That Fish Blog – Aquarium Advice and Information

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

Welcome back Brandon Moyer for Wednesday’s post.

Many aquarists are familiar with varying forms of symbiosis.  The most popular example I can give is that of the clownfish hosting in an anemone, providing the anemone with food and in return receiving protection from predators.  There are several different types of symbiosis, but both parties do not always benefit from these close relationships.

You may not be familiar with one creature that forms a symbiotic relationship with humans.  This relationship is considered parasitism, and we receive the poor end of the deal.  The species is called Vandellia cirrhosa, or candiru.  These fish live in the Amazon River basin, grow to about three inches in length, and prey on other species of fish.  They can detect the waste excreted by other fishes gills and follow the trail to find their prey.  Rather than eating the fish, they use barbs on their gill covers, or opercula, to wedge themselves under the opercula of their victim.  They then bite into an artery in the gills and drink the fishes blood.  They can completely gorge themselves with blood in as little as thirty seconds!

What makes the candiru even more horrifying is that their ability to detect waste from other fish also allows them to detect human waste, more specifically urine.  They have been known to follow the trail of urine of those who relieve themselves in the water and actually swim up the victims urethra!  If that doesn’t make you cringe, once in the urethra they are unable to turn around, and because of their spines they cannot back themselves out.  They eventually die in the urethra while the victim experiences swelling and obviously extreme pain.  The only way to remove the fish is through surgery. 

I hope you have enjoyed my first article on symbiosis.  Make sure to wear a bathing suit if you are ever swimming in South America.  Look for another blog on pistol shrimp and goby mutualism and pearlfish and sea cucumber parasitism.