Not sure because we don’t know lifespans of all species. But as much as I have read so far, I found “gastrotrich”.
The real record-holder
The real champion of the short life is the gastrotrich, a near-microscopic creature found in aquatic environments worldwide. The flat, transparent critters live their entire lives in less than a week, reaching sexual maturity in three days, reproducing by cloning and then dying a day or two later.
What animals live the shortest lives?I read about mayfly too. They live much longer.