Foreword: I'm aware of the limitations of the square-cube law involved in sizing an animal up, as these limitations have been relentlessly stressed in every discussion of realistic fantasy creatures. In this case, I'm discussing not an animal, but a slime mold with animal-like motility.


Slimes in fantasy video games are all rather similar to each other in appearance and behavior. They are macroscopic and generally sphere or droplet-shaped, and capable of moving under their own power, often elastically bouncing meters into the air. They absorb solid matter directly through their gelatinous surface, and it's often implied, stated or demonstrated that this is how they eat. Though they are extremophiles found in every biome (e.g. forest, tundra, island, volcanic mountain, swamp), they are fairly weak creatures, and they can be dispatched with the first sword you get.

I want to construct a more realistic slime. I've looked for the largest slime molds that exist in the real world and have identified Brefeldia maxima as a potential candidate for improvement. It's a single cell approximately a meter across, and it can weigh 20 kilograms. It's not spherical, though - it's a layer about a centimeter thick. Nevertheless, its size is interesting.

The traits I am looking for are:

  • Spherical, ovoid, or droplet shape. This conserves surface area relative to volume, as far as I understand it.
  • Able to move at a speed humans can observe. Bouncing is not required. It doesn't even need to be fast compared to a human, just visibly moving. But when I say "moving," I mean that the entire slime physically changes position, not that it grows towards food like a slime mold in a dish.

I do not need these slimes to have big cartoony eyes like they have in some games.

Naturally, some questions follow this:

  • What is the largest such a slime mold could be under the limits of the square cube law? Could it be a single cell? I assume this would be governed by the efficiency of transporting food chemicals within its mass and the strength of the protein fibers it's made from.
  • Would such a slime mold at such a size actually be as weak as video games imply, a one-hit kill? Or would it have to have a thick, stiff "crust" so it could support itself, and such a crust would be hard for a typical sword to cleave?
  • By what means does it eat? Does it need to be rooted in place for some time? Or does it sense food around it somehow and absorb the food through its surface? And if it has a crust, how would this be reconciled?
  • Could it possibly inhabit the wide range of biomes that fantasy games show? Or would its range be limited?
  • Is a macroscopically motile slime possible? How would it achieve this movement? Is there a way for a slime mold to produce motor impulse? I tentatively suspect rolling could be viable, but I don't know too much about slime molds. Maybe it would crawl like a slug?

Thanks in advance for your answers.

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    $\begingroup$ I suspect the largest problem you'll have is breathing. Single-celled organisms tend to respirate (either O2 or CO2) directly across the cell membrane, and this only works if everything is near the cell membrane. Insects breathe the same way, and the largest (giant weta) is only 35 grams. The only way around this is to have specialized circulatory and respiratory systems and oh look you're multicellular. $\endgroup$ – Azuaron Aug 28 '16 at 21:09
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    $\begingroup$ Is "flat puddle" an acceptable shape for your slimes? (I know some fantasy slimes, such as the infamous black puddings of D&D fame, adopt this form) $\endgroup$ – Shalvenay Aug 28 '16 at 21:19
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    $\begingroup$ @Shalvenay I'm really speaking to video game depictions such as the slimes in Final Fantasy, Dragon Quest, The Legend of Zelda, Maple Story, Terraria, and so forth. "Flat puddle" is out there, but I'm open to it. $\endgroup$ – undine_centimeter Aug 28 '16 at 21:32
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    $\begingroup$ @Azuaron So it's better then to have a larger surface area relative to volume? What about a knobbly surface? $\endgroup$ – undine_centimeter Aug 28 '16 at 21:32
  • $\begingroup$ Surface area certainly comes into it (lungs have something absurd like a square mile of surface area), but it's also about penetration. Passive systems can penetrate oxygen a couple centimeters toward the core, but no further, which is why everything bigger than the giant weta has a circulatory system. $\endgroup$ – Azuaron Aug 28 '16 at 23:29

If there is only the requirement to resemble the "slimes" of video game fame, siphonophores come immediately to mind, as characterized by the Portuguese Man-O-War. Siphonophores are a bizarre sort of "creature" with a poorly understood evolutionary methodology that can capture the imagination of a reader without unduly stressing their willing suspension of disbelief (WISOD?).

A Man-O-War consists of a diverse number of individuals that are morphologically separate sub-species called zooids; these zooids are so highly specialised that they cannot exist separately, and must combine to form the entity known as the Man-O-War to survive. Individual creatures that make up the Man-O-War include specialized eating, attacking, and reproducing zooids, and a single "gas bag" zooid to which they are all attached for mobility and coherence. How such a wide variety of highly specific, symbiotic evolutions came about is the subject of much research and debate, and is only lately becoming understood.

Seen that way, it is not hard to imagine a composite creature like this that resembles the video game slime. A central "water bag" creature in the centre, covered in eating/digesting zooids, with propulsion provided by undulations of the central organism. The long tentacles typical of hydrozoae needn't be present; you might posit that as this creature became adapted to tidal pools instead of the open ocean, the long tentacles became a liability instead of an advantage. The stingers could be retained, making the danger of encountering such a creature very immediate, but preserving the classic "slow digestion" feel of the creature. The stinging would be an enormous deterrent for most predatory species, but for a sword-using human, splitting the water bag gives you the "easy kill" of low-level monsters that you require.

The use of the stingers is, frankly, my favourite part, and the thing that brought the Man-O-War to mind. The usual explanation for how a slime could hurt you is through acid or some other sort of digestive juice, but that never made much sense to me. I like this better.

While this hypothetical creature probably would never inhabit deserts or volcanoes, separate morphs for freshwater, seawater, swamps, sandy or rocky tidal zones, and perhaps jungle or rainforest would not be out of line. Humidity would likely be your limiting factor, or for a more "realistic" creature, proximity to water.

This has answered all your questions, I think, except with regard to size. In moving from an aquatic environment to a terrestrial one, size limits are always hard to consider. My very unscientific approach tells me that water is pretty heavy to pack around. Purified fresh water is 1g/cc, so a ball of water 1m in diameter would be a whopping 523.6kg! Not terribly maneuverable for a creature motivated by undulation. Perhaps a multi-celled central water bag, with some chambers containing buoyant gasses like a Man-O-War has, could be used to reduce this density in a plausible way. Even so, there are likely no plausible giant slimes to be had with this hypothesis.

With regard to things that are slime molds in the biotic sense, colonies of such creatures acting as a single creature might be worth considering. The horror RPG Eclipse Phase has an alien life form evolved from slime molds called the Factors. I'm not sure I understand the biology as presented there, but in the rough sense, "individuals" of the species typically combine to form large colonies of mold. The individuals can operate independently, however, and are sentient. I don't like this direction much; the association with fungus, although recently genetically refuted, looms too large in my mind for me to find plausible.

  • $\begingroup$ Thorough and novel! I'm going to leave the question open for a while longer, but in the absence of any additional answers, I'll accept this. $\endgroup$ – undine_centimeter Sep 3 '16 at 21:38
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    $\begingroup$ maybe have them not be a "water bag" but something like a sponge, that absorbs water, and has a lesser density. And can stay about the same size, even dehydrated. but old question, so who cares^^ $\endgroup$ – Anakin Veganos Mar 21 '18 at 22:33
  • $\begingroup$ What about a gasbag? The creature morphs its shape by having the central gasbag put pressure on separate bags to create bulges etc. The gas makes it lighter, perhaps it could do electrolysis on water to create hydrogen gas and expel (or use) the oxygen. The hydrogen is attempted to be kept inside as long as possible to reduce the amount of electrolysis required. $\endgroup$ – Demigan Jun 28 '18 at 12:03
  • $\begingroup$ Just a minor correction of you want to be accurate; the different zooids aren't subspecies, since a subspecies is just a fairly discrete population within a species. The zooids just come in different morphs, of which there are seven. $\endgroup$ – SealBoi Apr 14 at 9:48

I always liked the idea of using a mutant of a translucent gastropod (slug) for my "slimes". They'd secrete an acidic mucus and have mostly translucent features save the eyes and possibly the brain (assuming you want it to have a "core") and it may have any number of eye stocks as it would be a mutation from the normal, garden variety slug.

It would also be very malleable, being able to squeeze into tight spaces and because its mouth is not obvious, it may seem to someone looking like it just "absorbs" its prey, when in reality it’s swallowing them and using this acid and radula (mollusk teeth) to break them down in its mouth and gut.


In a book i am writing the this is what the slimes are like. They can mold themselves into any shape and can eat using their whole body absorb flesh and bone using acids. They see using their hole body. They are split into cells each cell acting independently but working as a whole, the cells are the size of a tennis ball. They are sticky and can climb up walls and jump of ceiling onto their prey. They have sharp poisonous spikes that they can protract out there body to stab into their prey. They can split themselves into separate independent smaller cells. The only way to kill a slime is to burn it, poison it or stab it one cell at a time.

To humans they are one of them most deadly! creatures to exist.

  • $\begingroup$ That's interesting and I wish you all the best with your book. Your answer doesn't address the question asked though (which by modern standards is very broad and has multiple questions). $\endgroup$ – Hoyle's ghost Apr 14 at 10:48

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