The giants of the deep are so-called xenophyophores, sponge-like animals that—like amoebas—are made of just one cell. They were found during a July research expedition run by the Scripps Institution of Oceanography in La Jolla, California.
The animals are about four inches (ten centimeters) long—among the largest single-celled organisms known to exist.
The creatures were discovered at depths of 6.6 miles (10.6 kilometers). That breaks a previous record for xenophyophores found in the New Hebrides Trench at 4.7 miles (7.6 kilometers).
Xenophyophores represent "one of the few groups of organisms found exclusively in the deep sea," said Lisa Levin, a Scripps oceanographer who studied the expedition's data.
"If any creatures should be able to live at the ocean's greatest depth, then xenophyophores certainly should be among them."
(See "Life Is Found Thriving at Ocean's Deepest Point.")
Next, "Behemoth Nematodes"?
The Mariana xenophyophores were seen in footage from Dropcams, free-falling devices equipped with lights and digital video that were developed by the National Geographic Society. (The Society owns National Geographic News.)
Protected by thick walls of pressure-resistant glass, the Dropcams were baited to attract whatever marine life might be lurking in the deep. Expedition scientists also saw, for instance, the deepest-swimming jellyfish to date.
"The deep sea is the largest biome on Earth and holds much of the diversity on the planet—[yet it's still] largely undescribed," Levin said in an email to National Geographic News.
(Also see "Pictures: Hard-to-See Sea Creatures Revealed.")
According to Jon Copley, a marine biologist at the U.K.'s University of Southampton, "many of the major discoveries in deep-sea biology have come from making direct observations at the seafloor."
"The Dropcam is a great tool for the future, because it can help us see more of what's down there for less cost than using ROVs or submersibles," he said via email.
For instance, "finding xenophyophores far deeper than before shows how much we still have to learn about our oceans depths and their inhabitants."
Tullis Onsott, an expert in deep-sea microorganisms at Princeton University, also called the xenophyophore discovery "fantastic."
"Who knows what's next, behemoth nematodes?" he said by email.