The tiny, filter-feeding animal is only one-third of a millimetre long, barely visible to the naked eye. It appears to replace its guts, entire innards and hairy mouthparts several times in its life, and the dwarf male form appears to be blessed with two penises.
Its Danish discoverers believe it is far more than a new species, genus, family, order and class. They claim to have struck taxonomic gold, for this creature, which they call Symbion pandora, is a whole new phylum.
While new species are the tiniest twigs on the great tree of life, phyla are the huge main branches. They represent the biggest, earliest evolutionary steps as life forms radiated and diversified several hundred million years ago. There were believed to be 39 phyla - until this week.
Humans belong to the vertebrate phylum, which includes all the mammals, birds, reptiles, amphibians and fish. All the other phyla in the animal kingdom are creatures without backbones and the bulk of them can only live in water - where life began over a billion years ago.
This week's discovery was announced in the British science journal Nature.
Dr Geoff Boxshall, a crustacea expert at the Natural History Museum, in London, said: ''New species are found every day, but this is special.''