Marine worm

Any worm that lives in a marine environment is considered a marine worm. Marine worms are found in several different phyla, including the Platyhelminthes, Nematoda, Annelida (segmented worms), Chaetognatha, Hemichordata, and Phoronida. For a list of marine animals that have been called "sea worms", see sea worm.
Many of these worms have specialized tentacles used for exchanging oxygen and carbon dioxide and also may be used for reproduction. These specialized tentacles allow for gas exchange, further decreasing oxygen content in dead zones and in shallow water, which encourages plant and algae growth.
Polynoid scale worms are estimated to have arrived in deep sea ecosystems around sixty million years ago. Through the comparison of 120 genes, researchers came to the conclusion that genes related to DNA repair, recombination, and integration were only present in the deep sea polynoidae, which correlates with the idea that they have to adapt to deal with potential hypoxia in deep sea environments.
Some marine worms are tube worms, of which the giant tube worm lives in waters near underwater volcanoes, and can withstand temperatures up to . 
Some worms can live in extremely deep oceanic trenches, such as in the Pacific Ocean off the Galápagos Islands.
In recent years, marine worms (especially those found in the ocean) have been observed ingesting microplastic particles found in the oceans. This trend is concerning many scientists, as marine worms act as an important food source for many fish and wading birds. Marine worms are often keystone species in an ecosystem, and the introduction of plastic in the oceans not only diminishes the growth rates of the marine worms, but also affects the food chain of that ecosystem.