The Missing Planimal

October 2, 2008 6:45 AM

Why are there no species that implement both halves of the photosynthesis/respiration cycle?

It seems like it would be handy for an animal to have a photosynthetic capability, either naturally or as a symbiont. You wouldn't have to find food or stop to eliminate waste (as often), improving workplace productivity. Your exhalations, if you still had any, would be closer to the ambient air mixture, making detection harder for those species that do that (I'm looking at you, mosquitoes).

You could also imagine it evolving from the plant end of the spectrum. In fact, it would be easier, since plants already do both halves of the cycle but the respiration half is muted and part time. Why don't they go whole hog and get double the advantage? (One could argue that plants don't use a lot of energy, so why do it, but that seems backwards to me. If they have the energy available, wouldn't a species that DID do something with it prosper?)

It seems like there are so many advantages and yet it hasn't evolved even a single time higher than lichen. Why not? The only thing I can think of is that having the advantages of both means having the disadvantages of both. But there's no disadvantage to being able, but not required, to photosynthesize. You don't have to stand still or have leaves. Just have some chloroplasts in your skin (also protects you from UV!) and if you happen to be standing in the sun, use them.

posted by DU to science & nature (45 comments total)

Thinking of a photosynthesizing dinosaur, I just realized that it could be related to area vs volume. A photosynthesizing species does best with high area and low volume to maximize photons received but minimize the overhead. But in order for a moving animal to have high area, it might need to have high volume.

Still, it seems like an elephant, say, that has a fairly high area and stands in the sun all day, would be so much better off also doing photosynthesis that there must be something preventing it from evolving.
posted by DU at 6:51 AM on October 2, 2008

While I can't think of any species that do both offhand, coral is a good example of symbiosis between the two. The coral polyp is the animal, and it contains a one-celled algae in its sac. The algae provides oxygen and some nutrients while the coral provides carbon dioxide and different nutrients. It's also one of the reasons corals can only live at a certain depth, as the algae need a certain minimum of light.

link
posted by Gneisskate at 7:07 AM on October 2, 2008

Another example along the lines of Gneisskate's is the sloth. They don't groom themselves and have hollow hairs that have evolved to be good for growing algae. Sloths don't really use the algae for doing much other than being green to blend in with their habitat, though.
posted by burnmp3s at 7:12 AM on October 2, 2008

I'm no biologist but at a guess I would suspect that plants use orders of magnitude less energy than animals, and therefore that the energy gain from an animal that photosynthesised would be negligible at best. But I might be wrong! :)
posted by katrielalex at 7:14 AM on October 2, 2008

wasn't this a bad guy on Darkwing Duck? *googles* ah yes, Bushroot
posted by jrishel at 7:15 AM on October 2, 2008

There are many, many features of life that would be different if organisms were, say, intelligently designed. For example, the fact that humans share an orifice for both breathing and eating has caused no small amount of grief. There are many ways you can answer this question but one is that animals and plants diverged from each other in evolutionary terms long ago so that animals lack the biochemical framework to evolve photosynthesis. In other words rather than being an extension of a twig on the tree of life, a new animal species that incorporated photosynthesis would have to jump to an entirely different branch on the other side of the tree.

As for the plant side of the equation, such organisms do exist; they are called photoautotrophs. They have not taken over the planet because, basically, every other organism eats them in one way or another.
posted by TedW at 7:18 AM on October 2, 2008

...the energy gain from an animal that photosynthesised would be negligible at best

I did think of that as well but dismissed it. Any improvement is an improvement, right? But maybe not. One would probably have to try some actual designs and see what the energy would work out as to see if it's really worth it.

...a new animal species that incorporated photosynthesis would have to jump to an entirely different branch on the other side of the tree...

What about symbiosis? If I was colonized by cyanobacteria that paid for their ride by excreting glucose into my blood.... Maybe the problem here is that the cyanobacteria don't get anything out of the deal. If anything, they are worse off, because when I die they are stranded. OTOH, my body could be carrying "poisonous" oxygen away from them more efficiently and supplying high levels of CO2.

I don't get the photoautotroph link.
posted by DU at 7:27 AM on October 2, 2008

But there's no disadvantage to being able, but not required, to photosynthesize.

This is a bit of a misconception. Having to expend the energy and genetic material to synthesize enzymes that aren't used is costly, even fatal over evolutionary time scales. This can be seen in antibiotic-resistant bacteria, which thrive in hospitals, farms, and other environments where antibiotics are common, but when transported outside of those environments tend to quickly lose that resistance because the metabolic cost of synthesizing the unnecessary enzymes for resistance confers an evolutionary disadvantage. (This is not entirely accurate because there are a wide range of mechanisms for antibiotic resistance out there and resistance can be coupled with other, more adaptive traits such as increased virulence as in the case of MRSA. As is often the case in biology, the answer to a seemingly simple question has very deep roots.)
posted by TedW at 7:34 AM on October 2, 2008 [3 favorites]

I didn't know that about either coral or sloths. The fact that sloths are so close and yet still don't do anything with it only increases my curiosity.
posted by DU at 7:34 AM on October 2, 2008

Having to expend the energy and genetic material to synthesize enzymes that aren't used is costly...

Well, I didn't mean over evolutionary time. I just meant that if I'm an photosynthesizing animal, I don't have to be standing in sunlight 12 hours a day. I could graze on the grassland on Monday, then hide in a cave on Tuesday.

How much of the time are beavers using their big teeth, for example? It's just a tool they have available because it's a net profit. However, you may be right that there is no net profit here.

But I keep coming back to symbiosis. I personally don't have to have the genetic material to make the photosynthesis happen. And plants already do both, but (seemingly) underutilize the respiration half.
posted by DU at 7:38 AM on October 2, 2008

A large portion of animals spend every waking hour eating vegetation and distributing their seeds/spores/pollen... Isn't that kind of like a symbiosis? Without one, the other would die.
posted by erpava at 7:38 AM on October 2, 2008

I don't get the photoautotroph link.

Most plants do exactly what you propose; they use photosynthesis to synthesize sugars from water and CO2, then those sugars become the substrate for ATP production (cellular respiration) and the synthesis of all the other biochemical components of life. In other words, cellular respiration is not muted or half time in plants, it is an essential part of their biochemistry. All life on earth depends on the ability of these sorts of organisms to convert inorganic compounds into organic compounds, so on a global scale, we are all symbiotic with the organisms at the bottom of the food chain.
posted by TedW at 7:45 AM on October 2, 2008 [2 favorites]

One reason is that, by biomass, plants don't produce as much oxygen as most animals consume. A large tree with a 39" trunk at breast height produces only .31kg of oxygen per day, while an adult human consumes .84kg (source). To give you a sense of how large a 39" diameter tree is, a 10" diameter balsam fir has a typical dry mass of 250kg. Your hypothetical organism would need a massive amount of photosynthesizing biomass to support a significant animal component.

Caveats: trees are not the most efficient producers of oxygen by mass, and the hypothetical organism could have an animal component with a very slow metabolism.
posted by jedicus at 7:46 AM on October 2, 2008

Well, I didn't mean over evolutionary time. I just meant that if I'm an photosynthesizing animal, I don't have to be standing in sunlight 12 hours a day.

The thing is, evolution is the mechanism by which an animal would get the ability to photosynthesize, so to answer your question it helps to think in evolutionary terms. In other words, if there were 2 members of a species identical other than the fact that one could photosynthesize but the other could not, it could very well be the case that the metabolic expenditure needed to produce the enzymes needed for photosynthesis would not be recouped and that individual would be smaller, slower, and perhaps more likely to be eaten. Better to just get your carbohydrates by eating some plants.

Another thought is that photosynthesis and respiration accomplish two different things; photosynthesis produced sugars, respiration produces energy in the form of ATP. All living creatures have cellular respiration be it aerobic or anerobic; some also photosynthesize sugars and starches. The organisms that don't photosynthesize get sugars and starches by consuming either directly or indirectly those that do. (There are also chemotrophs that get sustenance directly from oxidation-reduction reactions, such as the bacteria that live near deep-sea vents, but I am leaving them out for simplicity's sake. Many scientists think that they are the type of organism that got the ball rolling in the first place billions of years ago, though, so I don't want to leave them out entirely.)

I hope I am not coming across as too critical because this is a really interesting question and thinking about it takes me back to my days as a biochem major. As I mentioned above it brings up some profound facts about life on earth that a lot of people don't know.
posted by TedW at 8:06 AM on October 2, 2008

trees are not the most efficient producers of oxygen by mass

Not by far, since the main mass (wood) does no photosynthesizing. And in any case, a better measure of efficiency is surface area. If my skin were photosynthetic and I walked around naked, how much oxygen would I produce from that?

the hypothetical organism could have an animal component with a very slow metabolism.

Only if that were it's only source of oxygen, something I am not proposing.

In other words, cellular respiration is not muted or half time in plants

It must be, or animals wouldn't exist, unless I'm misunderstanding something. If plants are respirating as much as they are photosynthesizing, where does the free oxygen come from?
posted by DU at 8:14 AM on October 2, 2008

Well, we can estimate the surface area of a human as 2 m²

Energy yield from photosynthesis at the equator is about 10 W m^-2 (this is averaged over a solar day)
So 20 Joules per second for an adult human at the equator.

At 4000 joules to the kcal, you're looking at a single 'calorie' every three minutes or so.

Of course, whether the photosynthesis happens in our skin or in a symbiote, there is also an energy cost, so this figure bounds the energy on the high side.
posted by atrazine at 8:19 AM on October 2, 2008

Only half the surface area is going to be facing the sun, so let's call it 1 food calorie every minute, which also takes some other costs into account. I think by "averaged over a solar day" you must mean solar angles, but not night time, so that's 12 hours (at the equator).

12 hours is ~700 minutes. 700 food calories in a 2000 calorie diet isn't insignificant. Even if you reduced that by a factor of 3 or so for time spent in shade, etc you'd still be talking 10% of my diet.
posted by DU at 8:27 AM on October 2, 2008

Only half the surface area is going to be facing the sun, so let's call it 1 food calorie every minute, which also takes some other costs into account. I think by "averaged over a solar day" you must mean solar angles, but not night time, so that's 12 hours (at the equator).

No, that's average over 24 hours.
posted by atrazine at 8:45 AM on October 2, 2008

No, that's average over 24 hours.

Are you sure? The solar constant is about 1000W/m², so to get 10W/m² over 24 hours, even at the equator, you'd have to be 20% efficient. I thought photosynthesis was more like 8-10%.

But if you are right, then my point is even better. That'd be 1400 calories of my 2000 daily taken care of.

Wait a minute--one of us dropped a 0. (10 watts/sqm * 1 sqm * 86400 seconds/day) = 864000 joules/day. (864000 joules/day)/4000 joules/kcal ~= 200 kcal.
posted by DU at 8:52 AM on October 2, 2008

DU: Why are there no species that implement both halves of the photosynthesis/respiration cycle?

Plants. There are also some motile green algae running around.

The basic problem is that photosynthesis doesn't produce that much energy compared to that cosumed by multicellular heterotrophs. Which is why ecologists sometimes talk about a food pyramid rather than a food chain. Herbivores have to consume many times their mass in plant matter just to support their metabolism.

Also, as mass increases, the ratio of surface area to mass generally decreases. So while unicellular Chlamydomonas and colonial Volvox can support their motility through photosynthesis, just about anything with muscles won't have enough surface area to support it.

The point of photosynthesis isn't to produce energy, or oxygen. The point is to reduce inorganic carbon into organic carbon. The reason why plants have a net oxygen output is because some (much) of that carbon is converted into cellulose, amino acids, lipids, nucleic acids, and just about everything else that structurally makes a plant.
posted by KirkJobSluder at 8:54 AM on October 2, 2008 [1 favorite]

If plants are respirating as much as they are photosynthesizing, where does the free oxygen come from?

Oxygen is produced because the glucose produced by photosynthesis is not directly metabolized via the Krebs cycle etc. as an energy source. Much of it is stored for use when there is insufficient light, for seeds, to make structural components of plants such as cellulose, and so on. Also, photosynthesis and respiration are not simply opposites in chemical terms, although the simplified chemical equations often used to describe them make it look that way. This site gives a more in depth look at this concept.
posted by TedW at 9:00 AM on October 2, 2008

Or what KirkJobSluder said.
posted by TedW at 9:01 AM on October 2, 2008

And of course, this is ignoring the basic problem of descent with modification. Just because something theoretically looks good, a family of organisms is SOL if there are no traits that can be bootstrapped towards that feature.
posted by KirkJobSluder at 9:01 AM on October 2, 2008 [1 favorite]

The point of photosynthesis isn't to produce energy, or oxygen. The point is to reduce inorganic carbon into....cellulose, amino acids, lipids, nucleic acids....

Good point, but you left off glucose. (Well, I guess it's part of at least some of the above list.) Symbiotic cyanobacteria could inject glucose directly into my blood.
posted by DU at 9:03 AM on October 2, 2008

I feel like I'm answering the same objections over and over again. The answer to my question must be "nobody knows".
posted by DU at 9:05 AM on October 2, 2008

DU, no, you'd have to be about 1% efficient which I've seen quoted as an efficiency figure.
I calculated starting at total solar flux, calculated roughly the % of it that was between 400-700nm (photosynthetic photons) (this is the photosynthetic photon flux, substituting here for the more accurate but harder to use phytometric irradiance) and then took 8% as the % of photons incident that are used in photosynthesis (this is established as a good rule of thumb at high irradiances).

(and yeah, the answer is, nobody knows)
posted by atrazine at 9:52 AM on October 2, 2008

The answer to my question must be "nobody knows".

Either that, or you have some serious misconceptions about evolutionary biology, and biology in general.

First, there are lots of organisms that engage in both photosynthesis and respiration. They are called plants. They don't respire away all of the carbon they take in, because they need some of it to build their own structure, and to store for future use as fuel, either for themselves, or for their offspring, or for animals who will help them have sex or spread their offspring.

So let's change your question. It seems you really want to know why there aren't any large motile animals with photosynthesis, as your examples are all things like humans, elephants, and dinosaurs. It is wrong to say that these species are more highly evolved than lichens, coral, or Venus fly-traps, but they are more complex.

Let's assume, for the sake of argument, that you're right that it would necessarily be an advantage for large motile animals to have photosynthetic abilities. I don't think this is correct, chiefly for the reason TedW explains in this comment. The adage that "there ain't no such thing as a free lunch" is especially apt here. But it doesn't matter; let's assume you're right anyway.

The ultimate answer is that evolution does not work the way you seem to envision. As KirkJobSluder points out, evolution is descent with modification, and this means that the traits of today's organisms will not necessarily be at a global maximum for fitness. In the fitness landscape, the species may merely be at a local fitness maximum. But it may be hard or impossible to get to the supposed global maximum where the elephant has the ability to feed on light. Such a development would not happen overnight. You can't dismiss this objection because you didn't mean "over evolutionary time" because that is the only timescale in which such an adaptation could develop.

If you are correct about this being a great trick that nature missed somehow (and once more, I don't agree), then it is just another argument against the existence of an intelligent designer who creates optimally efficient creatures.
posted by grouse at 9:54 AM on October 2, 2008 [2 favorites]

Good point, but you left off glucose. (Well, I guess it's part of at least some of the above list.) Symbiotic cyanobacteria could inject glucose directly into my blood.

Well, there you run into some other problems which is that you don't get that much glucose without some efficient gas exchange. Oh, and photosynthesis is less efficient at human body temperature.

I feel like I'm answering the same objections over and over again. The answer to my question must be "nobody knows".

And I feel like your "answers" are simply the repitition of the same ignorant claims that don't really address the very good answers you've been given.

The modern-jackass estimates of what yield you could get from symbiotic cyanobacteria are way off. Incident solar energy on the surface of the Earth is about 250W/m^2 for a perpendicular surface. Cut that in half again because very little of a human body is ever perpendicular to the sun, so about 125W/m^2. The photosynthetic efficiency of mature forest stands is about 3%. So let's just say 4W/m^2 for an ecosystem with multiple layers. But humans don't have 425 million years of evolution developing fine-tuned systems for maximizing efficiency of both gas exchange, or multiple absorption layers, so 4W strikes me as extremely optimistic. For a single layer, lets try on the order of 0.8W (being excessively generous). Not all of which will be pumped out into your bloodstream. So, even with a very generous parameters, 17kcal.

Or about the energy one can get from a single bite of kidney, or a quarter of a modern apple.

Which gets to the whole problem of this scenario. Why dabble in marginal autotrophy when there are millions of calories waiting to be eaten?
posted by KirkJobSluder at 9:57 AM on October 2, 2008 [1 favorite]

The ultimate answer is that evolution does not work the way you seem to envision.

I am completely clear on the idea of a fitness landscape. Which is why I have repeatedly stated that my scheme does not require an animal to evolve photosynthesis. Please search this page for the word "symbiosis".

...you don't get that much glucose without some efficient gas exchange. Oh, and photosynthesis is less efficient at human body temperature.

OK, now we are talking. I'm not clear where the gas exchange has to occur (in the cell? outside the leaf?), but the temperature is an excellent point. What about a cold-blooded animal, a la my dinosaur (if dinosaurs were cold-blooded...). Those big fins are a tempting place to put a solar panel.

The modern-jackass estimates of what yield you could get from symbiotic cyanobacteria are way off. Incident solar energy on the surface of the Earth is about 250W/m^2 for a perpendicular surface.

Speaking of modern-jackass estimates, incident solar energy on a perpendicular surface on the Earth is actually 1000W/m². Maybe you are talking about the band of energies that is used in photosynthesis? What if we combine brown and green (and yellow)? Or perhaps you've averaged over a day (but it should still be higher than that)? I'd call your number correct with an order of magnitude, but even a 4- or 5-fold increase would bring this into the realm of the very-important-over-evolutionary-time scale. 100 kcal doesn't sound like much, but a 5% decrease in the amount of effort I need to spend looking for food does.

Of course, any energy estimate is going to require some knowledge about how much energy is required to support the photosynthetic apparatus. FOR A SYMBIOTIC RELATIONSHIP, it seems like it would be pretty low. An algal growth on/in my skin would require little or no energy input from me, since the whole point is that it's getting it's energy from the sun.
posted by DU at 10:27 AM on October 2, 2008

DU, do you really think that the cells of vertebrates are set up so that you could inject a cyanobacterium into, say a human egg cell, and voila, the offspring would magically have the ability to get energy from the sun? You really think that you wouldn't need any changes to either the way the host cell or the cyanobacterium would work? If so, then you are laboring under a misunderstanding of cellular biology. Our cells are not set up to take in the energy output of a foreign endosymbiont, and the endosymbiont is not set up to produce this output.

If you understand this, then the changes can only develop through evolution. Keep in mind that the endosymbiosis of chloroplasts and plant cells has evolved over a period of hundreds of millions of years.

There are additional practical problems as well—how does the endosymbiont get in the cell in the first place? How do you keep the host immune responses from destroying it, or the host cells from committing suicide? How do you keep the endosymbiont alive over twenty years before it has a chance to be exposed to light? Sure, there are ways to do all these things but they are going to (a) cost energy, and (b) require evolution to get there.

Since you are so clear on the idea of a fitness landscape, you should understand that even if there would be a net gain eventually, it will be hard to evolve in that direction, as the benefits will not come overnight. Especially when the first response of any existing amniote to such an intrusion would be to kill the potential endosymbiont, cutting off this evolutionary path before it even begins.
posted by grouse at 10:51 AM on October 2, 2008 [1 favorite]

DU, do you really think that the cells of vertebrates are set up so that you could inject a cyanobacterium into, say a human egg cell, and voila, the offspring would magically have the ability to get energy from the sun?

Uhhh....no, I don't. Instead of assuming I'm an idiot and concocting an impossible scheme to attribute to me, assume I'm a non-idiot and think of a better answer.
posted by DU at 11:00 AM on October 2, 2008

Oh and that's disregarding another basic point: I'm not saying we'd have to start with the world as it is today and add a photosynthesizing animal. This would have happened back in the mists of time, when "animals" first evolved and were presumably still equipped with at least partially functional photosynthetic biochemistry (or closely related to cells that did).

In this interpretation, my question could be phrased this way: Why did animal life evolve to not include photosynthesis?
posted by DU at 11:09 AM on October 2, 2008

I'm don't think you're an idiot, DU. Not at all. I just think you haven't thought this through very carefully, and there is much about biology that you do not understand yet.

You seem to think that there is an unidentified potential endosymbiont already out there and ready to be taken into vertebrate cells through some unidentified mechanism, which can then provide energy to the host cells through an unidentified mechanism, and be nurtured by the host cells through another unidentified mechanism. You seem to think that all these unidentified mechanisms will (a) result in a net energy benefit to the host, and (b) already exist such that no evolution will be necessary for this system to stabilize.

It is your burden to identify the endosymbiont and these mechanisms, if you want this scheme to be viewed as anything other than impossible. I think their existence is exceedingly unlikely. This is why this sort of adaptation could only be viewed through an evolutionary lens, yet you have rejected, twice, the necessity of evolution to the production of this sort of system.

On preview:

Why did animal life evolve to not include photosynthesis?

This is a good question. The ancestors of animals may have been phototrophs or chemotrophs. The parsimonious hypothesis would be that they were chemotrophs, and they have just been doing fine over more than billions of years ago, consisting first on the primordial chemical soup, and then eventually on the energy fixed by their autotrophic neighbors.

The alternate hypothesis is that they were phototrophs, and lost the ability to get energy from the sun at some point. They may have evolved such that they could get energy from the sun or from energy-bearing molecules, and at some point moved into an environment where there was no sun, and the photosynthetic ability atrophied. Ever since then they have managed to survive.
posted by grouse at 11:31 AM on October 2, 2008

Your planimal does exist. It's made up of the plants and animals in a local ecosystem. They're already symbiotic; they don't strongly compete with each other. The various problems pointed out above are solved by having the two halves of the cycle embodied in two completely different physical forms. One immobile with lots of surface area, numerous, covering basically all available land area. One mobile and compact. The cycle including both of the specialized forms is more efficient and out-competes any single organism trying to do both.
posted by madmethods at 11:32 AM on October 2, 2008 [1 favorite]

Why did animal life evolve to not include photosynthesis?

According to this page, the earliest animals evolved in the depths of the ocean where there was insufficient light for photosynthesis. Among their closest modern relatives are the corals, which as mentioned above have somewhat regained the ability through symbiosis. Other animals evolved to eat photosynthetic beings (plants) which gave them more flexibility in terms of being able to move, hide in caves or other places out of the sun, and have shapes that do not require a high surface area to volume ratio, among other things.
posted by TedW at 11:35 AM on October 2, 2008

DU: OK, now we are talking. I'm not clear where the gas exchange has to occur (in the cell? outside the leaf?), but the temperature is an excellent point. What about a cold-blooded animal, a la my dinosaur (if dinosaurs were cold-blooded...). Those big fins are a tempting place to put a solar panel.

Within the cell actually. And with dinosaurs, again you run into the problem that for animals, surface area increases do not keep pace with increases in size.

Speaking of modern-jackass estimates, incident solar energy on a perpendicular surface on the Earth is actually 1000W/m2.

The actual figure varies with the Sun angle at different times of year, according to the distance the sunlight travels through the air, and depending on the extent of atmospheric haze and cloud cover. Ignoring clouds, the average insolation for the Earth is approximately 250 watts per square meter (6 (kW·h/m²)/day), taking into account the lower radiation intensity in early morning and evening, and its near-absence at night.

I'd call your number correct with an order of magnitude, but even a 4- or 5-fold increase would bring this into the realm of the very-important-over-evolutionary-time scale. 100 kcal doesn't sound like much, but a 5% decrease in the amount of effort I need to spend looking for food does.

100 kcal strikes me as incredibly optimistic, demanding almost as good efficiency as one would get from a mature ecosystem of trees.

And now that I think about it, autotroph-heterotroph symbiosis has certainly evolved in organisms other than lichen. Corals and vent tube worms are two examples. But both of these have specialized organs to support their autotrophs.

Which is what I suspect would be needed for this kind of a scheme to work. Remember that your skin is one of the most important and critical organs in your body. It is a sensory organ. It's pigmented to reduce the harmful effects of solar radiation. It contributes to the regulation of body temperature. The development of an entirely new layer of skin to accomodate cyanobacteria, tuning the immune system to avoid overgrowth and opportunistic infection, strikes me as a very expensive and risky prospect.

Why bother when you can incrementally change the metatarsals, or the angle of the thumb, or a lobe of the brain to get an edge?

In this interpretation, my question could be phrased this way: Why did animal life evolve to not include photosynthesis?

Well, it did in a few cases.

But if you mean "animal" as in "large multi-cellular beasts" the primary factor is that there just isn't that much energy there. And as TedW points out, not being chained to a sessile lifestyle of basking in light for tiny trickle of glucose and ATP allowed animals to expand into new environments.
posted by KirkJobSluder at 11:42 AM on October 2, 2008

You seem to think that there is an unidentified potential endosymbiont already out there and ready to be taken into vertebrate cells through some unidentified mechanism...

Wrong on at least 3 counts:

1) Endo
2) Already out there
3) Vertebrate

100 kcal strikes me as incredibly optimistic

OK, so let's talk about a smaller animal than humans. A quadruped will have more surface area directed skyward anyway. Maybe something like a turtle, which has a nice big, sky-facing shell, is cold-blooded and doesn't require a ton of energy to begin with. The sunlit-area-to-metabolism ratio is much more favorable here.

...again you run into the problem that for animals, surface area increases do not keep pace with increases in size. and being chained to a sessile lifestyle

I again reiterate that I'm not talking about a completely solar-powered animal. Plus I specifically mentioned the "fins" like on a Dimetrodon. The fin is facing the wrong direction, but if you imagine a turtle/flying-squirrel hybrid you get the idea.

Those who are saying how impossible this is should perhaps cast an eye over the working examples already given. The question is no longer "why don't these species exist" but "why aren't there more of these species".
posted by DU at 12:17 PM on October 2, 2008

Wrong on at least 3 counts:

1) Endo
2) Already out there
3) Vertebrate

OK, DU. If the symbiont does not already exist, then it would have to evolve. Cue evolutionary considerations, yet again.

Non-vertebrate examples already exist. You asked why something doesn't exist, when it already exists. I thought the charitable approach was to change your question so that it does not contain an incorrect premise. One that fits all the examples you used at that point, which were vertebrates. While you now acknowledge that this premise was incorrect, you would prefer to change the question in a different way (and abandon the examples about vertebrates): "why aren't there more of these species?"

The answer is the same as the answer to any question of why a particular adaptation does not exist in a species. Here the species you are asking about are all of the species that do not have this adaptation. There are several possibilities.

The first possibility, again, is that the "adaptation" actually decreases fitness rather than increasing it. I actually think this may be the case here. I am certainly unconvinced that this adaptation, if it provided any net benefit, would provide a benefit sufficient to exhibit selective pressure. If you assume for the sake of argument that the adaptation would improve fitness significantly, then you still have to get there somehow. I think you now acknowledge that such a thing would not happen overnight. And it seems that in the vast majority of cases, any of the necessary preliminary steps towards autotrophic ability either provided a selective disadvantage, or failed to provide a selective advantage sufficient to overcome genetic drift or other mechanisms of evolution. The other alternative is that they did not happen at all—that some of the necessary events might have been much more improbable than a mutation.
posted by grouse at 1:02 PM on October 2, 2008

Maybe there are better alternatives to food shortage?

I think we've established that photosynthesis can only contribute to a small proportion of total energy intake for an animal.

So, it's only an advantage if an animal can't get enough food to maintain its body adequately.

If so, there might be easier routes for an animal adapting to a food shortage. Dwarfism for instance. There are usually different alleles allowing for size differences within a population, so shrinking and growing can happen very quickly on an evolutionary scale. If food starts to get short, it might be more efficient for an animal to start shrinking than to start photosynthesizing.

This might also relate with the local-maxima thing. Suppose it would take 10,000 generations to evolve photosynthesis, but only 100 generations to shrink to a smaller size. You'd never get the 10,000 generation solution to the problem, since by then the animal would have adapted in size to the food shortage, and the selection pressures would be for something else entirely.
posted by TheophileEscargot at 2:03 PM on October 2, 2008 [1 favorite]

I don't have much to add, but I wanted to say that this was basically the take-home final exam for my biochemistry class. We had to determine if it would be possible to genetically engineer humans to be photosynthetic. The short answer is no (we don't have the surface area to make all the calories we need by photosynthesis).

I loved that class.
posted by jenne at 8:44 PM on October 2, 2008

There are species of slugs that can retain chloroplasts after they ingest them and they do photosynthesize: ie they have cells in their skin with chloroplasts that photosynthesize and the animals use the cells for energy. Most are sea slugs but I'm pretty sure there is a terrestrial species too.

The phrase you're looking for in terms of how this comes about is probably "adopting a genome" btw.
posted by fshgrl at 10:16 PM on October 2, 2008 [1 favorite]

Also, as KirkJobSluder says, photosynthesis might interfere with the current function of the skin. Most animals seem to find fur, scales or feathers useful for protection or insulation. Those might be more useful than photosynthesis, and it might not be possible to combine those functions.

Even with elephants and rhinos, they seem to a have thick skin which presumably has a protective layer of dead cells.
posted by TheophileEscargot at 11:29 PM on October 2, 2008

So, it's only an advantage if an animal can't get enough food to maintain its body adequately.

You may be on to something with your thinking about other, less major, solutions to an energy problem are probably closer in fitness-space. If there were a famine in the land, then it probably does make sense for animals to just be smaller. A decrease in size is more immediately useful than the first glimmerings of some complex biochemical addition.

Although what about the sloths? If their food sources started dwindling, wouldn't it make sense to somehow tap into the algae already living in their fur? But I suppose that's still harder than just not growing as large. Alternatively, it might make sense for the algae to "help" their host by injecting them with glucose.

Also, evolution can walk and chew gum at the same time. Just because the sloth is growing smaller doesn't mean it can't also send out feelers to the photosynthetic world, as long as those feelers are themselves advantageous. Which may not be the case, especially if it is trying to evolve from scratch (as grouse continues to assume I am restricted to).

There are species of slugs that can retain chloroplasts after they ingest them and they do photosynthesize...

Whoa. Pictures and more information (separate species and it looks like the former are exosymbiotic while the latter is endo).

Of particular note: ...the dark green sea slug can be sustained in culture solely by photoautotrophic COz fixation for at least 9 months if provided with only light and a source of COZ

The slugs in the picture seem transparent, other than the chloroplasts, so there's your multiple layers. So if you imagine my turtle/flying-squirrel hybrid as being (at least semi-) transparent, you could make this work as a significant diet source. But wait...can a terrestrial animal be transparent? Why are only sea creatures like that? UV protection? Is that mooted by having chloroplasts to absorb the energy? Maybe you could have a regular, opaque animal with a gel coating on its back full of photosynthetic goo.
posted by DU at 5:41 AM on October 3, 2008

I was just coming to post about those sea slugs; really interesting stuff that I hadn't known about before. As you mention, DU, the fact that there are photosynthetic animals out there just leads to more questions. That is why science is so fascinating to me. As to why there are so few photosynthetic animals and they are all sea dwellers, I would suspect the need to be small, have a large surface area, and be sort of transparent all point to a marine environment.
posted by TedW at 6:29 AM on October 3, 2008

This morning I realized a way to ask this question that might lead to fewer objections from the "photosynthesis doesn't provide much energy to an animal" standpoint.

Why don't plants exploit respiration more? There is indubitably an enormous amount of energy available in the free O₂ in the atmosphere. And plants can already use it. Why not more? Why didn't plants latch onto this mechanism and prevent animals from ever even evolving?

Not expecting an answer, I just wanted to document the question.
posted by DU at 5:03 AM on November 3, 2008