Decomposers are an integral part of any healthy ecosystem, breaking down plant and animal matter and releasing more simplistic nutrients that can be used as energy by other creatures. Underwater ecosystems are no different and feature several types of decomposers that also appear on land.
As on land, bacteria are one of the most prevalent decomposers in any underwater ecosystem. At any given time, bacteria cover anything and everything in a marine environment. Thus, as soon as a plant or animal dies, bacteria are often the first decomposers to get to work at turning that organic matter into the nutrients that other sea creatures rely on. Bacteria are known as "microdecomposers," because they are impossible to see with the human eye.
Another decomposer found underwater and on land, fungi vary in size from being a small microdecomposer to certain mushrooms that grow bigger than small mammals. When underwater, fungi are typically microscopic, though once they begin decomposing a plant or an animal, they grow a thick, visible gelatinous layer around the organic matter. This is much like the mold layers that grow on decomposing plants and animals on land.
Underwater ecosystems also contain worms that act as decomposers. Like their landed counterparts, many of these worms bury themselves beneath the surface of the underwater floor while they slowly move about, cleaning up detritus (waste) and leftover organic matter and turning it into something that other plants and animals can use. Marine worms are much larger than bacteria or fungi and considered macrodecomposers. They can be vibrant and colorful in appearance, which encourages some aquarium owners to keep these worms as useful pets that help keep aquariums clean.
Sea urchins, starfish and sea cucumbers are all echinoderms found exclusively in marine ecosystems. These colorful, often-symmetrical invertebrates are omnivores because they eat other animals and plants. Still, echinoderms also feed on and process the thick film of organic matter that coats underwater rock formations, making them also macrodecomposers that help sustain underwater ecosystems.