Xenophyophores are mysterious, poorly-understood marine protozoans that cover the abyssal plains of the deep ocean. They are gigantic single-celled organisms, among the largest in nature, with one species, Syringammina fragillissima topping out at 20 cm (8 in) in diameter. They are of variable appearance, including frilly shapes that resemble the Golgi apparatus within the eukaryotic cell, spherical sponge shaped, and even four-sided (tetrahedal) forms.

Xenophyophores are benthic bottom feeders, meaning they are found only in the very deep ocean. Instead of blood, xenophyophores are filled with cytoplasm, the intracellular fluid, and numerous nuclei which contain its genetic material. The entire ensemble is enclosed within an organic cement-like structure called a granellare. Xenophyophores move along the sea bottom like slugs, filtering sand for nutritious particles and tiny animals -- including nematodes, which are abundant on the abyssal plain -- using a slimy excretion.

Like amoeba, xenophyophores consume food by engulfing it using a structure called a pseudopodium. Most xenophyophores live right on the surface, although one species, Occultammina profunda is known to bury itself in sediment. Xenophyophores have a disgusting habit -- letting their fecal matter build on themselves when it mixes with their slimy secretions, gathering into long structures and strings. This is where the organism gets its name, which roughly means "bearer of foreign bodies" in Greek.

Xenophyophores are very numerous in some areas of the ocean floor, such as abyssal trenches, where their density may exceed 2,000 per every 100 square meters, making them the dominant organism in these ecosystems. They are capable of withstanding the extreme pressures on the ocean floor, which may exceed 100 times the pressure at sea level. Xenophyophores probably don't have many predators, as few predators live so deep that they can harvest the abyssal plains, although they do exist. The abyssal plains are among the most poorly explored areas of the planet, so there is much more to learn.