What’s up with the rori guts?

Three healthy rori… snuggling?… on the bottom of Rarotonga’s lagoon.



Walking along the beach in Arorangi yesterday, I (Tiffany) started seeing sea cucumbers that looked a little… unhappy. Sea cucumbers, called rori here, are often present on the sandy lagoon floor right by the shore, looking a bit like… well, look at the picture and you decide. They do move, but often slowly, and they quietly munch their way through detritus and algae stuck on the sand of the seafloor.


Another healthy sea cucumber, perhaps even attractive? This one is called a leopardfish or rori kuru, likely Bohadschia argus. This photo was taken in Aitutaki's lagoon.

They do an important job, but are often a bit overlooked or mocked for their shape. So when I saw the disturbed rori, my first thought was that some cruel passersby had walked along the shore stepping on or hitting the rori. It is easy to step on them accidentally when going in or out of the lagoon, and they can react by eviscerating – basically putting their insides on the outside to try to confuse away whatever is hurting them. I could see some bully thinking that this was great fun and then going along trying to squish all of the ‘gross slugs’. [Rori are not slugs; sea slugs do exist in their own awesome right.]

Here’s what I saw:

The pink stuff that you see here is usually on the inside… Pretty though, eh? It's likely the rori's breathing apparatus. The white might be a defensive sticky thread. There are actually two species of rori in this picture, with at least 3 individuals.

I was getting all fired up about an awareness campaign when the pattern stopped making sense: it wasn’t all of the rori within a couple of steps from the shore. There were other rori that did not appear disturbed, and the disturbed ones were found in patches along a very long stretch of shore, with some visible in regions that would not be easily reached by a casual thug.

More eviscerated rori along the shoreline.

I’m not ruling out the thug possibility, but there might be a natural cause. There were rori further toward shore and nearby away from shore that were not eviscerated, so it didn’t look like a reaction to a local change like salinity or some sort of algal stimulus.

Sea cucumbers are able to turn out their respiratory organs and/or their viscera, perhaps to avoid or confuse a predator. …but a single predator was not likely to attack the number of rori that I saw disturbed (more than 20 in a stretch of ~30 m) all at once, I think.

Surveying rori in Aitutaki’s lagoon. All those dark blobs you see in the foreground are rori.

There is a lot we don’t know about sea cucumbers (here’s one paper), but there has been a suggestion that sea cucumbers eviscerate seasonally, as an extreme sort of a deep cleanse. Waste by-products or toxins might build up inside the simple digestive system, and the cucumber might just throw it all out and start again fresh. Perhaps that is what was happening here.

Do you know more about sea cucumbers and why they eviscerate in groups?

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