by Matt Simon
All told, there are some six thousand threads per cup of slime, and these strands easily tangle in, say, the gills of a predator. The defense works so well that scientists originally began to figure the goo served this purpose—as opposed to the hagfish releasing a cloud and hiding in it like it was a force field—because they wouldn’t find the critters in the stomachs of fish, but instead in those of gill-less creatures like dolphins and seals. Another bit of good evidence is that the hagfish will only fire a mucous gland that something has agitated. Squeeze the creature and goo comes out at the points of contact, not anywhere else. Substitute your fingers for the jaws of a shark, and you have a well-targeted countermeasure against predation.
THAT TIME A DENTAL DAM LED TO A SCIENTIFIC DISCOVERY
One experiment that confirmed hagfish slime to be a defensive weapon is probably the most amazing study I’ve ever read, at least as far as materials are concerned. It involved a rockfish’s disembodied head, a hagfish, PVC pipe, a siphon, and an “extra-heavy dental dam.” Researchers wanted to see if the hagfish’s slime really had an effect on water flow over a fish’s gills. So they covered the end of the pipe with the dental dam, rammed the fish’s head through, propped its mouth open, added a siphon at the other end, and dropped the whole weird apparatus into a tank with a hagfish. The scientists switched the siphon on and our brave hagfish “was pinched on the tail with padded forceps to induce sliming.” The siphon sucked the resulting goo through the fish’s mouth and into the gills. The rockfish gave up its dignity and the hagfish its slime—not to mention the invaluable conclusion that its kind had in fact evolved to weaponize snot.
Seeing video of unfortunate sharks and fish tangling with the hagfish really makes you feel for the would-be predators. Their reaction comes almost instantaneously after the bite. The swirling cloud explodes, and there’s much head shaking and gaping and full-body convulsions and a rapid retreat, with the attacker trying desperately to dislodge the snot from its face. And if it fails, it’ll suffocate to death and sink to the seafloor to become a welcome buffet. The isopods and crabs and bacteria will come from all around—joined by, inevitably, the lowly hagfish.
Axolotl
PROBLEM: A leg is a terrible thing to waste.
SOLUTION: The axolotl salamander can regenerate entire limbs lost to predators.
Lucky for me, the salamander is already dead. And lucky for the salamander, really, because as it would turn out, I’m not that great at amphibian surgery. But I take a deep breath as a sharpshooter might, pull the limb nice and taut, and with scissors cut through it. Well, mostly through it. I’m looking through a microscope, so my perspective is all off. The scissors get through the humerus all right, the bone not so much snapping or cracking like I’d expected, instead just gently crunching. But the tips of the scissors meet before they’re all the way through the limb. In a panic, I pull the limb taut again and make another cut. But still unaccustomed to the magnified view, I miss again. So I cut again. And reorient again and miss again. And snip once more to finally liberate the limb.
I pull my eyes back from the microscope and look down at the four-inch-long salamander, all clad in white tissue paper save for the deep-red external gills erupting out of the back of its head and the bloodied bits around the stub. It’s an axolotl of Mexico. Cut off one of its limbs, and in a month it’ll grow right back, every bit as functional. Remove part of its brain or jaw or spine, and that’ll all grow right back, too. The axolotl can’t be bothered to not remain in one piece.
I should clarify here. I didn’t just desecrate an axolotl in some back-alley pet shop. I’m in the Limb Regeneration Lab at the University of California, Irvine, funded by the one and only US Department of Defense, which longs to bestow the powers of regeneration upon wounded soldiers. These are brilliant scientists who let me go to town on a dead specimen, as I am clearly unqualified to perform surgery on a live salamander. And these scientists don’t think it’s a matter of if humans can grow back limbs like the axolotl can, but when they figure out how to do so, for we have the exact same genes that give the creature these powers. And the scientists believe that these procedures on salamanders (the living variety, not the one I desecrated) will get us there.
THAT TIME THROWING A SALAMANDER IN A FIRE LED TO . . . ABSOLUTELY NOTHING
The salamander has long been revered in European folklore not for its powers of regeneration, but for its supposed immunity to the ill effects of fire, e.g., burning to death. Legend said it was born from flames, which is metal as hell but probably not true, considering that amphibians—with their moist, delicate skin—are among the animals most sensitive to fire. There was even a naturalist who put this all to the test: the great Roman writer Pliny the Elder. And by “test” I mean he tossed a salamander in a fire, with predictable results. Sadly, the path to human knowledge seems to be paved with things like charred salamanders.
Here’s the thing about evolution that never ceases to blow my mind. In a way we’re related to every single organism on Earth, be it an animal or plant or bacterium, because the tree of life began with one organism and over billions of years branched out into the incredible array of creatures we know today. Over the millennia, as creatures evolved and new species split off and went down their own lineages, they still shared what’s known as a common ancestor with the animals they diverged from. So we share damn near 100 percent of our DNA with chimpanzees because we have a relatively recent common ancestor, but we’re more genetically distinct from the axolotl, with which we share a much more ancient common ancestor. Yet still we have the genes required for regeneration. In a way we’re part salamander—we just don’t use those genes the way the axolotl does.
The benefits such regeneration bestows on the axolotl are obvious. The salamander is an aquatic species, with plenty of predators to worry about. (Well, it did worry about them at one point. The axolotl may well exist only in labs now, the expansion of Mexico City having forced it out of its lake habitats.) If a fish makes off with a limb, growing the thing back allows the salamander to keep clambering around the lake floor. And that’s not the axolotl’s only worry: They tend to eat each other’s limbs. Like, really frequently. So growing back amputated limbs helps the axolotl continue hunting in order to survive, not to mention partaking in a complicated mating ritual.
Sounds good, right? Let’s get us some regeneration, since, after all, we have the same genes? Well, it isn’t as easy as all that.
Instead of regenerating, we humans scar, and that’s suited us perfectly well over the course of our evolution. But the axolotl never, ever scars. Well, I’ll qualify that: Its regeneration is a series of steps, the first of which is a bit of scarring, but that disappears as the process unfolds. But why? Why do our bodies get so carried away with scarring, when the axolotl shows that it’s possible to switch at some point to regeneration? The science here is still murky, but a clue could be that when the axolotl is regenerating, its immune system takes a hit. The salamander is coated in a powerful antibiotic mucus to compensate for that, but we humans don’t have such a luxury. Growing back a limb won’t do you any good if you die of an infection first.
WHEN 894A MET 664A: AN AMPHIBIAN LOVE STORY
It was my great honor to witness the axolotl mating ritual in that university lab, in a tale I like to call “When 894a Met 664a: An Amphibian Love Story.” A technician has placed specimens 894a (to be known henceforth as “the male”) and 664a (to be known henceforth as “the female”) in a tank together with some nice rocks and plastic plants for decor.
At first the lovemaking is slow going. Neither moves much, instead mutely staring and flicking their fluffy gills back and forth. But then it happens: the first contact, as the male ever so slowly pulls his head closer to her, nudging her side. He then makes for her cloaca, jamming his snout under her body and nuzzling her naughty bits, nearly lifting her onto her side. But seemingly frustrated by the female’s indifference, the
male freezes, then violently flicks his tail to rocket away. He hangs out alone at the opposite side of the tank for a while. More stares between the two abound, until the male again approaches the female, turns to look at me as if to ask for help or at the very least some advice, for Christ’s sake, and vomits. Love story over: 894a and 664a, it would seem, were never meant to be.
What’s clear about axolotl regeneration is the importance of what are known as growth factors, proteins that promote the development of tissue. Take your tweezers and scissors and amputate an axolotl’s limb, and these growth factors recruit cells around the wound site to begin rebuilding the structure. It’s important to keep in mind that all manner of different materials are involved here—bone, muscle, skin, etc.—and the body needs thorough instructions to get the cells into the right spots. The salamander isn’t building out a uniform stick of flesh. It has to know when to start forming a joint and all that jazz.
Inducing such regeneration in humans isn’t about tweaking the genes we share with the axolotl. That’s complicating things. Instead, human regeneration will likely be about harnessing the power of the growth factors associated with those genes. So labs like this one are trying to decode the steps the axolotl takes to regrow a limb, so we might someday apply the growth factors to wounds in the right order.
It sounds like lunacy, I know. But again, these researchers think it’s only a matter of time before the axolotl helps unlock the secrets of human regeneration. And when we do, we can all look back on that day when I wasted these scientists’ time and held back the advance that much longer.
My apologies, humanity.
Cuttlefish
PROBLEM: For sea creatures, sometimes a shell just isn’t good enough for protection.
SOLUTION: Cuttlefish have evolved the animal kingdom’s most incredible active camouflage to imitate any kind of background in a flash.
When it comes to keeping from being eaten, critters have a couple of options. The hagfish has an active defense, opting to blow the nose that is its entire body, and the axolotl goes about things more passively, resigning itself to the occasional dismemberment. But the cuttlefish has a far more highfalutin strategy to survive predation: It avoids detection by deploying some of the most astounding camouflage on Earth.
Let’s pump the brakes a bit and back up some 500 million years to meet the ancestors of the cuttlefish and other cephalopods, like the octopus and squid. Far from sporting the floppy cephalopod bodies we see today, these ancient creatures protected themselves with strong shelled armor. There was a downside to that protection, though—it made them slow and ungainly, as the only remaining shelled cephalopod, the nautilus, demonstrates by awkwardly swimming around and bumping into reefs and things like it’s on horse tranquilizers.
So it could have been that a powerful predator evolved in the seas, a beast capable of crushing shells, and the sluggish cephalopods found themselves losing their advantage. These creatures, though, were no pushovers. They found other ways of surviving. They lost their shells and diversified into all manner of species, utilizing a wide variety of strategies (the cuttlefish’s ancestry is betrayed by its internal surfboard-shaped cuttlebone, which helps it regulate its buoyancy). Squids have their speed, and octopuses can squeeze themselves into almost impossibly tight crevices, but among cephalopods, the cuttlefish puts on the most audacious defense.
The cuttlefish never met a surface it couldn’t blend in with: algae, coral, sand—even an artificial pattern like checkers. The transformation is almost instantaneous, and it’s mind-blowing. Chase a cuttlefish into a field of seaweed and it’ll hunker down, modify the color and pattern and even texture of its skin, and lie still, swaying in tandem with the vegetation. There it’ll wait you out, but if you close in further still, it’ll give up the ruse and rocket away, leaving a cloud of ink in its wake.
LOVING WOULD BE EASY IF YOUR COLORS WERE LIKE MY DREAM
The cuttlefish is sometimes referred to as the “chameleon of the sea,” but that’s a bit of a misnomer, not because chameleons can’t swim too good, but because the two creatures have different uses for their camouflage: Chameleons don’t just change color to blend in with their environment. A good indicator of this is the fact that chameleons can be splotched with bright reds and blues and greens simultaneously, which wouldn’t do them much good as camouflage unless they’re sitting in a box of crayons. Instead, they’re using it to signal to potential mates and help with thermoregulation. If a chameleon is a bit chilly, it can darken its skin to absorb more of the sun’s energy, and if it gets too hot, it can dial down the color again. Think of it as having a good coat that never goes out of season.
The spectacle comes from three special layers in the cuttlefish’s skin. The bottommost layer is plain old white flesh. The next is a sort of iridescent surface that reflects light to provide green and blue. But the top layer is where things get interesting. It holds cells called chromatophores, each being a sac of a particular pigment—orange, red, yellow, brown, or black—to which tiny muscles are attached. When the cuttlefish wants to express a certain color, it’ll contract the muscles around those cells, opening up the surface area of these chromatophores as much as 500 percent to expose more pigment.
Because these cells are hooked up to muscles and nerves communicating with the brain, the cuttlefish is bestowed with camouflage so quick it doesn’t seem possible. The light show is so well orchestrated that some species can send pulsing waves up and down their bodies, perhaps to serve as a warning or, and I’m being totally serious here, hypnotize their prey. Divers have seen them flaring out their arms and deploying a rapid, chaotic light show as they approach a soon-to-be victim, then fire tentacles out that snatch the prey and reel it in.
Coordinating all of this is one magnificent brain. It should be noted that invertebrates typically aren’t the sharpest knives in the drawer compared to vertebrates like ourselves. And that’s okay, because they’ve got other things going for them, things like rugged exoskeletons and the ability to dry out and reanimate like the water bear. But cephalopods are a dramatic exception to the doltishness. They’re scary smart. In the lab, cuttlefish can learn to solve mazes, while it’s an all-too-common occurrence that octopuses figure out how to escape their tanks and wander down the hall. (All right, maybe not too bright as far as long-term goals go, but still . . . ) Such smarts are indispensable when hunting, but the cuttlefish’s formidable brain also has to coordinate its defense, processing its surroundings and translating that into camouflage.
They’re such accomplished camouflagers that they can even outwit members of their own species. The male giant Australian cuttlefish, for instance, cross-dresses to get laid. Big males hold sway over the females, guarding them and attacking any rivals that approach. But smaller males will modify their color to mimic females, and also morph their arms in such a way as to appear to be holding an egg like the ladies do when they’re not keen on mating. The sneaky male thus hits a middle ground: The dominant male won’t attack him because he thinks the imposter is a female, but he won’t try to mate with him because he thinks he’s an unreceptive female. So the cross-dresser slips under the big male and mates with his female, passing along his genes for brains, not brawn.
IF IT HAS “COLOSSAL” IN ITS NAME, IT’S PROBABLY BAD NEWS
I don’t want to sound like an alarmist, but you should never go in the ocean ever again, for the cuttlefish has enormous cousins that will eat you: the colossal and giant squids. Okay, fine, there’s never been an attack on a human, but these squids can balloon to sizes that defy belief. The giant squid reaches forty feet long (though, in fairness, a good amount of that is just two long tentacles), and although the colossal squid grows to just fourteen feet, it’s far bulkier than the giant, weighing in at over one thousand pounds, making it the heaviest invertebrate on Earth. The giant squid has suckers with serrated edges (their mortal enemies, the sperm whales, often have ring-shaped scar
s around their mouths), but some of the colossal squid’s suckers sport something far more sinister—swiveling hooks that sink into flesh. And here you were thinking I was an alarmist.
Oh, I almost forgot: Cuttlefish are also color-blind. I’ll let that sink in, then disappoint you by saying that no one is quite sure how that’s possible when the creature is perfectly matching its color to its surroundings. A 2015 discovery regarding the California two-spot octopus, though, could provide a clue. Researchers removed patches of the octopus’s skin—which is also packed with chromatophores—and exposed them to light in the lab. The chromatophores expanded on their own, obviously without the help of the creature’s eyes or brain. Furthermore, the scientists found that the octopus’s skin is loaded with the same light-sensitive protein you’d find in eyeballs, so it would appear that somehow the skin itself is processing color. Whether the ability also extends to cuttlefish is to be determined, but I’m betting it’s likely.
The cuttlefish may not be seeing color with its eyes, but what it is most certainly picking up are contrasts in the environment. Curiously, for all its transformations, the cuttlefish is working with only three pattern templates of varying contrasts. There’s “uniform,” which is pretty much a single color; “mottle,” which looks like static on a TV (remember the days of static on TVs?); and lastly, “disruptive,” which splits the skin into a checkered pattern. Using these templates and adding in the appropriate colors, the cuttlefish can dissolve into whatever background it sees fit, deploying the uniform pattern for something like monochromatic sand, the mottle for more multicolored sand, and the disruptive for more complex backgrounds like coral.
