Journey to the Back Passage of Holothuria

Image: Klaus Stiefel

We're talking anus. Sea Cucumber anus.

Few creatures have as fascinating and well-trafficked an anus as the Sea Cucumber. While most of us use our anuses to purge ourselves of nasty stuff we hope never to see again, Sea Cucumber anuses lead much richer, varied lives.

So let's don two or three pairs of rubber gloves and take a good look at the ins and outs! Maybe we'll wear some goggles, too.


THE INNIES

Video: ilmare77
This is extremely intimate. We must love each other very much

Oxygen
First things first: Sea Cucumbers breathe through their anus.

Between the end of their intestine and the actual anus is a muscular chamber called the cloaca.

Image: BIODIDAC

Attached to this chamber are a pair of branching structures known as the respiratory trees, which extend far into the body. They look something like the bronchi and bronchioles of our very own lungs, the point being to amass a large surface area in a small space. Water is drawn in through the anus and pumped into the respiratory trees, where oxygen diffuses across the thin walls and into the bodily fluids.

This requires the anus to maintain a constant, rhythmic expansion and contraction, like a more disgusting version of the gentle rise and fall of a sleeping kitten's chest. It also means that all gasps of joy and sighs of relief are WRONG. I sure hope they never sneeze!

Image: Ken-ichi Ueda
Parastichopus californicus

Food
It has been discovered that at least some Sea Cucumbers feed through their anus.

Breathing through your anus might seem like a good idea if you want to devote the front half your body to eating, eating, and nothing but eating. Sea Cucumber mouths do indeed have a much more monotonous existence than the other end. However, I guess with all that fresh seawater flowing in through the back passage the Sea Cucumber thought it was a waste not to filter out any food that might have come along with it.

Some scientists cottoned on to this possibility and decided to check it out. They picked up some Giant Californian Sea Cucumbers (Parastichopus californicus) and put them in a tank. Then they applied radioactive tracers to some algae and added it to the water. The radioactive tracers allowed them to find out where the algae was without having to tear open the Sea Cucumbers and look for it with their eyes.

Over the next 26 hours they checked those Giant Californians to see where the algae had gone. Turned out it was not only going into their respiratory trees, it was also getting absorbed by the tissue there! The algae was most abundant in a collection of blood vessels called the rete mirabile, which joins the gut to the respiratory trees. So it looks like food was going in through the anus, getting absorbed by the respiratory trees and then passing on to the gut via the rete mirabile.

They decided to call this "bipolar feeding". Sea Cucumbers still take in most of their food through the mouth, but it's nice to see that there are anuses out there who are willing to give something back.

Video: liquidguru

Crabs
Sea Cucumbers get crabs. Anal crabs.

A number of creatures can be found relaxing on the soft, spongy skin of Sea Cucumbers. Some of them at least occasionally approach the anus. And some of a particularly "adventurous" nature (to put it kindly) dive straight in! Perhaps they're gathering food? Maybe it's a cry for help? Or are they just enjoying a massage from the rhythmic expansions and contractions?

But they wouldn't go all the way in would they? Clambering about in the guts of a Sea Cucumber... What kind of reprobate would do that?

Video: OceansIQ

Pearlfish
Ah, yes. Anal fish.

Pearlfish are an assortment of species in the Carapidae family, found in tropical waters the world over. They're long and eel-like, with a translucent, scaleless body.

Some have the dignity and self respect to lead the free-swimming life, looking after themselves and seeking out their own food in this big, bad world. Others are commensal, living within clams and sea squirts but without causing their host any particular harm. They've been found inside oysters, which is how they got their name.

Image: Randall, J.E.
Onuxodon fowleri, 10 cm (4 in) long, lives in clams

Some are commensal with Sea Cucumbers. No smooth nacre or pearls here, they live in the cloaca and come out at night to forage for crustaceans and such to eat. Others are more of a homebody and prefer to eat in. Unfortunately there's no pizza delivery (would they like anchovies, I wonder?), so they just eat bits and pieces of their home, the Sea Cucumber.

These Pearlfish escape the cloaca by biting through the walls of the intestine or respiratory trees to enter the body cavity, which is the big, open space that contains all the internal organs bathed in coelomic fluid. Here the Pearlfish has access to all those organs and can nibble on them as they please. For some reason they have a particular fondness for Sea Cucumber gonad. Seems odd, but then quite a few humans have a fondness for sea urchin gonad, so I guess echinoderms just have delicious gonads.

Image: Alessandro Pagano
Carapus acus, 20 cm (8 in) long, lives in Sea Cucumber anus

Pearlfish usually enter the Sea Cucumber tail first. The Sea Cucumber can tightly shut the entrance but... they breath through that thing, remember? So eventually they have to open it up at least a bit, and the Pearlfish quickly fills the gap with its slender tail. The Sea Cucumber must open a little wider to breath and the Pearlfish inches its way a little further in. The process continues until the Pearlfish has enough room to disappear right up the pooper.

Image: Andrey Ryanskiy / Fishbase
Encheliophis gracilis, 30 cm (1 foot) long, likes to eat gonad

It must be quite spacious in there because some gonad munchers are a foot long! Like, for example, the so-called Graceful Pearlfish (Encheliophis gracilis). Yeah, "graceful". I suppose they have a very elegant way of shimmying down the intestinal tract.

Even worse, some Pearlfish cohabit! Male/female pairs hanging out in a Sea Cucumber. There's one for the advice columns. "I met this girl and we're getting on great but I have a problem: I live in an anus. When should I tell her?"

Eventually they turn that thing into a love nest and mate in the privacy of their anus, surrounded by internal organs and coelomic fluid. Sounds like a good time to evaluate the trajectory of your life. Be that as it may, tiny eggs soon hatch into larval Pearlfish that float around in the ocean for a while before they descend to the bottom of the sea and search for the bottom of a Sea Cucumber.

Some Sea Cucumbers have a way of keeping Pearlfish and other problematic loafers out:

Image: Klaus Stiefel

Anal dentata?


THE OUTIES

Image: Pierre Pouliquin

Innards
Sea Cucumbers spill their guts.

You'd think getting your gonads eaten by a fish would be a disastrous setback from which one could never return. Sure, it opens up career opportunities as a eunuch, but that seems cold comfort. Sea Cucumbers have an ace up their... up there: they can regrow their gonads. They can regrow quite a few internal organs, actually. This talent has inspired a certain laissez-faire attitude when it comes to the integrity of their physical body.

Few animals would consider tearing themselves open and throwing their guts in the face of an attacker. Even fewer would do it by firing it all out through their bum. What kind of disgusting, bottom-feeder would-

Video: NatGeoWild

oh, of course.

Attached to their left respiratory tree is a collection of fine tubes called the Cuvierian tubules. When attacked, some Sea Cucumbers contract their muscles so that the walls of their cloaca tear open, and the ends of the tubules erupt from the anus. Water from the respiratory tree is forced into the tubules so they greatly extend in length. They're sticky, so any crab or fish gets organs stuck to their face. And their toxic, so it's a faceful of sticky, toxic organs that came out of an anus. These are the kind of dramatic teaching aids you get in the University of Life.

The tubules become detached from the Sea Cucumber, who walks away to regrow them all in a few weeks. I knew it would be bad news if they sneezed!

Video: japflare

Faeces
Hmmm? Oh, yeah! Faeces! The stuff that was always supposed to come out of this thing. I never thought I'd be so glad to see it. The thing with Sea Cucumbers is their faeces isn't normal. No. It may in fact be some of the most important faeces in the world...

Sea Cucumber's don't so much eat their food off the floor as eat the actual floor. They bung huge quantities of sand and mud down their gob and digest whatever edible bits and pieces happen to be sprinkled among it. And if they end up consuming other people's faeces along the way then that's fine, too. It might even be a good thing, since it'll be covered in... "delicious" bacteria.

Image: Matthew Hoelscher

This complete lack of gastronomic sophistication means Sea Cucumber's have a huge effect on their local environment. Their feeding habit is really a sort of cleaning service. Detritus covered in bacteria and creepy-crawlies goes in one end, and from the other comes cylinders of sparkling, clean sand. This way they control nutrient levels and prevent the kind of overbearing algal and bacterial growth you get in eutrophication. The effects of eutrophication are at their worst in enclosed environments like lakes, but it can also be a problem in marine areas.

In coral reefs, Sea Cucumbers chomp on the sand and rubble which is in large part composed of bits of coral skeleton. That's calcium carbonate which has been weathered and eroded from the coral. Sea Cucumbers munch through it all, dissolve the calcium carbonate in their gut and release it into the sea through the other end. This makes it available to yet more corals, who can absorb it from the water and use it to construct their stony skeletons. It may even counteract the growing acidification of the sea, since calcium carbonate is alkaline.

Sea Cucumbers don't always use their anus to eject faeces but when they do, it changes the world.