Bearer of foreign bodies
In the crushing pressure and total darkness of the Mariana Trench, scientists have documented single-celled organisms called xenophyophores at depths of 10,641 meters (6.6 miles). These creatures are a type of foraminiferan, giant amoebas that construct shell-like structures called tests. Their scientific name means "bearer of foreign bodies," which refers to their method of building these tests. The organism extends parts of its body, called pseudopodia, to select and cement together particles from the seafloor, such as sand, minerals, and the skeletal remains of other microscopic organisms.
The largest species, Syringammina fragilissima, can reach 20 centimeters (about 8 inches) in diameter, making it one of the largest known single cells on Earth. Xenophyophores found in the Mariana Trench have been measured at over 10 centimeters (4 inches). Inside the protective test, the organism itself is a mass of cytoplasm containing millions of nuclei, a structure known as a coenocyte or syncytium. This organization allows a single cell to manage functions over a very large area, exceeding typical biological constraints on cell size.
Engineers of the deep seafloor
Xenophyophores are detritivores, feeding on the sediment and the organic matter, or "marine snow," that drifts down from the upper ocean. They are thought to envelop food particles with their pseudopodia, similar to how a common amoeba feeds. They are found exclusively in the deep sea, typically at depths between 500 and 10,600 meters, across all major ocean basins.
Despite their simple nature, these organisms affect their environment. The physical structures of their tests create microhabitats on the otherwise flat, muddy abyssal plains. These structures provide shelter and surfaces for other deep-sea life, including isopods, copepods, and nematodes. Studies have shown that areas with high densities of xenophyophores host three to four times the number of other benthic invertebrates compared to adjacent areas without them. By creating these small pockets of biodiversity, they are ecosystem engineers in one of the planet's most extreme environments.