Could this filter feeder have a hybrid lung-stomach organ?

Question

The creature is a small reptilian creature, about 15cm long. It feeds on aeroplankton, which in its environment are very plentiful. Other than that, its environment is essentially the same as Earth's tropical rainforests. The creature itself has similar requirements to the reptiles of Earth

It has no stomach, intestine, or other digestive organs, instead having enlarged lungs. These lungs would be used both for gas exchange and for feeding, with the lung-tubes being adapted into something like a combination of an alveolus and a microanimal stomach

Could such an animal realistically survive, or would this lung structure not be able to supply the creature with enough food/oxygen?

Answer 1

Yes, this can work... if the aeroplankton is thick enough and nutritious enough, and the creature has some way of absorbing sufficient water. Living near waterfalls or in a humid environment would probably suffice. For aeroplankton to exist, high humidity is also likely.

As the OP says, this would require lungs that serve two purposes, those being gas exchange and nutrient collection.

I think that most people would be sufficiently familiar with the respiratory function of lungs that I don't need to go into too many details. That just leaves the matter of nutrient collection.

To understand how this creature would feed, we must first understand what it feeds upon. Aeroplankton would seem to be microscopic organisms light enough to float in the air and live there, being either phytoaeroplankton photosynthesising for their living, or zooaeroplankton feeding upon the floating microscopic plants. Either way, these microscopic organisms would need to be small, lightly built, and have a large surface area in order to be able to float in the air on the air currents. This would make them quite flimsy - even the plants - so they wouldn't be as tough as organisms that rely upon their exteriors to protect them at the expense of increased weight or density. They would likely reproduce very rapidly in order to compensate for the likelihood that many will die from the many hazards in their environment.

So, we have either single-celled or tiny multicellular organisms that float in the air. To be able to survive, they must exist within a certain size range dictated by the air pressure and gravity, not too heavy to float, yet not too light to sacrifice efficiency. That they will occur in a limited range of sizes (whatever that range may be in their world's conditions) is an important factor in this animal's strategy.

It may not be known to many people that aerosol medication can be targeted to affect a particular part of a patient's respiratory system... but what does this have to do with this filter-feeding creature? The mechanism of this selectivity is the droplet size of the aerosol. The smaller the droplets, the further into the respiratory system they will travel before colliding with the wall of the respiratory system.

So, knowing the size of the aeroplankton, we can work out how far into the creature's lungs they will travel, and the size of the air passages at which a collision will most likely occur, and likewise, the creature can evolve to maximise the surface area of its breathing passages at the point and passage size at which the aeroplankton will most likely impact.

Higher up in the lungs, where the passages are larger, and only a few aeroplankton are likely to impact the walls, the walls may be dry and not sticky, to allow the aeroplankton to bounce off and continue further into the lungs.

At the point in the creature's lungs where most aeroplankton will impact, the walls would be coated with a layer of mucus designed to trap any aeroplankton that touches it. Beneath that mucus would be cells evolved to devour the trapped aeroplankton and digest them, along with cells to produce the mucus.

Beyond the point at which the aeroplankton is trapped and digested, the narrowing passages would carry air - now free of aeroplankton - to the respiratory part of the lungs, where gas exchange would occur.

So... one "organ", the "lungs" would serve the purpose of both respiration and digestion.

As to how this might evolve, consider the consequences of not filtering out the aeroplankton. Any creature which lives in an aeroplankton-rich environment would risk respiratory infections from a failure to filter out the tiny organisms. Some of those creatures which could perform such filtration may have done so by digesting the trapped aeroplankton in-situ. With sufficient aeroplankton trapped, the creature's metabolic requirements may have been partially satisfied, and as they became able to trap more aeroplankton, its metabolic requirements would have been fully satisfied, leading to the degeneration of the normal, unnecessary digestive organs.

I should point out that such a lifestyle would involve mainly sitting still and breathing, and would suit a low-metabolism, cold-blooded creature. It may well be camouflaged and/or contain toxins to hide from - and/or discourage -predators. Having to move to escape a predator would take a lot of energy that would take a long time to replace given its feeding strategy.

Answer 2

I don't think it can work.

You are assigning the same structure to two different and conflicting use cases:

• gas exchange requires an environment where gases from the atmosphere can be quickly exchanged with those contained in the blood stream
• nutrient exchange requires an environment where a nutrient rich liquid solution can exchange substances with the blood stream.

While the gas mixture can be brought in contact with the gas exchange almost immediately, basically requiring only some filtration step to remove suspended particles, the liquid solution cannot be simply obtained by soaking the food in water. You need substances to break down the macromolecules into smaller components which can pass through the blood barrier, in a process called digestion.

Answer 3

Perhaps, but you'll need to change the workings of your lungs a bit.

This creature, if I were to design it, would indeed have large lungs, most likely body-length, but the lungs would have separated lung chambers and would require that breathing be a one-way air passage through the creature's body, with the first chamber taking in fresh air and being the primary respiration chamber with minimal nutrient absorption ability, which will pass on its air to the second chamber which is slightly less adapted to respiration and slightly more adapted to nutrient absorption, and so it will go on from chamber to chamber until the last one has the most absorption ability but little to no respiration ability, which will then finally pass on the by now useless air to what is essentially a colon with waste storage and disposal properties.

With every breath this creature takes air will pass from one chamber to the other, and with every breath it takes it will expel the waste air, meaning that it'll be tooting almost constantly, but hey, its primary method of survival is breathing so... yeah.

Answer 4

Sponges and Corals:

PROBLEM: This is implausible but not technically impossible. There would be a very high threshold on a reptile to evolve the structures you would need, and this is the biggest reason it wouldn't work. Bacteria, fungi, viruses and the like would be constantly attacking the lungs. Gas exchange would be relatively inefficient, as would feeding.

SOLUTION: The likely path for your organism is to degenerate from a motile reptile to a low- non-motile filter-feeder organism. On land, this doesn't happen, because generally food doesn't just drift by (okay, a few times a year species will have spores, seeds, or reproductive swarms). Even herbivores need to walk about to eat. If it had periods of anoxia (like after inhaling too much food) it would need to suffer little long-term harm, so a brain would be best small or non-existent. Low motility also reduces the requirement for oxygen by not burning calories.

ENVIRONMENTAL PRESSURES: In this environment, these organisms will likely move as little as possible, and get small. They may live collectively, but small size means they can get up in the winds near the tops of trees to maximize feeding and stay away from predators. They will develop spines, poisons, and possibly shells. They may form symbiotic relationships with trees that grow protective niches for these organisms to live in, probably trap water or have sap drained for liquid, and in exchange the plants get all that yummy waste from digestion.

MECHANICS: There would need to be AT LEAST excretory organs, which would likely be the vestiges of the digestive system so metabolic waste products could be disposed of. They would be exquisitely vulnerable to disease, since lung structures are in direct contact with the particulate of the aeroplankton. An extremely robust system of macrophages would likely evolve to clear this matter and fight infection. The immune system would need to be massive and put buzzards to shame. I might suggest a double-filtration (a bit like a vacuum) where an initial screen (aerovilli) trapped the particulates and fed them to tiny digestive sacks. and a subsequent screen (alveoli) exchanged gasses. This would, admittedly, be more like a digestive system, but if such a filtration system in the lungs evolved first, then became a digestion system, it would lower the evolutionary threshold and make the whole thing a bit more plausible.

ALTERNATIVES:

• An alternative approach is to still have a digestive system but filter all those particulates out more like a whale before the particulates reach the lungs. In an environment full of particulate, this would evolve from a protective structure screening material from getting into the lungs.
• OR your organism could use some kind of net-like structure (easily evolved) to catch food, then clean the food off the nets like a spider. The net could be built (like the spider) or part of the body (like a frill or network of feather-like structures).