The most easily recognized symbiotic relationship in the marine world include clownfish with anemones and Gobies with Pistol Shrimps. Both animals benefit from a partnership, both have roles to play.
There are a few symbiotic relationships that are oddities. One of them is the Butt Fish.
Better known as the Silver Pearlfish, (although better remembered as the Butt Fish), these guys are long, skinny, eel-like fish that use a cucumber's hiney to get comfortable. Yup, they use the arse of the cucumber as a home. They lounge in these host quarters by day and come out at night to feed on small crustaceans.
This odd pair is rare in the hobby, but aquarists can find that the fish has hitchhiked in their tanks inside the unmentionables of the cucumber they purchased. Imagine explaining to your kids what the new fishy is doing.
So what do they do in there?
It is thought that they reproduce in the cucumber in addition to making it their habitat. Some species of the Pearlfish are not content by the protection their host offers, and are parasitic to the point of eating their hosts genitalia. It is not uncommon to find a PAIR of Pearlfish living inside Mr. Cucumber's bottom.
The odds of keeping the Pearlfish alive long term in the home aquarium is thought to be possible only with a cucumber host alongside with it.
Check out this video from Emmy Award-winner Richard Fitzpatrick: