How could this slippery, elusive slime be so important to science?

Leviathan Press:

In The Matrix, when the protagonist Neo wakes up from his "dream" of machines and programs, in order to show that he has returned to the real world, the first thing he discovers after waking up is that the real world is full of mucus.

© Giphy

Although no one would like this slimy feeling (it may even be disgusting), we still have to ask: What is the nature of mucus? Why is it so common in the biological world? What is the significance of having the characteristics of both fluid and solid?

For example, our noses and sinuses are always filled with sticky mucus, and for good reason. The nose's primary function is of course to breathe in clean air, but it also has to act as a filter, removing as much of the air as possible from the air. When we cough or have a runny nose, the mucus we expel from our bodies is actually a normal response of the respiratory tract to expel external irritants and pathogens.

The weather in Morro Bay, California, was perfect, the coastline was picturesque, and the wildlife looked like something straight out of a Disney movie: sea otters frolicking with their pups in the surf, herons basking on the beach, and seals showing their bulging bellies in the sun.

Yet, in the tranquility of Morro Bay lurks a monster straight out of HP Lovecraft, as slimy and slithery as a creature from Sartre's nightmares. Nothing could be more alien than this creature. Two hearts? Tentacles sprouting from its head? Four rows of fangs? A smile on its face? But would you call it a smile? Or a face? Yes, it is a hagfish.

Weapons-grade slime: When threatened, hagfish (like this one) can release molecules from their skin that turn water into a slime thick enough to fill a shark's mouth. The US Navy is even experimenting with using hagfish slime to stop suspicious ships without resorting to force. © Frank Fennema/Shutterstock

Author and Nobel Prize winner John Steinbeck disliked the creature. He found it "disgusting" and "revolting," but also said that his close friend, marine biologist Ed Ricketts, "didn't find it that way because there was something about the hagfish that he found fascinating." I find them fascinating, too.

When threatened, hagfish can instantly condense large amounts of water into a thick mucus. © Andra Zommers

There are a few peculiarities about the hagfish, including its German name, Schleimaal (slime eel) ; a misleading one, since this spindly creature is not an eel, nor is it a true fish, as its English name implies. In fact, hagfish are members of the same family as the equally unpleasant, parasitic lampreys, the only remaining members of a primitive class called cyclostomata, a name that means "round mouth."

The name is more suitable for lampreys, which have a disc-shaped suction mouth with many teeth that allows them to cling to fish and tear the flesh from the side. Previously, a surprising discovery confirmed the unique evolutionary history of cyclostomes[1]. The study found the first fossil of a hagfish from 100 million years ago, which was well preserved with traces of mucus, like "a sneeze in stone". It highlighted the close relationship between this creature and lampreys and proved that they are not the primitive ancestors of our vertebrates, as some scientists had previously speculated.

The lamprey is a cyclostomum. It has no jaws, but instead has sharp teeth, a trait that is characteristic of its ancient fish ancestors. Its gills are in a primitive bag-shaped state, with seven gill holes on each side, arranged behind the eyes. © High Country News

Unlike lampreys, hagfish seem to be harmless most of the time, living in the deep sea and feeding primarily on carcasses. On land, they are only seen by people like me who seek out wholesale fishmongers on the streets of Morro Bay.

Sandy Winston is one such wholesale fishmonger. On this dreary December afternoon before Christmas, he was kind enough to let me into his yard. Inside, hundreds of hagfish were writhing in two oversized metal containers, fed by large pipes. But it wasn't always that way, because the creatures quickly transformed the liquid into a thick, gel-like substance. I saw it happen, and felt it myself, because soon I was groping around in the clear goo, trying to grab a hagfish with my bare hands.

They were harder to catch than I thought, because they never stayed still but would slip through my fingers. Not only that, but the slime was a problem, it was so tough that I could lift it like a thick, dense fabric. It was so sticky and stringy that it formed a web between my fingers, and it was so sticky that I couldn't wash it off.

Without a word, Sandy's coworker, Becky, handed me an old rag stiffened with slime, which I used to scrub the slime off my hands. She was blond and smiling, and she stood beside me, also rummaging through the slime. She was in charge of the animals, and at the moment, she was looking for a dead hagfish. "I can smell it," she said.

Why take a dead hagfish surrounded by its own carrion-eating cousins? "They don't eat each other," Becky said.

This close-up of a hagfish bears a striking resemblance to the poster for the 1990 horror film Tremors. © BRANDON D. COLE/CORBIS

Not many creatures would prey on the hagfish, however, as its loose skin makes it difficult to catch, and its defense is extremely slimy.

When threatened, hagfish release extra-long molecules from their skin (normally stored in a space-saving spindle shape, waiting to be put into use), which then burst into rapid bursts, instantly condensing large numbers of water molecules into a dense mucus, forming a suffocating cloud of gel that is even strong enough to plug a shark's mouth.

There are tens of thousands of fibers in each liter of hagfish slime. They are long and thin, but tough and elastic, a bit like strong silk or synthetic fibers. As pure natural molecules, they may point the way to a new ecological textile fabric.

© Knowable Magazine

But there's more: The U.S. Navy is experimenting with using military-lab-grade hagfish slime to stop suspicious ships without resorting to force. Current interception tactics involve firing plastic ropes that slow down ships by tangling around their propellers, but are difficult to untangle afterwards. In contrast, weapons based on synthetic hagfish slime might be able to expand underwater into a ball of slime to stop suspicious ships, then dissolve away without leaving any residue.

It would be a modern re-telling of the mysterious slimy ocean described by the ancient Greeks and Romans.

"For more than 2,000 years, geographical texts have mentioned a 'frozen sea' that would stop ships from travelling further or make navigation more difficult,"

"The phenomenon is also mentioned in medieval literature, where one reads of a 'frozen sea' or 'immovable sea', which was given the Latin name 'Morimarusa' (Sea of ​​the Dead)," German historian Richard Hennig wrote in 1926.

Who knows how many other unique, useful slimes exist beyond our ken? If the hagfish is the king of animal slimes, then its kingdom includes the entire natural world—and every species in it. Biological slimes are not obscure, occasional special cases; they are the rule, a necessity for survival.

In all my years studying this fascinating substance, I have yet to encounter an organism without mucus, and I doubt such "minimalist" organisms even exist. This is not surprising, because for nearly every evolutionary question, nature has an answer in mucus. Invertebrates in particular rely on the substance for locomotion, communication, reproduction, self-defense, and even hunting, while jellyfish, comb jellies, and other zooplankton are made entirely of a gelatinous mesoglea, Mark Denny of Stanford University wrote in a seminal 1989 article.

The all-encompassing medium of life: mucus is just as important for microorganisms. And what about us, the so-called higher organisms? Mucus is just as important for us vertebrates and plants. We, too, use mucus in countless ways, but it is not so easily noticed in us.

© Science

This concealment is necessary for terrestrial life, because mucus, which contains a lot of water, dries quickly in the air. Any organism that uses mucus on a large scale on land would prefer to hide it inside its body or, like plants, in the soil, where it is easier to control water loss.

The only mucus surfaces we humans display publicly are our eyes. They are covered in a thin mucus membrane, which is protected from dehydration by a lipid layer. Perhaps it is because of mucus's mostly secretive nature that we have long failed to notice that it is an important and complex substance.

© Medical News Today

So, what exactly is slime?

It’s the default term for an unknown but slow-flowing liquid or a spooky soft solid. It’s something in between, a feeling and a description of a substance, but there’s no typical slime.

Depending on its origin and function, it hides behind many nicknames such as "gel", "biofilm", "mucilage" and "glycocalyx", in ecological communities such as "biological soil crusts" and in phenomena such as "marine snow".

But how do you sort out the differences and commonalities between these concepts? Most colloids are lumped together as “slime,” even in scientific publications, but their inner molecular lives are rarely studied—or at least they were until a few years ago: Now, more and more researchers studying specific slimes are connecting with their peers to share their insights.

© CAMP

Adam Braunschweig of the City University of New York leads an international collaboration that aims to study the mucus of different animals and harness their designs to develop new technologies. This is a worthy goal because, as they write in one article, these mucuses “have a remarkable diversity of functions, including lubricants, wet adhesives, protective barriers, and mineralizers.”[2] The new discipline even has a name: “mucomics.”

But even if specific mucus can be identified by its structure and function, how can we come up with a unifying definition for all types of biological mucus? The answer may be as slippery and elusive as the substances themselves, but at least all kinds of mucus share some important common features in terms of composition, structure, behavior, and function.

It may be easier to unpack this from the outside in, starting with function: despite their diversity, biological mucus primarily act as lubricants, adhesives, and selective barriers. They also have other functions, such as moisturizing or mineralizing, but these functions can generally be classified into the above-mentioned major categories, which themselves are not completely independent and clear.

I have yet to encounter a creature without slime, and I doubt that such "minimalist" creatures even exist.

So far, we have only understood and studied the functions of mucus in a few species. Snails, for example, can hang on to surfaces with ease, as well as crawl on them, by secreting a different kind of mucus glue. They also cover internal surfaces such as their digestive tracts with mucus as a barrier – just like humans and many other creatures.

But what if a single cell is the entire organism, and if it has no tissues or surfaces to protect? Microbes may be the most primitive and skilled slime-makers of all living things. They come together to build slimy cities for themselves, or biofilms, anywhere there is water and a surface to cling to. Microbial slime is ubiquitous in the environment, affecting habitats from deserts to coasts by gluing together sand, sediments, and other substrates, often at the interface of air, land, and water.

Despite having no nerves, muscles or even a brain, sponges have the ability to expel clumps of mucus from their bodies in a manner similar to a sneeze. Scientists have long known about this behavior, but until now, exactly how it happens has remained a mystery. © Current Biology

Next, let's consider behavior: How do mucus act as lubricants, adhesives, and flexible barriers? This is due to their viscoelastic properties, which means that they have properties of both fluids and solids.

In many cases, organisms can tune this behavior—fine-tuning the fluidity, viscosity, and density of mucus, making them extremely adaptable to changing needs. The behavior of hydrogels depends on the duration and strength of certain forces acting on them, which is the main reason why they are so diverse and adaptable as lubricants, adhesives, and barriers—even though they are pretty much just water. Their characteristic slow flow, or “viscosity,” depends on the substance’s internal structure and components.

According to German microbiologist Hans-Curt Flemming, "slime is almost just hardened water". This stiffness is partly due to a three-dimensional framework that holds the water in place - it keeps it in molecular chains. In other words: the water wants to flow, but is held firmly, if somewhat elastically, by the molecular framework, which explains its behavior that is closer to that of a solid substance. This network consists of polymers, which are long chains of molecules that are cross-linked to one another. They are unique in their ability to hold together unusually large amounts of water, at least when it comes to highly functional mucus produced by living organisms.

Although science is only beginning to get its hands on this slippery, elusive slime, this isn’t the first time it’s intrigued and used by humans.

© Simia Attentive/Shutterstock

In bright sunlight, the yellow mucus of the murex turns a deep purple. In ancient Roman times, the mollusk was hunted by the thousands (and later by Europe’s royal families) for use in luxurious gowns of imperial purple (also called royal purple). The once-humble European gull clam (Pholas dactylus) suffered a similar fate as a treasure. These creatures use their elongated shells to carve holes in stone, where they spend their lives hiding. Despite the ingenious burrows they dig for themselves, the Romans still discovered them, and Pliny the Elder wrote that at nighttime gull clam banquets, guests’ mouths, hands, and clothes would glow in the dark, splashed with the gull’s bioluminescent mucus.

It seems we may be learning from the past, seeping into the future: We’ll use and appreciate nature’s gels in more surprising ways—and hopefully more restrained ones.

References:

[1]www.pnas.org/doi/full/10.1073/pnas.1814794116

[2]pubs.acs.org/doi/10.1021/acsbiomaterials.0c00713

By Susanne Wedlich

Translated by Kushan

Proofreading/Lean Bamboo and Bean Curd

Original text/nautil.us/the-importance-of-slime-287047/

This article is based on the Creative Commons License (BY-NC) and is published by Kushan on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan