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This question is in relation to speculative biology/evolution: A genre of science fiction and hypothetical branch of science which seeks to explore "what if" questions about life using established principles of biology, ecology, evolution and related scientific fields.


An in-depth explanation for the proposed planimal life cycle, and comparisons to life on Earth:

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Depiction of ancestral planimal life cycle

The ancestral condition for Earth animals is to have a motile juvenile stage called the larva and a sessile adult stage, and while not all animals' adult stages are sessile, all Earth animals begin life motile. By contrast, the ancestral life cycle of these planimals has them begin life as a sessile, photoautotrophic juvenile stage which I shall call a polyp, which matures into a motile, heterotrophic adult stage. This life cycle may be likened to Earth cnidarians, however even cnidarians begin life in a motile larval stage called a planula, while these planimals begin life sessile. Early stem-planimals and even a few planimal clades do not have a motile adult stage, and are effectively just "plants," which to my knowledge is unlike anything seen on Earth.


Proposed hypothetical evolution of planimals:

  • 1). A heterotrophic eukaryotoid microbe consumes and undergoes endosymbiosis with a cyanobacteria-like photoautotrophic microbe.

  • 2). The now-photoautotrophic eukaryotoid evolves multicellularity, and attaches to the benthos of the photic zone using a holdfast.

  • 3). In order to increase reproductive success the organism develops a strobila much like many cnidarians on Earth. The segments of the strobila, called proglottids, break off of the main body once mature and become planktonic; acting as vehicles for the genes of the whole organism to allow for a faster propagation of gametes; protecting the gametes from predation until coming into contact with another proglottid and exchanging gametes. Proglottids may be capable of some movement via flagella/cilia, however do so rarely.

  • 4). In order to further increase reproductive success the proglottids develop a highly motile, more animaloid mode of existence in order to more easily reach other proglottids to mate with. As a consequence the proglottids become primarily heterotrophic to fuel their now highly active lives, although they may still supplement their diet with some level of autotrophy. By this point the organism has become a planimal.


As far as I know, there exists no organism on Earth which has evolved in such a manner as this. Could such lifeforms exist on an alien world, or are they not naturalistically possible?

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Since there is no exact analog on Earth, I can speculate that the closest thing to this plant/animal organism are corals. Unlike your creature, the corals rely on a symbiont algae which takes hold inside the coral. Every coral larvae that hatches must settle down and wait for the algae to set inside it. After all, it is speculated that mitochondrial organelles were independent cells which developed a symbiant relationship with all living cells today, up to the point that they live exclusively inside the cells. Corals don't move much beyond extending their filter-feeding mouth-parts to extract drifting organic matter and contracting into their calcareous shell. They are cold-blooded too. Despite all this, the symbiant algae merely supplements the needs of the coral. The coral has to filter some food in addition to the food it photosynthesizes.

In order to make your creature live off the resources, you must make sure you follow some basic rules:

  • Fast metabolism takes energy.
  • Movement takes energy.
  • Maintaining body heat takes energy too.

Your creature may have two different phases depending on how much energy each phase requires.

The motile adult must look for a mate. It swims or digs tunnels underground or moves on the surface, depeding on where it lives. This phase requires energy. The adult must feed off available resources and photosynthesis may not be sufficient. In addition, its compact shape, aimed to better locomotion means its surface area for photosynthesis is just too small. The animal must feed off organic material and photosynthesis may be insignificat at that phase. On the other hand, after the adult mates, it releases a spore that develops into a sessile creature that behaves just like a plant.

The sessile creature has long and wide extensions which act as "leaves". It may either grow underground (like grass rhizomes) extending its appendages above the ground or grow upward (having a trunk like a tree), and towering above others to compete over light. It must conserve energy, so it will not move except in order to adjust its leaves to the position of the sun (as plants do). To make the most of sunlight, the sessile creature must slow down metabolism and focus all its metabolic activity on growing and recovery from injury (by grazers) just like plants.

Using the analogy of "alternation of generations" in ferns, one can see the two phases as the sporophyte (adult) and gametophyte (plant-like). Not exact analogy, but quite similar.

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  • $\begingroup$ *photosynthesis WILL not be sufficient $\endgroup$
    – Hobbamok
    Oct 5, 2023 at 15:05
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Totally plausible

The one obstacle for a successful photosynthetising animal is that in most cases, photosynthesis would not provide even an insignificant amount of the calories an animal needs. Mandatory XKCD: Green Cows:

Well, a 450-kilogram cow just wandering around in a field might eat about 10 kilograms of dry matter a day, extracting on the order of 50 million joules of metabolic energy. So photosynthesis could only make up about 4% of the required intake—saving only a few handfuls of grain.

However, your animal is not only probably much lighter, it has a much slower metabolism too. It most certainly is a poikilotherm.

Just so you know, there are photosynthetising animals in our own world. No seriously, I am not stoned out of my mind (right now). As it turns out some sea slugs are able to steal chloroplasts from the algae they eat, and the chloroplasts last for quite some time while feeding the slug. This phenomenon is called kleptoplasty.

Your creature could either store a lot of chloroplasts during the motile generation to pass along to the sessile generation (and have a way to keep the chloroplasts alive), or the polyp could steal them from unicellular algae (or have a symbiotic relationship with them)... or simply have evolved in a way that allowed it to make its own chloroplasts. Coming from a different branch of the protists from which actual animals evolved.

It will be more plausible if your polypoid is a mixed autotroph/heterotroph. Photosynthesis only provides you with calories, nutrients underwater are more easily obtained from other living matter.

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About as plausible as a regular animal

Animal life is incredibly specific and complex: There are many features, such as well-ordered growth, fast and easily motion, and complex intercell communication, all of which are required for anything animal-like to form. Having chloroplasts on top of all that wouldn't be much of an addition

As for the specific life-cycle, it's pretty much already occured with jellyfish, replacing the polyp's filter-feeding with photosynthesis won't be much of a change

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    $\begingroup$ I think I should've specified that the life cycle depicted above is the ancestral condition for an entire kingdom of organisms, and that they even undergo terrestrialization multiple times like Earth bilaterians have. For example: A "tree" on such a world could strobilate or fragment, and the resulting piece which falls off could be an insectoid, or a parasitic worm, or even a large vertebrate-analogue, so essentially nearly every "animal" on this planet begins life as a "plant." $\endgroup$ Oct 3, 2023 at 13:14
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    $\begingroup$ @LordMalecith I'd expect a large quantity of this kingdom to loose either the sessile or the motive stage in their lifecycle, just because it doesn't fit into too many environments but this origin for a kingdom (and therefore more widespread versions upholding that lifecycle) are realistic. I pity the early biologists of that world $\endgroup$
    – Hobbamok
    Oct 5, 2023 at 15:07
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    $\begingroup$ @Hobbamok There would indeed be quite a few clades whose life cycles have been altered, although to my knowledge it'd be easier to lose their motile stage via neoteny than the other way around (In fact, I don't even know if a term exists for such a process nor even if said process exists). It's likely that most terrestrial planimals would retain their polyp stage, which would certainly be interesting to see. I wonder what flowering planimals would look like? $\endgroup$ Oct 5, 2023 at 15:31

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