Regeneration

Going with your gut? Not if you are a sea cucumber

This creature poops out its organs when spooked! But can it grow them back?

Eisapour, Mina, et al. “Digestive tract regeneration in the posteriorly eviscerating sea cucumber Holothuria parva (Holothuroidea, Echinodermata).” Zoomorphology (2021): 1-15. https://doi.org/10.1007/s00435-020-00511-3.

The sea cucumber is a strange creature in many ways. For one, it does bear a strange resemblance to the  cucumbers you would put on your salad. Don’t be misled by the name! Sea cucumbers are animals, not plants. But much like their plant namesakes, sea cucumbers don’t have any protruding body parts – or even eyes! They only have tiny feet that they use for crawling around on the ocean floor like defenseless sausages, looking for bits of debris to munch on. 

All in all, sea cucumbers may look rather unremarkable. But they possess a fascinating – and a rather disgusting – ability. 

Keeping your cool as a cucumber

The mouth of a sea cucumber looks a little bit like broccoli. The anus is on the opposite side of its body. Different species of sea cucumbers can eject their organs either through their mouths or anuses, or both. Image by Clker-Free-Vector-Images from Pixabay

If a mere, defenseless sea cucumber is startled or thinks that a predator is in hot pursuit, what can it do to protect itself? Does it swim away very fast? Hide in a cloud of ink? Crawl between some rocks? Not at all. Our sea cucumber does a perfectly sensible thing in its worldview. Full of composure and quiet dignity, it propels out its insides from, well, its rear end. 

Presumably, a sludge of organs sent flying scares off any predators. And that makes sense – if your empanada suddenly ejected its meaty filling, you wouldn’t eat it either.

After surviving a close encounter, our sea cucumber should be relieved.  But it still finds itself in a pickle. It survived – but its guts are now gone.

Worry not! Sea cucumbers can grow back their digestive organs, also perfectly normal in their worldview. To better understand how they manage such a seemingly impossible task and how long it takes, a team of Iran-based scientists traced gut regeneration in Holothuria parva, a previously understudied sea cucumber species native to the Persian Gulf.

A gut reaction

The scientists collected sea cucumbers near at the Port of Deyr in Bandar-e Deyr, Iran. “Bandar” means “port” or “city by the sea” in Arabic. Image source: Google Maps

The researchers collected sea cucumbers at the seashore of the Persian Gulf, near the Dayyer port in Iran. 

To get the animals to expel their organs, the scientists injected them with a solution of potassium chloride. After only a minute, the sea cucumbers excreted their internal organs, including half of the breathing organ and most of their digestive tract. They then were kept in tanks in the lab and allowed to regenerate. 

After several weeks, the researchers put the sea cucumbers into deep anesthesia, cut them open, and evaluated how well the animals had managed to regrow their organs. 

Regenerating on an empty stomach

Undaunted by their predicament, the cucumbers began regrowing their gut as soon as three days after self-eviscerating. Over the course of two months, the empty voids in their body cavities filled out with regrowing rudiments of organs.

The digestive tract first grew out as a thin rope. It then hollowed itself out to make a tube. Just over three weeks after gutting themselves, the cucumbers regrew their intestines, from the mouth to the backside opening.

At this point, sea cucumbers could finally enjoy a meal. Traces of food in the intestines were a clear sign to the scientists that the regrowing organ was perfectly functional! And after two months, the new digestive tracts looked very much like the original ones.

But can they do it again?

Can sea cucumbers regenerate repeatedly?

That’s a great question! It seems that the answer is unclear – especially for Holothuria parva, whose regeneration was only described in this study for the very first time.

Wouldn’t it be so unfortunate if a spooked sea cucumber were forced to expel its brand new digestive tract but couldn’t regrow another one? Luckily, there’s a good chance that sea cucumbers should be able to manage repeated regeneration. This is likely because much more complex animals like amphibian salamanders can regenerate their amputated legs several times. But we won’t know for sure until scientists make some sea cucumbers ejaculate their insides over and over again.

Until then, let’s admire sea cucumbers and their creative strategy for survival – and hope that they don’t go too hungry while regrowing their intestines.

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