@hiba_moktar123 Thank you for asking!! Nutricula is the scientific genus for the so-called "Immortal Jellyfish"! This jellyfish has a very unique ability: the adults, in order to heal themselves (either from damage or old age), will revert back to their polyp (baby) phase, allowing them to live potentially indefinitely! Unlike the quirk in the story, Turritopsis (taken from the scientific name, N. turritopsis), the actual jellyfish can be killed through outside means, like being eaten.
As for the actual chapter titles, most of them are jellyfish! Some have specific species that fit the chapter, some are just random species, and a few chapter titles aren't jellyfish at all, but a different sea animal entirely! ;)
I'm so glad you like the story, and I'm so thrilled you asked this question!!! I'm taking a small break for now, but I'm itching to get back to the next chapter!! ...Once I figure out a certain scene order.