Sea cucumber and coral. Photo by Teddy Fotiou.
Sea cucumber and coral. Photo by Teddy Fotiou.

Evolution is blind and random, so it does not always make sense. Thus, over the course of time, evolution has spawned some truly unusual critters. Case in point: sea cucumbers, pearlfish, and their peculiar relationship.

Sea cucumbers are lumbering echinoderms that breathe and poop through their anuses. Pearlfish are skinny fish that lack dorsal fins…and class. When the two animals meet, something truly bizarre occurs.

You see, some species of pearlfish and are parasitic. Their slender bodies allow them to take up residence inside various invertebrates, including clams, seastars, sea squirts, and sea cucumbers.

Pearlfish that live inside sea cucumbers are probably the trashiest of all fish in the ocean.

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In a gross violation of privacy, the pearlfish enters the sea cucumber’s body through its anus and makes its home inside. Then, in an even bigger violation of privacy, the pearlfish feeds upon the sea cucumber’s gonads, which are the organs that produce the sea cucumbers’ sex cells (eggs or sperm), and it may even invite over a few friends. One biologist recorded 15 pearlfish living inside the anus of a single sea cucumber!

Watch the video below to see more:

This relationship between two species is not new, and it is known as symbiosis. In these types of interspecies relationships, both species tend to benefit, such as the relationship between clownfish and sea anemones. However, in this case, the relationship is one-sided and only benefits the pearlfish. So, in addition to having a fish living inside its butt, the sea cucumber gets jack squat. That means the pearlfish is nothing more than a parasite. Unfortunately, the sea cucumber cannot do anything about it and must “grin and bear it”.