[AutumnBison (OP)]

As far as I know, Porifera (sponges) and Cnidaria (jellyfish, corals and hydroids) can thrive in conditions where oxygen contents are low.  Jellyfish, in particular, thrive better in warmer, oxygen-lower water than cooler, oxygen-higher water.  And sponges were around when oxygen contents really were lower than they are today.  But is that everyone?  What other animal phyla, if any, can thrive in water that have low if not no oxygen content at all?

[Kryptid]

Some loriciferans can live their entire lives in anoxic conditions: https://www.nature.com/news/2010/100406/full/464825b.html

The crucian carp can survive anoxic conditions for at least 140 days: https://en.wikipedia.org/wiki/Crucian_carp#Physiology

[AutumnBison (OP)]

Some loriciferans can live their entire lives in anoxic conditions: https://www.nature.com/news/2010/100406/full/464825b.html [nofollow]

The crucian carp can survive anoxic conditions for at least 140 days: https://en.wikipedia.org/wiki/Crucian_carp#Physiology [nofollow]

Could the phyla from either species be around in periods of Earth's history when the oxygen levels in the water really were lower than today?

[chris]

Just adding to the point about carp - last year we spoke to Michael Berenbrink, from the University of Liverpool, who showed that carp adjust their metabolism under hypoxic conditions in a similar way to yeast and begin to excrete alcohol! But don't drink the duckpond in spring, the total amount produced is very low! https://www.thenakedscientists.com/articles/interviews/goldfish-swap-oxygen-alcohol