An impossible habitat
More than 3,000 meters below the surface of the Mediterranean Sea lies the L'Atalante basin, a depression on the seafloor filled with a brine so dense it does not mix with the water above it. This brine has a salinity of 365 g/L, about eight times saltier than normal seawater, and is completely anoxic—it contains no oxygen. The basin formed when ancient salt deposits dissolved, creating a permanently stratified, oxygen-free lake on the ocean floor. This extreme environment, containing toxic sulfides, was long considered inhospitable to any form of animal life.
In 2010, a research team led by Roberto Danovaro reported a discovery from the basin's dark, sulfidic sediments: three species of multicellular animals, or metazoans, that live their entire lives without oxygen. These animals belong to the phylum Loricifera, a group of microscopic invertebrates also known as "corset animals" for their protective outer shell, or lorica. The three species found in the L'Atalante basin were Spinoloricus cinziae, Rugiloricus nov. sp., and Pliciloricus nov. sp. Prior to this, scientists believed that multicellular animals could only tolerate anoxic conditions temporarily. The discovery of loriciferans completing their full life cycle—evidenced by the presence of adults, larvae, and shed exoskeletons—in a permanently anoxic place was unprecedented.
Life without a powerhouse
Nearly all animals depend on aerobic respiration, a process that uses oxygen to generate energy inside cellular organelles called mitochondria. The loriciferans of the L'Atalante basin have a different strategy. Electron microscopy of their cells revealed a complete absence of mitochondria. Instead, their cytoplasm contains organelles that closely resemble hydrogenosomes. These are energy-producing structures found in some single-celled anaerobic organisms, which function without oxygen. This makes the loriciferans the first known multicellular organisms to have adopted a cellular machinery for an anaerobic existence, altering the understanding of the limits of animal life.
The animals themselves are less than a millimeter long. They possess a head with a mouth, a brain, a digestive system, and a complex exterior cuticle called a lorica. Despite their tiny size, they are complex organisms that have adapted to one of the most extreme environments on Earth. The L'Atalante basin is now is a biodiversity hotspot for this phylum, with three distinct species in its toxic, oxygen-free sediment.