Axolotl Tank-mate Grossology

About a week ago, Flopsy the axolotl got a big tank cleaning. Instead of just a water change, Animal Caretakers Katie, Maida, and Lauren pulled out and scrubbed her rocks, replaced a bunch of sand and knocked back the algae that grows no matter how much cleaning the tank gets.

The tank looked wonderful and doing water changes was a dream. But then on Tuesday, a young girl from a Pacific Science Center summer camp called us over to point out some very concerning looking worms.

Our axolotl tank contained a dozen or more planaria (Planaria torva), flatworms, wandering around the glass. The camp student was quite concerned because similar worms were present in her fish tank before the fish died and she wanted to save Flopsy from such a fate.

We did not recollect ever seeing anything like planaria in our tank before. We promptly alerted our veterinarian, Dr. Maas, at ZooVet Consulting and sent him photos. Dr. Maas’ reply was reassuring, fascinating and gross.

Reassuring – The planaria we saw are freshwater flatworms that live in the water, eat debris, and are not parasites of the axolotl. They can coexist.

Fascinating – Planaria turn out to be an incredibly interesting animal. With their axolotl tank-mates, they share the ability to regenerate tissue, and they do it to a much greater degree. Planaria that have been cut into pieces can regrow new animals from the parts. Planaria that have been carefully cut to split their bodies can grow back the damaged tissue, including growing two heads!

Furthermore, they don’t have any specialized breathing apparatus. We have lungs; many animals have gills or tracheae of some kind. Planaria use diffusion to move oxygen in and out of their cells. Their skin, constantly bathed in water, lets in new oxygen and carries away carbon dioxide. If their bodies are too large or thick, some cells will be too far away from the oxygen. Therefore planarians are flat for a reason – so they can breathe.

Gross – Planarians have a simple body plan, with cavity acting as a gut. They eat and defecate through the same opening, a tube midway down their body. We know sea anemones do this, but it’s easier to imagine a stationary, circular animal using a big hole in the center for digestion. Because planarians look more like other bilaterally symmetrical animals we are familiar with, it is surprising that their guts are so simple.

Fishing all the flatworms out of the tank would be very difficult and we don’t want to medicate unless there is a good reason. So as long as they can share the tank safely, the flatworms, in all their weirdness, get to stay.