The ocean floor has "almost no oxygen," not "no oxygen." There remain oxygen-burning creatures adapted to living beneath in hypoxic conditions.

I dunno if hypoxic is the right term here as it’s usually taken to mean oxygen deprivation / insufficiency, which isn’t what’s happening here, as evidenced by organisms being adapted to the conditions.

The oxygen concentration is actually quite a bit higher in most hadal zones (> 150 µmol/kg [1]) than oxygen minimum zones that exist in the middle layers (< 45 µmol/kg [2]). It's definitely not a hypoxic environment relative to the rest of the ocean.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5829614/

[2] https://web.stanford.edu/group/MicheliLab/pdf/oceanographica...

Where does the oxygen at the bottom of the ocean come from I wonder?

I guess it could be like, in oxygen-poor places, animals are more likely to evolve to need less oxygen. Some (all that we know of so far) will do this by adapting to extract oxygen more efficiently and to use less energy, and others might do it like this parasite, by finding ways to extract energy directly from a host.

It seems that it would still be surprising that such a parasitical organism could exist in such an environment.

Presumably, if an organism is breathing in a near-zero oxygen environment, they are probably just getting the oxygen they need. If there's enough oxygen to support a parasite stealing from the first, there was enough oxygen to support the parasite getting its own oxygen.

It seems to me much more likely that we'll find these parasites where there is plenty of oxygen:

These organisms didn't evolve as a way of dealing with having no oxygen. They evolved because they adapted so that they did need to bother with mitochondria, because there was a plentiful supply of ATP they could steal. Since they didn't need to burn oxygen, they could do without. And the environment they are in has so much oxygen that the host organism doesn't suffer.