Meet the sea cucumber!
BBC Two's Natural World explains this "cumbersome oddball" (cu-cumbersome?) spends most of its time "sucking sand in through one end of its body, extracting anything nutritious from it and passing what it doesn't want out the other end."
Meet the pearlfish!
Natural World explains these slender and slithery fishes go to "extreme depths" to find a safe and secure place to hide from predators.
Wanna take a wild guess where this pearlfish hides?
Yep. Up the sea cucumber's butt.
Natural World notes the pearlfish is actually rather "clever" for
performing a colonoscopy on living in the sea cucumber because most sea predators are repulsed by it. Both creatures remain safe, even though one creature takes the backdoor to sanctuary.
But... the pearlfish benefits more from its commensal relationship with the sea cucumber — and will spread its wealth.
Not only does the pearlfish squat in the sea cucumber, but it also invites its friends and family to stay in the occupied asshole.
Kind of an ass-inine thing to do.