Marinology

Octopuses have three heart and nine brain.

(Nine brains are better than one: An octopus’ nervous system | biomechanics in the wild. (n.d.). Notre Dame Sites | The Notre Dame WordPress Community. https://sites.nd.edu/biomechanics-in-the-wild/2021/04/07/nine-brains-are-better-than-one-an-octopus-nervous-system/)

Even brainless jellyfish sleep at night

https://studyfinds.org/jellyfish-sleep-study/