What is the biggest facultative anaerobic metazoan known to science? Related: What's the biggest obligate anaerobic organism discovered till now? I had always assumed that the _Riftia_ tube worms were obligately anaerobic since they lived next to anoxic volcanic vents, but a quick literature search showed it to be a facultative anaerobe, being able to survive at least 60 hours without oxygen. If "facultative anaerobe" is defined as being able to survive at least 24 hours in the absence of molecular oxygen in an otherwise normal environment, are there any metazoal organisms larger than _Riftia_ (which are up to 1.5m long)?

I think a tapeworm is your best bet. For tapeworms specifically, I found this textbook, Animal Physiology: From Genes to Organisms, which claims:

> Active facultative anaerobes include some parasitic worms such as tapeworms that can survive without oxygen indefinitely...

I was not able to find any primary research on the subject either @WYSIWYG (other than a paper from 1951 that doesn't seem to be available anywhere), but if the textbook's claim holds, I can't think of anything larger than a tapeworm.