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.

Dolania americana  has the shortest lifespan of any mayfly: the adult females of the species live for less than five minutes.
[11]5 minutes 300 seconds.
Hardly worth the effort.


5 minutes. What can you do in 5 minutes.
Have sex, lay eggs and die.
Talk about getting down to the nitty gritty of life.
The Mayfly however does have a longer life in the nymph stage.