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Plaul, S. E., Pastor, R., Díaz, A. O. & Barbeito, C. G. 2016. Immunohistochemical and ultrastructural evidence of functional organization along the Corydoras paleatus intestine. Microsc. Res. Tech., 79, 140–148. doi: 10.1002/jemt.22614

Plaul et al. wrote: ABSTRACT
The Neotropical catfish,

Corydoras paleatus

(Callichthyidae) is a facultative air-breathing teleost that makes use of the caudal portion of the intestine as an accessory air-breathing organ. This portion is highly modified, being well vascularized with capillaries between epithelial cells, which makes it well suited for gas exchange. Instead, the cranial portion is a digestion and absorption site, as it has a typical intestinal epithelium with columnar cells arranged in a single row, villi and less vascularized tunica mucosa. Therefore, the intestine was studied by light and electron microscopy to assess differences between the cranial, middle and caudal portions. To characterize the potential for cell proliferation of this organ, we used anti-proliferating cell nuclear antigen antibody and anti-Na+K+-ATPase monoclonal antibody to detect the presence of Na+/K+ pump. In C. paleatus it was observed that cell dynamics showed a decreasing gradient of proliferation in cranio-caudal direction. Also, the intestine of this catfish is an important organ in ionoregulation: the basolateral Na+/K+ pump may have an active role, transporting Na+ out of the cell while helping to maintain the repose potential and to regulate cellular volume.

From the paper's discussion section: "this study shows that, along the phylogeny of some teleosts such as C. paleatus," ...

• "the intestine has acquired the role of an accessory respiratory organ, probably as a response to unfavorable environmental conditions, while maintaining the morphological characteristics for perform the absorption process."
• "the cranial portion of the intestine is the site of digestion and absorption, since its structure is the one typical of all vertebrates."
• [The middle and caudal portions of the intestine] "perform the function of a respiratory organ because their mucosa becomes thinner, villi are not observed, gas bubbles are present and cuboidal or squamous enterocytes are in close contact with the capillary network, which may even reach the lumen of the organ."
• "the caudal portion [has] morphological characteristics, such as the abundance of mitochondria and the complexity of the basolateral folds, which would be associated with osmoregulation, recirculation of water, and ion transport"

It is not news to me that the posterior intestine can be used as a accessory respiratory organ, but I was under the impression that fish were like aquatic reptiles and absorbed oxygen gas from water entering the cloaca (like an enema for breathing; see Why Do Some Turtles 'Breathe' Out of Their Butts?).

By contrast, this paper and the sources it cites state that the fish gulp air and pass the air through their gut all the way to the posterior end of their intestines, where gas exchange occurs and then they "fart" out the used air. That is not at all what I thought happens. I am curious about the transit time of such gulped air: Assuming that the air must either move WITH the digesting food in the gut (which could take minutes or hours to reach the posterior intestine) or move PASSED the digesting food (floating around and past the partially digested meal), how does this become metabolically efficient? If oxygen levels are low, how long can the fish wait for fresh air to reach their middle and posterior intestines? I suppose that as long as the fish is continuously gulping air, there will be a constant supply of oxygen available to the posterior intestine - in that case this seems very reasonable. But if the fish were to experience acute (sudden) hypoxia, I would imagine that the transit time for air to reach the intestine from the mouth might be insufficient.... Clearly there's a lot we don't know here, but it is fascinating.

I don't own paleatus, but I do own aeneus, which (according to the authors) is also capable of this. So I'm going to be looking for fish farts in the future.

Cheers, Eric

I don't own either of those species but have definitely seen my Corydoras fart on numerous occasions. After feeding, they dart up to the surface to take a breath, then return to the substrate and have a good fart (within a few seconds, rather than hours later).

I've seen peppered cories farting. Anyone that's ever kept weather loaches, as I did for a while, will have also seen these fart, I even stumbled upon a report online where mackerels farting, as far as I can remember, were confused with enemy subs being picked up by sonar equipment.

Now, I've seen bubbles rise, but I've always thought they were burps, not farts (released from the attic, not the basement, so to speak). I thought the fish gulped air and swallowed it into their swim bladders for gas exchange (although this gives me pause - do corys have swim bladders connected to the pharynx, which could be filled by gulping, or do they have isolated swim bladders with no patent duct system for air movement?), and then expelled it afterward like a person exhaling (which I would have referred to as "burping," although anatomically it would be exhaling and not burping in a technical sense if the air entered the swim bladder instead of the stomach).

Mol_PMB wrote:After feeding, they dart up to the surface to take a breath, then return to the substrate and have a good fart (within a few seconds, rather than hours later).

Exactly, with such a rapid gas release time, it's difficult for me to imagine that an air bubble passed from mouth to anus, moving passed all the food in the gut. That's why I thought the fish was exhaling/"burping" - such an action would not be impeded by food in the gut, nor need to move around such food. But again, apparently I stand corrected by the corys.

This paper is interesting and relevant:
Trade-off between digestion and respiration in two airbreathing callichthyid catfishes Holposternum littorale (Hancock) and Corydoras aeneus (Gill)
David I. Persaud Indar W. Ramnarine John B. R. Agard
Environ Biol Fish (2006) 76:159–165
DOI 10.1007/s10641-006-9019-2

Not seen my hoplos fart any air release appears to be a burp.

This is the abstract of the paper:

Abstract In callichthyid catfishes, the posterior intestine is modified to function as an air breathing organ by being air-filled, thin-walled and highly vascularized. These modifications make it unsuitable for digestive functions and digesta has to be transported quickly through this region to minimize disruption of vital respiratory functions. However, the weak muscles of the wall of the respiratory intestine make this problematic. It is hypothesized that the unidirectional ventilatory
air current within the respiratory intestine is responsible for the quick transport of digesta through the respiratory intestine. To verify this, movement of digesta through the alimentary tract was examined in Hoplosternum littorale and Corydoras aeneus that were either allowed to breathe air or prevented from air breathing. When air breathing was prevented, digesta was not transported to the rectum in H. littorale and there was a 94% reduction in the amount of digesta in the rectum of C. aeneus. This study suggests that the anterior digestive intestine facilitates the passage of air although it is filled with digesta. The anterior digestive intestine packages digesta into a string of slightly compressed boluses, creating an air channel in the digestive intestine thus allowing air to pass unimpeded.


I have downloaded the full paper but I don't think I can post it here. There are diagrams of how an air passage is maintained through the intestine.

Regards,
Paul

Thanks, Paul. I'll look that up.

Cheers, Eric

Somebody needs to do a "slo-mo" video of this on their 'phone - I'm very familiar with the "up to the surface, gulp, drop and bubble" behaviour in Corys (they do it in the wild and makes them possible to collect even in very muddy water as they tend to come to rest straight down from where they hit the water surface) but I can't say I've really looked.

Jools