sea mats.

Marine Fungi 
Fungi are heterotrophic organisms characterized by a chitinous cell wall including fungus-like protists.

Marine Microbes 
Marine microbes are marine microscopic organisms. Marine microbes can include parasites, bacteria, archaea and animal-like protista.
Commonly known as comb jellies. Ctenophora looks like jellyfish but zoologically they are not because they lack the characteristics cnidocytes.