Animal That Lives Without Oxygen.

In 2010, it seemed that biology textbooks would have to be
rewritten. At the bottom of the Mediterranean Sea, in one of the most extreme environments on Earth, one research team found evidence of an animal able to live its entire life without oxygen.
Not one of the other million or so known animal species can do that. Oxygen, in some form, is often assumed to be vital for animal life. Yet the existence of these creatures seemed to blow a hole in this theory, with far-reaching implications for our understanding of life on Earth.
The tiny Mediterranean animals belong to a group called the loriciferans – an animal group so unusual that it was not discovered until the 1980s.

Because the mud at the bottom of the L'Atalante basin is completely devoid of oxygen, the team did not expect to find "higher lifeforms"