Hi folks, Brandon here. I recently re-read my article on the Candiru, and their strange relationship with humans reminded me of another strange relationship between two aquatic creatures. Some of my favorite marine animals are sea cucumbers, a type of echinoderm related to sea stars and sea urchins. There are varying types of sea cucumbers, some of which sift through the sand, filtering out organics and leaving sand or silt pellets behind. Others have fan-like structures that protrude from their mouth that they use to filter small particles out of the water for food. All sea cucumbers share one characteristic in common; they breathe through their anus using respiratory trees to extract oxygen from the water. They expand and contract their lower intestinal tract, very similar to how our lungs expand and contract, to take in and expel water. They can even spray water several feet when exposed to air. Sea cucumbers are fascinating animals any way you look at them.

Another interesting animal that shares a close bond with certain types of sea cucumbers is the pearlfish. There are many different species of pearlfish, all of which share the same characteristic long, slender body shape. Pearlfish seek out shelter from sea cucumbers, but instead of sharing the same hiding place like pistol shrimp and gobies, the pearlfish will actually retreat into the anus of the sea cucumber. It’s very strange to watch, but amazing nontheless. The pearlfish will back into the sea cucumbers anus tail first where it is then safe from predators.

It is unclear whether this relationship is commensal or parasitic. Some pearlfish have been known to nip at the respiratory system of the sea cucumber, but it does not seem to affect the host in any way. Most sea cucumbers will expel what is known as cuvierian tubes (sections of the respiratory tree) when they are stressed to deter predators from eating the cucumber. These sections of the respiratory system naturally regenerate over time, so the pearlfish’s nipping does not seem to affect them in any way.

I hope you enjoyed reading about this bizarre but fascinating relationship.

Until next time,

Brandon