Scientists have found the first multicellular animals that apparently live entirely without oxygen. The creatures reside deep in one of the harshest environments on earth: the Mediterranean Ocean's L'Atalante basin, which contains salt brine so dense that it doesn't mix with the oxygen-containing waters above. Previous samples taken from the water and sediments in the basin showed that single-celled life was present, but a new study published this week in BMC Biology has identified multi-cellular animals that apparently live and reproduce in the sediments under the salt brine. Italian and Danish researchers describe three new species of tiny animals called Loricifera. The animals took up radioactively tagged leucine (an amino acid), and a fluorescent probe that labels living cells, evidence that they were alive when they were collected. The researchers also found examples of individuals that contained eggs and evidence of apparent molting, which led them to conclude that the animals spend their whole lives in the harsh sediments. The creature's cells apparently lack mitochondria, the organelles that use oxygen to power a cell. Instead they are rich in what seem to be hydrogenosomes, organelles that can do a similar job in anaerobic (or oxygen free) environments. The find could help scientists understand what life might have looked like in the earth's early oceans, which also had very little oxygen.