Xenophyophores – The World’s Largest Single Celled Organisms

Xenophyophore

A large 20-cm wide Xenophyophore. Xenophyophores are single cell animals called protists. As benthic particulate feeders, xenophyophores normally sift through the sediments on the sea floor. and excrete a slimy substance; in locations with a dense population of xenophyophores, such as the bottom of trenches, their density may be as high as 2000 per 100 square meters. Atlantic Ocean, Mid-Atlantic Ridge. 2005 July.
Credit: IFE, URI-IAO, UW, Lost City Science Party; NOAA/OAR/OER; The Lost City 2005 Expedition.

Xenophyophores, the world’s largest single-celled organisms, grow up to eight inches in diameter. More than 42 species have been discovered in deep parts of the oceans.

When they were discovered in 1889, they were believed to be sponges. They grow in a variety of shapes and colors and do resemble sponges in appearance. Xenophyophores live in every ocean and have been found in waters nearly seven miles deep.

Their preferred habitat are the abyssal plains, where they constantly dig in the ocean bottom for food. They feed by engulfing their prey, similar to an amoeba. For this reason, they were long thought to be amoebae, but genetic studies place them as a specialized group of Foraminifera.


Many other animals seem to depend on xenophyophores for habitat creation. Isopods are often found living where xenophyophores have recently dug and brittle stars are often found sitting on top of or underneath the xenophyophores.

Xenophyophores with Brittle Stars

Mountains in the Sea Expedition. Phenomenal line-up of organisms representing the great diversity on Balanus Seamount – a strange spoon worm, an elegant sea pen, a stalked crinoid, and two xenophyophores (large one-celled organisms) with brittle stars. New England Seamount Chain. May 2004.
Credit: Mountains in the Sea 2004. NOAA Office of Ocean Exploration; Dr. Les Watling, Chief Scientist, University of Maine.

Very little is known for certain about the life cycle of xenophyophores. Their fragility makes it difficult to harvest them for captive study. While they are abundant on the deep ocean floor, they are not found in any other habitat.

Xenophyophore in Claw

A large 20-cm wide Xenophyophore. Xenophyophores are single cell animals called protists. As benthic particulate feeders, xenophyophores normally sift through the sediments on the sea floor. and excrete a slimy substance; in locations with a dense population of xenophyophores, such as the bottom of trenches, their density may be as high as 2000 per 100 square meters. Atlantic Ocean, Mid-Atlantic Ridge. 2005 July.
Credit: IFE, URI-IAO, UW, Lost City Science Party; NOAA/OAR/OER; The Lost City 2005 Expedition.

Photos courtesy NOAA Photo Library

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Author: Heather Carr

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