This method of alcohol delivery is, from a certain very twisted point of view, quite clever. The gut is a tube. This means that regardless of which orifice alcohol enters your gut,you can still uptake the alcohol into your system and enjoy the intoxicating effects.
Now, a new paper from Jaeckle and Strathmann looks whether sea cucumbers might be able to ingest food via the back door. It turns out that sea cucumbers have an odd system for taking up oxygen. They have respiratory trees that connect to the anus, shown below:
Consequently, the anus isn’t just an exit point for undigested waste in sea cucumbers. There is much more activity than simple expulsion, with water being moved to and fro to allow for respiration. Given this, there would seem to be much more opportunity for nutrition to enter the digestive system through the anus.
To test this, they performed two experiments. In the first, they put giant California sea cucumbers (Parastichopus californicus) in tanks with algae that contained a heavy carbon isotope (14C). Let’s look at Figure 2.
What the authors are trying to show is that the closer you are to the respiratory tree, the proposed entry for the food, the more of the heavy carbon isotope you find. After 24 hours, though, the level is nearly as high in the digestive system as the respiratory tree. But a close look at the figure starts to see some significant shortcomings.
First, the more disturbing thing about this graph is the error bars. There aren’t any. There is no indication of sample size anywhere in the paper. There are no statistical analyses, either. I suspect that each data point was pulled from one individual. If so, that is a huge problem that makes it difficult to conclude much of anything.
Then, notice the time collection intervals. You get a lot of measurements in the first 8 hours, then nothing until 25 hours. I am betting the #OverlyHonestMethods version of this would read, “We did most of the work in one day, went home to eat and sleep, and came back the next morning.” That day long cycle might matter for the results, depending on the behaviour of the sea cucumbers. Are they nocturnal? Do they feed differently in the day that the night? Those sort of questions are not answered. They also don’t appear to take any steps to try controlling for ingestion through the mouth.
In the second experiment, they exposed the animals to large molecules containing a lot of iron. The iron allowed them to stain it later to see if it had been taken into the tissues.
I don’t like this experiment as much as the first one, because it seems unlikely that sea cucumbers are hanging out in regions that are rich in large nutritional molecules. The previous experiment seems to better represent actual ingestion of food in cells: the sort that a sea cucumber might encounter in the wild.
Again, there are some shortcomings in how they present their results. In their figures 4 and 5, Jaeckle and Strathmann show that there are bits of respiratory tree tissue that have stained blur for iron, indicating that those molecules were incorporated into the animal. But... the authors show only the positive experimental stains. While they say in the text:
In the respiratory trees of control animals, there was no equivalent presence of the blue reaction product.
It would be much better if they also showed the negative control in the pictures.
And another weird thing that you can see in the acknowledgements is that this work was done in 1996. While this is not a record for delay between the experiment and publication, over 15 years in the waiting has to be in the top one percent of waits.
The authors note that even if the sea cucumbers were able to retain all the food in the water, the amount of food they would be getting would probably not be that large. The authors do raise the possibility that this unusual way of feeding might allow sea cucumbers to get different kinds of food than they would get by ingesting it through the mouth. But this is speculative, and it seems likely that anal feeding would contribute at best only a small amount of the animal’s nutrition.
This paper makes a plausible case for this sea cucumber species to be able to get some nutrition via the anus, but it is very limited in what you can conclude from it.
P.S.—And if all that didn’t make sea cucumber anus remarkable enough, it can also provides a home for fish.
Jaeckle WB, Strathmann RR. 2013. The anus as a second mouth: anal suspension feeding by an oral deposit-feeding sea cucumber. Invertebrate Biology: in press. DOI: 10.1111/ivb.12009
Giant California sea cucumber photo by Ken-ichi; sea cucumber diagram from here.