Tiny animals can live without oxygen

We knew some single-celled organisms could live without oxygen. But multi-celled animals? Some tiny creatures of the phylum Loricifera have "learned" to live in anaerobic (oxygen-free) mud on the floor of the Mediterranean Sea. They manage without mitochondria, the organelles universally found in the cells of other animals which use oxygen to produce energy. Scientists who didn't know this was possible are now excited about looking in other environments for similar animals, while others ponder the implications for life on oxygen-free worlds.