Friday 8 May 2020

It's a plant! It's an animal! It's a bitroph!

Click to enlarge; Source: wikipedia

Several years ago, a species of sea slug had its day of fame on internet sites specialising in scientific news. Those sites all showed a bright green flattened blob. like the image above. This sea slug was green because it performed photosynthesis, which animals are generally not supposed to do.

I guess everyone interested in speculative biology sat up straight, because a lifeform that is part animal and part plant exudes ‘alienness’ through every pore. But was the flow of alienness coming out of those pores accompanied by oxygen, as in plants, or by carbon dioxide, something more befitting an animal? 

The slugs of the genus Elysia get their photosynthetic ability by feeding on algae. Algae, as the well-informed readers of this blog will know, perform photosynthesis in intracellular organelles called chloroplasts. The slugs eat the algae, but rather than simply digesting the chloroplasts too, they envelop then through phagocytosis, and keep them alive, in their own bodies. From then on the chloroplasts are called ‘kleptoplasts’, or ‘stolen plasts’.

It turns out that the photosynthetic slugs can live quite well in the dark, so they do not critically rely on photosynthesis. They do use photosynthesis as an auxiliary power source, mostly when they are starved anyway. When the slugs are kept in the dark AND starved, the number of kleptoplasts decreases, so the slugs then apparently disassemble the then useless chloroplasts and get a final energy boost from the hapless organelles (Cartaxana et al  2017).

Plant-animal combinations are not novel in speculative biology. Actually, there is a group of creatures  on Furaha called, for the time being, ‘mixomorphs’. They probably share characteristics with plants as well as with animals. The ‘probably’ is in there because I always had the uneasy feeling that a plant-animal combination might not work. After all, Earth is not filled with such creatures, doing whatever it is ‘plantanimals’ do when they are not just sitting in the sun. Does their absence mean that they do not make sense?

The concept of animals performing their own photosynthesis certainly sounds like a good idea. Earth plants take in carbon dioxide (CO2), water (H2O) and sunlight and turn them into carbohydrates. Because they turn nonbiological material into carbohydrates, they are called ‘autotroph’. Animals cannot do that and require some ready-made carbohydrates as a source of carbon, making them ‘heterotroph’. By breaking up those carbohydrates animals get materials for their own bodies, producing H2O, CO2 and energy. An animal is a plant in metabolic reverse, in a way.

Why not do what the slug does and cut out the middle man? This plant-animal chimaera could use photosynthesis as an auxiliary and cheap way to store free energy in carbohydrates, giving it an edge over animals that have to hunt, chew and digest to get any carbohydrates. They would even have an edge over plants in that a major problem with photosynthesis for plants is that there is so little CO2 in the air. The animal part of a chimaera would produce more then enough CO2 to boost photosynthesis of the plant part.

Click to enlarge; source: wikipedia

Autotroph + heterotroph = bitroph
There is a nice scheme on Wikipedia explaining the full nomenclature of how lifeforms get energy and carbohydrates. There are three big two-by-two divisions, shown above. These result in six fragments of phrases: hetero- vs. auto-, chemo- vs. photo-, and organo- vs. litho-. There are eight possible combinations. Our garden-variety plants (sorry for that pun...) are ‘photo-litho-auto-troph’, while ordinary animals are ‘chemo-organo-hetero-troph’.

This nice scheme seems to cover all the possibilities, creating a challenge for speculative biology lovers: where should we classify animals that can photosynthesise? Note that there already are lifeforms that cannot build their own carbohydrates and yet use photosynthesis: photo-litho- and photo-organo-heterotrophs. However, they are all bacteria, and to increase the ‘alienness’ level we want creatures we can see without a microscope, and that we can stroke, or supply with compost. Or both. Also, as these creatures would run both energy pathways, they do not fit in the scheme. They might be labelled ‘autoheterotroph’; I can't say I much like the term ‘plantanimal’. Let’s introduce ‘bitroph’ to emphasize the dual energy principle (without also adding 'photo-organo-litho-chemo-').            

Bitrophy in practice

'Bitrophism' needs consideration of energy requirements. The first question is how much energy you get from a leaf, or a standardised area performing photosynthesis.  Luckily, that information was already available on my bookshelf, in ‘Energy for animal life’ by the late R. McNeill Alexander (if you want to give your speculative biology a scientific edge, get his books). 
In bright sunlight the flux of light on the surface of the Earths is about 1000 Watt per square meter, and with that light intensity the rate of photosynthesis reaches a maximum of 21 Watt per square meter. This ratio of 21 to 1000 shows, again, how inefficient photosynthesis is. Mind you, this light flux is the maximum value in Alexander's biome, which was England. Just outside the atmosphere you get 1370 Watt per square meter. Obviously, seasons, clouds, latitude, and the time of day all influence the amount of sunlight the surface actually gets. For now, let’s go with that value of 21 Watt per square meter.

The next question is how much energy an animal actually needs. That also depends on many things, such as its activity, but it's minimum level is largely fixed: the ‘minimal metabolic rate’ describes the energy requirement of an animal doing nothing, except being alive. This rate depends on two factors.

The first is the type of animal: warm-blooded animals such as birds and mammals burn energy at much higher rates than other groups, such as lizards, fishes, etc. For two animals that have the same mass, a mammal uses almost 5 times the energy of a lizard (even one warmed up to 37 °C), and 12 times the energy of a crustacean at 20 °C.

The second factor is mass: a 100 kg animal will use more energy than a 10 kg one. However, it needs less than 10 times as much. As Alexander remarked: ”Weight for weight, it is a great deal cheaper to feed elephants than mice.”  The relationship between minimal metabolic rate (MMR) is an exponential one, and has the form

MMR = a (body mass) ^ b

(formatting is difficult here; the '^b' part means 'to the power of b'

The exponent ‘b’ differs somewhat between animal groups, but lies close to 0.75. The fact that it is less than 1 explains why large animals have a lower metabolic rate per kg than small ones. The factor ‘a’ is the one that differs between animal groups (it is 3.3. for mammals, 0.68 for warm lizards, and 027 for crustaceans.

Click to enlarge; copyright Gert van Dijk
The image above provides the Minimal Metabolic Rate the rate for mammals, (warm) lizards and crustaceans, all ranging from 0.1 to 1 kg. The crustaceans burn the least energy, and bigger animals need more energy than small ones.

But we wanted to get to photosynthesis; remember that one square meter of photosynthetic area provides 21 Watts, so I provided an additional y-axis on the right, which is simply the left y-axis divided by 21. The right one tells you how many square meters of photosynthetic area we need for each point on the graph. A 1 kg mammal will need about 0.16 square meters of ‘leaf’. That corresponds to a square with sides of 40 cm. Examples of 1 kg mammals are seven-banded armadillos, muskrat, pine martens, platypuses, meerkats and European hedgehogs. Just picture one of those them with a 40 cm by 40 cm parasol to catch sunlight. A large fruit-eating bat may also have a mass of 1 kg; it needs a large wing area anyway; hmmm...

Anyway, as I found it difficult to imagine how large that actually is, I assembled a mock animal with a mass of 1 kg (the volume can be calculated because the animal consists of spheres and cylinders; its density is 1.05). I used mammal characteristics to calculate the disc it needs to provide the energy for its MMR.

Click to enlarge; copyright Gert van Dijk

The image above shows such a 'Disneius solamor'. The small squares on the ground are 1x1 cm, and the larger ones 5x5 cm. The animal is 21 cm long, and the radius of its dark green 'sun disc' ('antenna'? 'leaf'?) is 22 cm. It needs that to power its MMR. A general human provides additional scale. Hm; the animal does not look very elegant, and that large 'leaf' looks rather vulnerable.

But we are not done yet. The calculations so far used maximum light settings, which is not realistic. And how about the effect of mass? How about animals that are thriftier with energy than mammals? How about more efficient photosynthesis? I suspect that this post may already have passed the 'maximum allowed complexity per unit of enjoyment ratio' (MACPUOER), so I will stop here. But I will very likely return to this theme.

PS. Although I welcome the large number of questions the blog has recently received, many had nothing to do with the post under which they were asked, and many could easily have been answered by using the blog's search options. So from now on I will be less likely to answer such questions.  Surely you would prefer me to spend my time working on The Book or on writing posts?


Anonymous said...

The first thing that came to mind were the garden worms from "The Future is Wild", which were described as living in dark caves underground and only basking in the surface to sun themselves. Given that slugs in darkness digested their chloroplasts I guess this isn't a particularly feasible lifestyle for a photosynthetic animal?

was said...

i look into this option for classical trope: The Green skin humanoid from Mars or outer space. one idea i had was to use the chloroplast as UV protection. on other side as carbohydrates production for animal muscle, while the plant part get the produced CO2. but this let to too complex biochemistry, next to that is the fan service of scary clothing Green female humanoid noew make more sense she need sunlight...

tribbetherium said...

How about an animal resembling the gliding Draco lizard, which has foldable sails supported by ribs that act as makeshift wings? Perhaps a photosynthetic creature could have a similar adaptation, opening its sails to bask in the sunlight and folding them away when not in use.

Sigmund Nastrazzurro said...

Anonymous: I had forgotten about those. Remember that the MMR of crustaceans is about 12 times lower than that of mammals. That translates to an also 12 times smaller 'leaf' to power the MMR, which is at the very least less ridiculously large than the mammalian I showed. I will show examples of such effects in the next post on this subject.

Was: Ah yes, Star Trek's green women ('Orions') and little green men in general. The green girl in star Trek indeed was not covered much. It's a funny idea to consider photosynthesis as the reason for that.

Tribbetherium: exactly! Such organs should preferably be foldable. But whether they are derived from ribs or anything else, is completely open. The 'foldable' idea is what made me pause at bats: their wings provide a large foldable surface anyway.

Kwalla Bird said...

How about a plant-animal symbiont that has an animal playing host to plants that provide it nutrients from photosynthesis, and in return the animal carries the plant to new sources of light and water? the question would be how such a symbiosis would arise in the first place, maybe a buffed-up version of "algae growing on sloth fur" thing? Would also be interestinf if the plant could also produce toxic, distasteful substances to protect both itself and the host

TheWingedScourge said...

Interesting! It does bring up the idea on whether "mobile, autotroph" makes much sense given that most large lifeforms on earth are sessile autotrophs or mobile heterotrophs.

Perhaps they are only mobile when immature, moving only to disperse and then permanently root themselves in place at maturity (and probably digest their own brain like sea squirts do)...

Sigmund Nastrazzurro said...

Kwalla Bird: I guess it depends much on what type of plant you envisage: the algae you described are easy to transport, and so they are doable, but a plant with a root system would be almost impossible to move from one spot to another.

TheWingedScourge: the mobile vs sessile issue is indeed the major breaking point: are there no mobile autotrophs on Earth because their metabolic problems cannot be overcome? And it is interesting you write about a transformation, because that is exactly how the only 'mixomorph' I have yet worked out lives it life; but instead of a juvenile and an adult we are talking about diploid and haploid generations.

Trish said...

I wonder how bitrophs would work as large aquatic animals. imagine a bitrophic whale-analogue, i guess they'd be restricted to near the surface as there's less sunlight further down

legless centipede said...

You mentioned before in a previous ask about a planet with "no animals", with the scenario that at the cellular level, the lack of photosynthesis meant the complete lack of life. So then all organisms would be photosynthetic.

The question then is: with the absence of non-photosynthesizers, would any evolve into photosynthetic "animals", i.e. organisms that move and feed on other organisms in addition to using sunlight as energy? Would there be any motivation or advantage to being able to move and eat, or would it just be a waste of energy and pressure toward sessile life-forms?

You did mention that the lack of "animals" would also mean the lack of flowers and fruit as well...

Keenir said...

Lots of great food for thought; kudos!

Perhaps we need to think smaller. Maybe the photosynthetic animals wouldn't use sunlight to power their entire metabolism, but instead would use the solar energy to fuel one or two particular metabolic processes (and-or, solar power as the equivilent of blood that fuels mosquitos)

Some plants *do* move, in the sense that, as they grow longer and longer, getting further away from where they were planted, they put down new roots...and some species atrophy the older roots as new roots are put down. (I think bananas are one such plant)

But otherwise, plants don't need to engage in much locomotion, because their growth includes both where they are in space & where they were. Look at a photo of kudzu carpetting trees and phone animal to cover all that, would need either to be part Blanket Octopus with organic drapes, or have an aphid's reproductive system.

makes sense, that in a non-animal world, there would still be herbivores...even on Earth, we have plants that are parasitic on other kinds of plants (largest flower is entirely inside another plant, except for the flower itself)

We do actually have some Earthly examples of clades that became totally sessile, and then produced mobile forms:
tunicates -> salps
crinoids -> starfish

cool!, and a photosynthetic whale could litterally follow the sun - going against ocean currents, planetary rotation, or whatever.

Anonymous said...

sorry, that 10th one was me.


elepus said...'s an interesting point how there are no large photosynthetic animals on earth. however, it's probably worth considering that there must be some sort of physical constraint among earth animals perhaps that hinders their evolution? sort of like how there are no large animals alive today that have more than four limbs: not because that that isn't feasible in principle: it's just that the land vertebrates ended up with four and were constrained by their anatomy. perhaps on a cellular level something similar happened to animals, perhaps the differences between plant and animal cells that made chloroplasts inefficient?

Kevin the horse said...

I wonder if flying or swimming animals would benefit more from being bitrophic, as they already have large surface areas for wings or fins that could double as a photosynthetic "leaf"?

And to add on to Trish's bitrophic whale idea, perhaps it has thin, prominent dorsal fins full of chloroplasts, that from a distance would give it the appearance of a ship with bright green sails? Perhaps it could also feed upon plankton as a secondary energy source, but unlike Earth whales rarely if ever submerges more than a few meters.

Sigmund Nastrazzurro said...

Trish: well, first there is the issue how the 'leaf' size scales with body mass; it may well turn out that the idea is better suited for smaller than for larger masses. And secondly, there's also that the intensity of light decreases quickly with depth. But those are factors that can be investigated.

Legless centipede: on a planet with plants but without herbivores, evolving the ability to move and eat plants would convey an enormous advantage. But you must always realise that the earliest stages towards an ability should already convey some advantage. An organism that can eat plants but cannot move to them would be pointless. But if there is another reason to move, such as to get better light, than 'eating' could evolve later. But we haven't settled the issue yet whether bitrophs make sense... The post is about the energetics only, but there are wider issues here, such as whether plant-metabisms allows active mobility (in the sense of movement of the entire organism on a timescale measured in seconds or minutes. The chasm on Earth between photosynthetic/immobile and heterotroph/mobile seems very wide. Why is that?

Anthony: quite true: in the post, I described bitrophy as a state in which photosynthesis is an auxiliary power source, not the only one. If it is the only one, then you can still consider whether such beings could be mobile.

Elepus: that is the theme of the chasm I meant above. You might be right in that we are looking at a fixed state. Perhaps photosynthesis on Earth is simply not efficient enough to provide one organisms with the energy for animal traits such as active movements. Or there simply never was any multicellular bitroph being with the potential to become macroscopic.

Kevin the horse: I think so too. That's why I alluded to bats. But there is one other thing that bothers me a bit. Light obviously needs to reach chloroplasts to work, so the living cells cannot be protected by scales, hair or a thick epidermis. That might make photosynthetic organisms vulnerable (unless they are covered by highly transparent protective tissues).
I like the manta/whale thingy with solar sails (if their large size does not make them impossible). But why limit them to green?

sidney-the-aussiedude said...

I wonder if a desert animal would be a good candidate for a bitroph. In the desert there is plenty of sun but little food, so perhaps a bitroph with perhaps a Dimetrodon-esque sail or fennec-like ears could take advantage of the abundant solar energy?

They can't really subsist on it alone, but it serves mostly as a secondary power source to keep them going for long periods without food, perhaps for weeks at a time while they trek through the desert in search for a meal?

Cephalopop said...

I don't think a lack of body covering would be much of a problem for a bitroph: frogs have thin permeable skin after all but they are one of the most diverse and successful vertebrate groups even after reptiles and mammals dominated the land, they seem to deal with it just fine and some frogs even thrive in the desert.

Alternatively, snakes have no eyelids and their eyes are protected by a transparent scale. Perhaps a similar, harder version of this could protect a bitroph's chloroplast organs?

Snailord said...

hmm, kind of brings to mind the groveback from expedition, though it's pretty unlikely to have a rooted plant symbionts whose growing roots risk penetrating its tissues to fatal results?

(also, on the topic of grovebacks, does the idea of a rear "skid" to support its weight make sense?)

Sigmund Nastrazzurro said...

sidney-the-aussiedude: dependable sunlight would certainly be a factor in favour of hot desert bitrophs. The 'leaves' would probably have to be movable not only to fold out of harm's way, but also to hold them perpendicular to the sun's rays.

Cephalopop: true, but I cannot help wonder how well frogs would do if their physiology allowed them a tougher skin... In plants, the cells performing photosynthesis need to be subjected to air as well as light, and lose water too to help them cool. In bitrophs, CO2, O2 and water could be transported by blood, so the chloroplasts do not need direct access to outside air. A transparent integument of some sort might help. It's composition is open to evolutionary creativity.

Snailord: the groveback never struck me as 'acceptably improbable'. Did the trees actually grow into the animal itself? s for the rear skid, it's been a long time since I looked up the costs of friction, but they must be gigantic in such a large animal. I loved the paintings of Expedition, but not the biology.

Plantypus said...

Could branching filaments (think something resembling axolotl gills) be able to provide a massive surface area for photosynthesis? Perhaps augmented with structures that refelect and refract light to maximize the absorbed light input?

Anonymous said...

so, sort of like the narrower fern leaves? sounds reasonable.


Sigmund Nastrazzurro said...

Plantypus: very interesting ideas.
- Gills are optimised for gas exchanged to start with, which in a bitroph could facilitate the supply of CO2 to the gills for photosynthesis.
- Reflecting light back through the layer containing chloroplasts -or their analogues- would help increase the amount of light. I like that concept very much. Such reflective layers can be found behind the retina in some animals (cats!), but I have never heard of such layers in plants. Perhaps the reason for their lack that photosynthesis efficiency saturates anyway, even when light increases. We really need better photosynthesis!

Anthony: sorry, but I missed the jump to ferns; did it have to do with the branching structure??

Anonymous said...

I was just trying to think of if anything photosynthetic was structured at all like gills, or at least looked a little like them. (the mimosa plant's leaves resemble them, imho)


Abby said...

Another scenario to consider is when the planet is tidally locked to its sun. If it is assumed that the total instellation is approximately the same as on Earth then the planet could still be habitable. However, regions even quite far from the sub-stellar point would still receive more energy than anywhere on Earth as there is no night. This would allow a photosynthetic area of a certain size to gather more energy over time than on a non-tidally locked planet and thus support a larger animal than would otherwise be possible. Whether or not this make a motile photosynthetic organism more or less likely is another question though.

Anonymous said...

One key fact that all discussions thus far (that i can recall, at least) is that photosynthetic organisms only have two options:
* photosynthesize & breathe at the same time, thus losing lots of water vapor and such.
* hold their breath while photosynthesizing, building up toxins while conserving water. (found in cacti and quillworts)...breathe at night when they can't photosynthesize or lose water.


ankalagon said...

Would an ecosystem with no true plants but an abundance of insect like bitrophs be sustainable? If the "producers" of the food web are mobile would it make the "herbivores" technically predators?

Sigmund Nastrazzurro said...

Abby: absolutely! I already prepared some figures for the follow-up post to explain some features about insolation on a planetary surface, and had decided to mention tidal-locking as an aside, focusing more on axial inclination and orbital eccentricity.

Anthony: spot on! The next post should see a summary of various factors affecting the efficiency of photosynthesis in bitrophs, and after that I may need perhaps a third post, focusing on solutions. I am becoming convinced that, for bitrophy to work, the model should not be a conventional plant growing out of a conventional animal's back...

Sigmund Nastrazzurro said...

ankalagon: I see no reason why such an ecosystem would not be sustained. But terms like 'herbivore' and 'carnivore' would not do justice to creatures eating bitrophs. Actually, if a conventional herbivore is in fact an 'autotrophivore', and a carnivore is a 'heterotrophivore', then you should also have 'bitrophivores'. Solved!

giraffes are a hoax said...

"conventional plant growing out a conventional animal's back" lol that just sounds like a bulbasaur lmao.

the question is if bitrophs would make more sense as the animal cells themselves actually photosynthesizing, ot if they have a symbiotic relationship with micro plants (sort of like our gut bacteria) that do the photosynthesis for them. in the case of the latter there must be a way to pass on their microplants to their offspring...

David said...

ANKALAGON: Yes, such a scenario exists in real life, in the open ocean. There are no rooted plants in this ecosystem, but there are tons of floating phytoplankton, which feeds the zooplankton, which feeds small fish, which feeds bigger fish, which feeds dolphins and sharks and seals and whatnot. So yes, that would be a completely feasible idea.

The question then is, what adaptations would their "grazers" have to feed on the insectoid bitrophs? Perhaps long sticky tongues and narrow mouths, like anteaters, to slurp them up in large quantities?

Abbydon said...

It seems to me that when people talk about "plantanimals" they are mostly interested in multicellular photosynthetic plants that act like animals and can move somewhat quickly. After all, animals that act a bit like plants are already somewhat common in two phyla: Porifera (sponges) and Cnidaria (sea anemones, corals and jellyfish). They are filter feeders but some also host algae or cyanobacteria as endosymbionts to harvest solar energy as discussed in this paper on
Photosynthetic symbioses in animals. Similarly, the combination of autotrophy and heterotrophy is widespread among plankton.

This leads to several related questions:

1. Why would an organism that uses solar energy for a large portion of its metabolic needs still need to consume organic matter?

2. Why would an organism that consumes organic matter for a large portion of its metabolic needs still need to use solar energy?

3. Why would such an organism remain motile?

Perhaps the oriental hornet (Vespa orientalis) could provide some inspiration to address this as it appears to be rather unusual.

Keenir said...

Excellent points, Abby!

as to #1, to get ahold of vital nutrients? (ie, carnivorous plants)

as to #2, maybe the faster it warms itself up, the more of an advantage it has over other species that don't use the sun to warm up. (think of ectotherms on Earth)


Sigmund Nastrazzurro said...

giraffe...: in true bitrophs -if those prove to be viable- the photosynthetic organelles would surely be transmitted to their progeny, just like all their other organelles.

David: before we know what 'bitrophivores' look like, first we would have to have some idea what the bitrophs themselves look like. I personally have not reached that stage yet.

Abby: as I wrote in the blog, the purpose of this thought experiment was indeed to see whether auxiliary photosynthesis might work for larger actively mobile organisms. The paper you mentioned (thanks) has a very telling phrase in the abstract, in that the animal lineages with photosynthesis have a large surface area. That is where I expect problems to arise.
As for your questions, the common answer is probably that this will happen if bitrophy provides an optimum solution, i.e. if the organism obtains the most gain for the least investment. For instance, if you obtain most of your energy needs from light, it may pay to use heterotrophy when there either is little light of when more energy is needed then photosynthesis can provide. (Alexander wrote a nice book on that subject too: 'Optima for animals'.)

Anthony: yes, and you may think of other examples too. For instance, motility prevents animals to be eaten on the spot like plants.

dancing marblebill said...

Interesting stuff as always, it's beautiful to see how a sci-fi world building project takes every small detail and truly fleshes out its world to the roots. Would be nice to see Furaha in other media, perhaps a mockumentary series told from the POV of scientists studying the planet.

Also, as a token of appreciation, have a dancing marblebill.

Sigmund Nastrazzurro said...

dancing marblebill: that is fascinating, very enjoyable and completely unexpected. Petr once made origami versions of Furahan animals, also including an marblebill. This is equally surprising!