So in the course of procrastinating last night (if only I could get paid to do it, although probably some people would think that's exactly what I get paid to do) I ran across something interesting.
You can basically divide all life on Earth into two groups: the eukaryotes, and everything else (which includes, mostly, bacteria and the archaea.) Eukaryotes include anything --single or multi-celled --with a membrane-bound central nucleus ("eukaryote" is from a Greek root, meaning, roughly, "good kernel.")
Virtually all eukaryotes --humans, animals, plants, fungi, many protozoans, and so on --also contain in their cells, tiny organelles called mitochondria; mitochondria are, essentially, cells-within-a-cell that allow the use of oxygen for the production of energy. (ADP+P-->oxidative phosphorylation-->ATP anybody?) Mitochondria actually existed as free-living organisms at one time; around 2 billion years ago, a bigger cell tried to eat one. The proto-mitochondrion, however, resisted digestion, and became an endosymbiont --its energy producing capabilities traded off to the host cell in exchange for a supply of glucose and, one presumes, some measure of protection from harm.
The other major endosymbiont in eukaryotic organisms is of course the chloroplast, which is found in plants, algae, and even in some unicellular eukaryotes. The chloroplast uses sunlight, water, and carbon dioxide to generate energy and make food (glucose.) An interesting thing about chloroplasts is that there has been an example of the beginnings of an endosymbiotic relationship between chloroplasts and a host cell fairly recently, in evolutionary terms --Paulinella, a photosynthetic amoeba, appears to have, at some point, ingested another photosynthetic microorganism and, apparently knowing a good thing when it saw it, decide to hang on to the chloroplasts.
For the longest time (since high school biology, basically) I was under the impression that was it: the only endosymbionts used in energy production were chloroplasts and mitochondria. As it turns out, however, there is another: the hydrogenosome. Hydrogenosomes, like mitochondria and chloroplasts, appear to be the result of endosymbiosis, and they require no oxygen. Until recently, they were thought to exist only in protozoa (they're the main energy producing organelle in Trichomona vaginalis and Plasmodium falciparum, the latter being the parasite responsible for most deaths due to malaria.)
However, in 2010, scientists investigating the depths of Mediterranean found the first known multicellular organism using hydrogenosomes --which is also the first multicellular animal known that spends its entire life in an oxygen-free environment. The critter in question is Loricifera --there are three species currently known to exist, and they're rather appealing looking, with fronds extending from a protective shell (albeit they're tiny, the biggest run to about a millimeter: giants among their kind.) The ones discovered live in a huge pool of ultra-salty water deep below the surface, at around 3000 meters, in the L'Atalante basin --the boundary between salt and fresh water, or halocline, prohibits mixing of salt and fresh water, so below the halocline the water is totally oxygen free.
Apparently they eluded detection for so long thanks to their rarity, and the extremely firm grip they keep on the bottom gravel.