A medical student who put their points into the wrong branch of the skill tree.
Editor’s note: That Time I Got Reincarnated as a Slime, which began airing this October, adapts the light novels written by Fuse and illustrated by Mitz Vah. It follows Satoru Mikami, an ordinary office worker who is stabbed to death by an assailant and reincarnated in another world as a slime, where he strives to build “a country where all races can live happily together.” Yet what caught my attention more than the plot was the fictional creature itself—the slime. Once, it too was a horror of the Cthulhu Mythos, rather than XP fodder for generation after generation of sword-wielding heroes. This article will briefly trace the slime’s origins and attempt a biological explanation—with a bit of wild speculation.
The Origins of Slimes: From Monster to Cutie
As the word slime itself suggests, slimes are often conceived as translucent, gelatinous or mucus-like creatures with soft, elastic bodies. Under the influence of the Dragon Quest series, slimes in Japanese anime, comics, and games have a relatively stable shape, usually somewhat like a teardrop, with some kind of transparent membrane covering their surface. Slimes in American works, by contrast, are often shapeless and more fluid, capable of forming tentacles and other appendages. The slime in this anime naturally belongs to the former type; the latter, however, is where the fictional slime began.
The slime as a fictional creature can be traced back to At the Mountains of Madness, the novella published in 1936 by Howard Phillips Lovecraft, creator of the Cthulhu Mythos. It features a monster called a shoggoth, one of the “multicellular protoplasmic masses” produced when the Elder Things created life for food. Able to form all sorts of temporary organs, shoggoths served the Elder Things as slaves, performing heavy labor beneath the sea. Over time, they began to develop intelligence and tried to break free of their masters’ control. They rebelled in the middle of the Permian period, only to be suppressed by the Elder Things. In the process, however, shoggoths evolved the ability to live on land and imitate the Elder Things, eventually destroying the Elder Things’ civilization.
It was a terrible, indescribable thing vaster than any subway train—a shapeless congeries of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and unforming as pustules of greenish light all over the tunnel-filling front that bore down upon us, crushing the frantic penguins and slithering over the glistening floor that it and its kind had swept so evilly free of all litter. Still came that eldritch, mocking cry—“Tekeli-li! Tekeli-li!”
—H. P. Lovecraft, At the Mountains of Madness, XI. Chinese translation: Zhuzi (竹子).
(Shoggoths look pretty grotesque. If you’re curious, look them up in an encyclopedia—think slimy tentacles and pustules.jpg.)
Although the shoggoths in At the Mountains of Madness already supplied an early version of the creature, the name “slime” first appeared in American writer Joseph Payne Brennan’s 1953 story “Slime.” In it, a volcanic eruption brings the slime onto land, where it attacks humans.
By Joseph Payne Brennan; 1953.
Slimes in Dungeons & Dragons have various resistances depending on their type, and they too are troublesome opponents for players. The 222nd layer of the Abyss is even named the “Slime Pits.”
In 1981, riding the wave of D&D’s popularity in North America, Wizardry: Proving Grounds of the Mad Overlord—the first game in the Wizardry series—was released, with slimes as low-level monsters. By then, a slime was no longer the terrifying creature that sent your sanity (SAN) plummeting at a glance. Anyone could pick on it. Yet it still had the oozy body of an American-style slime.
It was Dragon Quest that truly transformed the slime’s public image. Credit must go to the game’s lead creator, Yuji Horii, known as the father of DQ:
“I was really hooked on Wizardry … There’s … slime-looking characters … so I got the inspiration from it. I was doodling the slime-looking character and I took it to Mr. Toriyama, who did the character design, and he made it the Slime we see today.”1
Left: Toriyama’s design. Right: Horii’s rough sketch.
Horii himself admits, though, that he originally imagined the slime as a sticky mass of gelatinous matter. Akira Toriyama “took it upon himself” to make it teardrop-shaped instead. Horii pronounced the design “perfect,” and the DQ slime’s appearance was settled from then on. Its teardrop silhouette, unchanging gaze, and fixed smile make it look friendly and “just asking for a whack”—an ideal target for players to level up on. That flash of inspiration from the old master, together with Dragon Quest’s immense popularity in Japan, gradually brought the Japanese-style slime into the mainstream. New creations based on it (girl versions?) have appeared in an endless stream: Suu from Monster Musume, SCP-999, and so on. Add slimes in the broader sense—shoggoths such as Tekeri from Uchi no Maid wa Futeikei (literally, “My Maid Is Shapeless”)—and the slime family is positively thriving, an impressive clan indeed.
What Exactly Is a Slime?
Whether teardrop-shaped or a sticky, shapeless mass, slimes show at least some characteristics of life. If they really existed, what kind of organism would they be? In fact, setting aside such supernatural abilities as swallowing everything in sight, firing black lightning, and suddenly assuming human form … the anime provides quite a detailed account of slimes, enough for me to infer what kind of organism a slime is.
In appearance, the anime’s slime is a uniform mass of transparent blue gel, normally shaped like a slightly flattened sphere. It is denser than water and sinks when submerged. Obviously, a slime of this size cannot possibly consist of a single cell: in evolutionary terms, it comes after protozoans.
Among invertebrates, the circulatory system first appears in the phylum Nemertea—the ribbon worms, a group between flatworms and annelids. It is a primitive closed circulatory system, with colorless blood flowing through vessels. Slimes have no blood, so they would be more primitive than ribbon worms.
Subtitles: “Can’t slimes float? Am I really going to die this time?”
On-screen text: “Creating a body that requires no blood.”
In the anime’s first episode, slimes have no eyes. Eye-like structures appear in later episodes, possibly because Rimuru has used the skill “Mimic” (nothing so miraculous exists in the real world, of course), or perhaps simply to make the character’s actions easier to convey. In animal evolutionary history, flatworms already have primitive eyespots that distinguish light from dark. These structures are regulated by the Pax6 gene and are homologous to vertebrate eyes. Some cnidarians have simple eyes, but most species have only opsins with which to detect light. This places slimes before flatworms in evolutionary terms, leaving us with just two major groups to consider: Porifera, or sponges, and Cnidaria.
Subtitles: “I have no mouth or eyes … Come to think of it, have I been breathing? And I don’t need to go to the bathroom.”
Slimes have a simple digestive system, with neither a mouth nor excretory organs, from which we can infer that digestion takes place within their cells. There is one little glitch, though: a slime has no mouth, yet it has a stomach for storing whatever it consumes. No real organism seems to match this arrangement exactly. If we settle for something less exact, sponges have neither mouth nor stomach, only a simple differentiation into an outer layer and an inner lining; cnidarians have both a mouth and a stomach, with a primitive digestive cavity, and carry out both intracellular and extracellular digestion. On this evidence alone, it is hard to say which organism’s structure is closer to a slime’s.
All species in Porifera, however, live attached to a surface, and they look nothing like slimes. Their asymmetrical bodies, porous surfaces, and internal channels do not fit. Cnidarians are mostly radially symmetrical, with bodies divided into an epidermis, a mesoglea, and a gastrodermis. The mesoglea is filled with gelatinous material secreted by ectodermal and endodermal cells, giving them a transparent, gel-like appearance. Like slimes, they also have remarkable powers of regeneration. All this seems to suggest that cnidarians are real-life versions of slimes.
Are cnidarians a hundred-percent match for slimes? Obviously not. I think the fictional slime also borrows from organisms other than cnidarians.
Cnidarians have stinging cells, or cnidocytes, all over their surface, with the greatest concentration on their tentacles. These cells contain venom and a stinging thread. When stimulated, the thread everts, injecting venom into the prey to paralyze or kill it outright. The tentacles then draw the prey into the stomach for digestion.
When feeding, a slime stretches its body around food—as when Rimuru swallows Unlimited Imprisonment in episode two—or absorbs it through its surface. In episode five, for instance, Minister Vesta pours a drink all over Rimuru, who immediately identifies its type and alcohol content.
This way of feeding is quite unlike that of cnidarians, but closely resembles that of a group of organisms called slime molds. And, as luck would have it, even the name fits: slime mold! Whittaker’s five-kingdom system places them in Protista, whereas Ainsworth assigned them to Fungi in 1973. In the latest, tenth edition of Dictionary of the Fungi, slime molds still belong to Protista. In fact, they have characteristics of both protists and fungi, and their taxonomic position remains unsettled.
The slime mold’s life cycle has a haploid stage and a diploid stage. Its spores release either flagellated cells called myxoflagellates or unflagellated, amoeba-like cells called myxamoebae. The two forms can change into one another depending on the moisture in their surroundings. When two cells of the same form meet, they fuse to produce a diploid zygote. The zygote’s cell division is quite unusual: the nuclei divide, but the cytoplasm does not, eventually producing a multinucleate mass of protoplasm, or plasmodium. Larger specimens can even have ten million to a hundred million nuclei sharing a single cytoplasm, forming an enormous fan-shaped structure that envelops the surfaces of objects. When nutrients are scarce, the slime mold can form fruiting bodies and release spores, beginning another cycle of alternating generations.
A slime mold’s plasmodium can move toward food. Studies of one species, Physarum polycephalum, show that its protoplasmic streaming is driven by pressure gradients generated by the contraction and relaxation of actin. The flow can reach speeds of 1.35 millimeters per second (by comparison, plant cells move at about 2–78 micrometers per second).
Like the slime in the anime, the seemingly lowly slime mold turns out to be a “Great Sage”: it can solve complex neural-network problems. Scientists led by Atsushi Tero described their findings in the 2010 Science paper “Rules for Biologically Inspired Adaptive Network Design”. The star of the show was once again Physarum polycephalum. Taking advantage of its tendency to avoid light and move toward food, the researchers placed it at a point corresponding to central Tokyo, used different intensities of light to simulate Japan’s terrain, and placed food at the locations of major districts to represent the subway system’s “profits.” The resulting network closely resembled the actual layout of Tokyo’s subway lines, and even had better fault tolerance. Why do elaborate computer models fare worse than a hungry “slime”? No one knows. Perhaps it is because the slime’s ancestor, the shoggoth, was a highly intelligent organism?
For many devoted Rimuru fans, Rimuru has long held the title “King of Cuteness.” But should he/she/it really be classified as a female character or a male one? Cnidarians have two sexes, male and female—nothing too complicated. In slime molds, however—again taking Physarum polycephalum as our example—sex is determined by three loci, matA, matB, and matC. Both matA and matB have thirteen alleles, while matC has three. The random combinations across these three loci yield a staggering sexes … and an individual of any one sex can reproduce sexually with any of the other 506 (your crowd certainly gets around). Perhaps the only thing to put in Rimuru’s “gender” field is “Rimuru,” just as “Hideyoshi’s gender is Hideyoshi.”2 Whether a slime is fundamentally a cnidarian or a slime mold, it has long been a fixture of anime, comics, and games as the lowest-level monster, bringing us joy—and perhaps XP. In this anime, it finally turns the tables. When we see a slime nestled among beauties, we may well smile warmly and sigh:
“Even a slime has it better than we do.”
Subtitle: “I want to stay in this paradise forever.”