In what must qualify as one of the strangest and most complicated life-cycles around, Symbion pandora spends its entire existence as one of many forms living around a lobster’s mouth.
There’s a particular set of challenges living underwater, on an animal that molts its shell periodically, and relying on said animal for food. This tiny creature has evolved the ability to both clone itself asexually, undergo sexual reproduction when necessary, release cloned larvae by essentially vomiting them up from inside itself, and matching all of this to the molting phases of the lobster.
Just watch the video already. This thing is truly Pandora’s bug.