Symbion. A whole new phylum called Cycliophora was erected when these things were discovered in 1995. They have only ever been found on the mouthparts of a single species of lobster, eating the crumbs as they pass by.
They have an incredibly complicated life-cycle.
The simple bit is when feeding adults reproduce asexually.
But then they produce a male who swims over to another feeding adult who has a female inside her. And then the male produces two more males who do the actual fertilising.
The fertilised female escapes her mother and rests on the lobster, but doesn’t actually feed. Instead, she produces a larva that eventually swims off to finally, AT LAST, find a whole new lobster of the exact same species to settle on and grow into a new feeding adult.
What a palava!