by Matt Simon
Such is the existence of the tongue-eating isopod, Cymothoa exigua, an ill-behaved crustacean that invades a fish’s gills and latches on to its tongue, consuming it and replacing the organ with itself. Here it’ll work as a prosthetic tongue, staring out of the fish’s mouth with one hell of a view (well, until it goes blind, at least—more on that in a second) before making its exit and leaving the fish to starve to death. Having trashed its home, the isopod may lose its security deposit, sure, but it gains the ability to pass its genes down to the next generation.
First, the isopod must find its victim in the vastness of the sea. Juveniles only have so much energy to burn, so while they spend the first few days of their lives swimming around searching frantically for a fish to invade, if they’re unsuccessful they’ll switch to passive mode and wait for a host to approach, then ambush it. They’re looking for chemical cues, and when the passive juveniles catch a whiff of something, they’ll whirl back into action and look for silhouettes passing against the sun above. Target acquired, the isopod makes a break for the fish and wiggles its way into the gills.
All tongue-eating isopods are born male. When one arrives in a fish and finds that none of its compatriots are there yet, it stays male. But if another male arrives, the first isopod switches into a female and makes its way onto the tongue, digging into the tissue with seven pairs of incredibly sharp claws. She’ll live here for the rest of her life, so she has no use for eyes, which fade away as she develops. She also loses the ability to swim.
She begins feeding with five sets of jaws, a few of which are shaped like needles, slicing open the tongue and sucking out the blood. Slowly the tongue atrophies away and the isopod takes its place, as the fish uses the parasite to grind food against the roof of its mouth. Thus the isopod, like the caterpillar-enslaving wasp, is able to keep its host alive and well to support its own reproduction, for the time being, at least. Plus, the parasite gets both food and shelter.
A VALENTINE’S DAY MOST TRAGIC
Isopods get way bigger than the tongue-eating variety. The giant isopod sticks to the deep seafloor, dining on carcasses falling from above and growing to over a foot and a half long. You never know when you’re going to stumble across your next meal down there, and accordingly the giant isopod can go a very, very long time without eating. One individual in captivity in Japan, known somewhat half-assedly as “Giant Isopod No. 1,” went on a five-year hunger strike before dying on Valentine’s Day, 2014. Perhaps it was lovesick, or maybe it had something to do with not eating for five years.
But what’s the point of the sex changing? This happens to be a rather brilliant strategy in the open ocean for a creature that’s dependent on another to survive. If two isopods are lucky enough to wind up in the same fish, it’s all for naught if they’re both female or both male. And so the tongue-eating isopod has evolved to break the rules, switching sexes to help guarantee the ability to mate.
When the blind female is ready to release her young, she may be able to somehow detect when the fish is schooling and do it then, saving her kids the grief of wandering around searching for a host. Now finding herself in an empty nest, the new mother finally releases her grip, either popping out of the fish’s mouth and sinking like a stone or allowing the fish to swallow her. Not that the meal is going to do the fish any good. Unable to feed without a tongue, it’ll starve to death.
Weirder still is the lifestyle of one of Cymothoa exigua’s cousins, Cymothoa excisa. Unlike the former, the latter doesn’t totally destroy its host’s tongue and replace it. (And even then, Cymothoa exigua doesn’t necessarily always do that. It targets several kinds of fish, but only fully replaces the tongue of the rose snapper. Why that is isn’t yet clear.) Instead, Cymothoa excisa politely sips on its host’s tongue blood, allowing it to live. This means it gets extra time for mating, so more and more males show up in the gills. Indeed, they may be getting in line. When the female eventually dies, the male that mated with her may then turn into a female and move onto the tongue, and the next male in line steps up to mate with her, creating a perpetual chain of misery for the fish.
If that all wasn’t bad enough, it seems that overfishing is making matters even worse for fish species that fall prey to tongue-eating isopods. A 2012 study found that a population of fish in a protected environment in the Mediterranean were infested 30 percent of the time, compared to almost 50 percent of their counterparts in heavily fished waters. It may be that in these times of overfishing, natural selection has been favoring smaller fish that reproduce faster, which have an edge on their slower-growing comrades. And it could be that these smaller fish are unable to mount good defenses against the isopods, ending up infested more often.
A KAMIKAZE CRICKET MOST INFESTED
A parasite like the tongue-eating isopod that eventually kills its host is known as a parasitoid. Keeping its host alive can be in a parasite’s best interest, on account of it providing food and shelter, but once the invader is able to reproduce, fulfilling its purpose in life, the host will have fulfilled its own purpose and the parasite is free to dispatch it (and technically become a parasitoid).
Still, though, some hosts can end up surviving horrific trauma. There’s a parasite called the horsehair worm, for instance, that invades crickets and mind-controls them to kamikaze into water. At this point the foot-long worm erupts out of the cricket’s exoskeleton and squirms away. In one lab, a scientist witnessed thirty-two such worms piling out of one sad, sad cricket, which somehow survived. Intriguingly, there’s a six-foot species of horsehair worm out there, but no one knows what kind of giant insect it parasitizes. But boy, that giant insect certainly knows it.
So envy not the fishes. Although they freely swim Earth’s beautiful oceans, they also have to deal with not only us, but one hell of a parasite. Feel free to envy the tongue-eating isopod, though. Free room and board with an incredible view? We could all only be so lucky.
Pistol Shrimp
PROBLEM: The seafloor is a war zone.
SOLUTION: The tiny wonder known as the pistol shrimp forms huge societies ruled by a king and queen and takes up residence in sea sponges, where soldier shrimp stand guard, weapons in hand.
I’ve hitherto presented a view of nature that’s less than pleasant, that’s all just murder and tongue-eating and zombifying and whatnot. And that’s because it is less than pleasant out there. While Mother Nature solves problems, in so doing she necessarily creates problems, and thus does the push and pull go on and on, with critters evolving weapons and their prey evolving defenses. Really, amid all that, in the natural world you’d be lucky to die of old age. That’s a crappy situation and it sucks and I’m sorry, but as Alfred, Lord Tennyson, rightly put it, nature is red in tooth and claw. So it’s no wonder that some creatures have found fit to band together. After all, as the old cliché goes, there’s safety in numbers (this is a neat little writer’s trick, by the way—if you admit you’re using a cliché, you don’t get dinged for using clichés).
And there are few gatherings more peculiar than those of the highly social pistol shrimp, which, as their name suggests, have evolved some of the most powerful weapons on Earth. Individually, they’re intimidating, but as a group they make up armies and establish forts other creatures would be damned fools to try to conquer: These creatures form the sea’s only monarchies, on par with the gatherings of ants on land. And their collective gunfire is deafening.
The weapon responsible for all the racket is the pistol shrimp’s enormous, grotesque-looking claw (only one—its other is a smaller pincer), which in some species can grow to half the length of the animal’s body. By contracting certain muscles, the shrimp brings back one half of the snapper, which has a protruding bit known as a plunger, into a locked position while the second half, which has a socket, remains immobile. When the shrimp contracts another muscle, the two halves slam together, with the plunger striking the socket with so much force that water flies
out of it at 105 feet per second. The impact forms what are known as cavitation bubbles, and when they collapse, they heat the surrounding water to 8,000 degrees Fahrenheit and send out a shock wave so powerful that it can instantly kill prey. Strangely, it’s not the impact of the claw itself that makes the noise, but the violent collapse of these bubbles.
“THE CACKLING AND CLUCKING OF A BARNYARD FULL OF CHICKENS”
Pistol shrimp make such a racket that they’ve been known to disrupt sonar equipment. In World War II, a constant crackling noise nonplussed one American submarine skipper in Indonesia, a din he chalked up to the Japanese having “some newfangled gadget that they drop.” As the Milwaukee Journal recounted in 1956 with enviable and perhaps unnecessary thoroughness: “The sounds were described in terms that were both poetic and harshly blunt: Such as coal rolling down a chute, fat frying in a pan, the dragging of heavy chains, croaking, moaning, whining, grunting, drumming, a subdued steamboat whistle, the rasping of a saw on a strip of steel, the cackling and clucking of a barnyard full of chickens, the ‘put put’ of a poorly running outboard motor, a badly hurt and groaning man, and so on.”
In subtropical shallow waters, the shrimps’ collective snapping produces much of the ocean’s ambient noise, a sort of burning-twig crackle. And the sound—my God, the sound. A single snap can hit 210 decibels. For perspective, the American Speech-Language-Hearing Association considers a lawn mower’s 106 decibels to be “extremely loud,” and a firecracker’s 150 decibels to be “painful.” That’s as high as they bother going. The pistol shrimp’s snap is literally off the charts. (Another writer’s trick: If it’s literal, it’s not a cliché.)
Now, we humans use guns to kill things and maybe sometimes fire in the air when we’re really excited about something, as does the pistol shrimp with its own weapon (the killing-things bit, not the other part). But for the shrimp, a gun is so much more: They’ll use snaps to communicate, and in fact they have sensory hairs all along their pistols that help them detect the shock waves. The shrimp also use them to fight, not necessarily maiming each other but instead just kind of blasting off warnings. Some species can even excavate rock to form a home with successive blasts of cavitation bubbles.
But back to the monarchies. A handful of these pistol shrimp species form what are known as eusocial societies inside sea sponges. And that’s really, really odd. Such organization is expected for things like ants and termites (but not bees, as it happens—most of their species are solitary), but this is unheard of in the oceans. The society is all structured around a queen, who’s far larger than any of her subjects. After all, the bigger a female is, the more young she can bear. She’s the only one who mates—with her king, of course—ruling several generations of offspring, like a bossy grandmother of sorts. And while she towers over her underlings, she lacks a snapping claw, suggesting that she relies on the colony’s soldiers for protection.
HERE, HAVE THIS TOKEN OF EVERLASTING ENTRAPMENT
Another intriguing species of shrimp takes up residence in the Venus’ flower basket sponge, which looks like a long tube of glassy mesh. These so-called wedding shrimp enter as a young pair, eventually growing too large to exit through the sponge’s lattice. So they’ll mate and spend the rest of their lives trapped inside. In Japan, a traditional wedding gift is the flower basket sponge, complete with its expired inhabitants. If there’s a better metaphor for marriage than that, I don’t know it.
While these warriors may be small, they have the advantage of numbers. And for pistol shrimp, the claw isn’t only for intimidating each other. It’s an alarm. Should one of the soldiers come across an intruder trying to weasel into the sponge, be it another species of pistol shrimp or something like a fish with a mind to wipe out the colony, it’ll snap a rhythm to call in reinforcements. And when they arrive, they’ll all join in on the rhythmic snaps to drive the enemy away. It appears to be quite effective, in particular when driving off other species of pistol shrimp. They’ll also do this to fend off the sponge’s own enemies, things like sea slugs, thus paying their rent.
Where this gets a bit complicated is that in the animal kingdom, selfishness typically rules. Individuals are in it to pass down their own genes at whatever cost, not risk their lives for anyone else. So if you’re a eusocial pistol shrimp, and you can’t mate, why on earth would you stick around to protect your queen and siblings? It would seem to defy Darwin’s idea of evolution by natural selection.
The answer is something called kin selection. You don’t have to personally pass down your genes to pass down your genes. If you can help guarantee the survival of your kin, in a roundabout way your own blood will make it into the next generation. Yeah, you don’t get to have sex, that’s a bummer. But hell, if you’re a social pistol shrimp you’ve got a sweet pad and a lovely family. What more could a crustacean ask for?
Sociable Weaver
PROBLEM: The desert is somewhat . . . toasty, not to mention full of snakes.
SOLUTION: A nest might do nicely. But even better? The biggest bird nests on the planet, climate-controlled structures so big they pull down trees.
Squatters, mutters a bird known as a sociable weaver. Bums. Miscreants. All of them. Invading anuses and mouths and sponges. Try building something for a change. And off the busy little bird flies to grab yet another twig to help construct the most incredible nest on Earth—hands down. I’m talking twenty feet long. Thirteen feet wide. Seven feet thick. All told, it’s two thousand pounds of nest that’s home to up to five hundred birds living in one hundred chambers. Really, it’s not a nest. It’s a mansion so big that it’ll sometimes topple the tree it’s built in. But should the nest avoid that fate, it can stand for a century in the plains of southern Africa, serving as a home for generation after generation of sociable weavers.
There are no queens here to boss everyone around. This is an anarcho-syndicalist commune, baby, through and through. The birds work together, scouring the landscape for twigs to weave into the massive structure, as well as grass to line their chambers, which actually aren’t interconnected. Instead, each has a single entrance at the bottom of the nest. Situating the chambers like so helps keep the rain out and makes it that much harder for snakes and birds of prey to infiltrate the structure. Interestingly, though, while the sociable weavers will team up to chase away invading pygmy falcons, at times they’ll let the predators move in. Sure, the falcons might pick off a weaver from time to time, but keeping them around has its benefits, too: They prey on snakes. Think of it as a protection racket, however fragile the alliance may be.
As with pistol shrimp society, what seems to hold the weaver group together is kin selection. By working cooperatively to construct such a huge nest, the weavers have advantages over birds that individually build small nests. With such a mansion, weavers enjoy excellent insulation in addition to protection from predators. Nighttime temperatures in their habitat can plummet to below freezing, but by huddling together in their chambers, they can crank up the thermostat to a toasty 70 degrees Fahrenheit. And when things heat up during the day, the nest retains the coolness of the previous night.
WAITER, THERE’S SPIT IN MY SOUP
Asia has a bird whose nest may be dwarfed by the sociable weaver’s, but it’s far more valuable. The edible-nest swiftlet constructs a treasured little home that sells for a thousand dollars a pound in China, where it’s made into soup that’s said to have medicinal properties. Only a chucklehead would pay a grand for a bundle of sticks, though, right? These turn out to be very special structures: Instead of building with sticks or grass like other birds, the swiftlet uses its sticky spit to spin a cup-shaped nest on cave walls. The nests are so sought after that in Indonesia, folks are spending as much as sixteen thousand dollars to build three-story concrete nurseries for the swiftlets. The creatures come for the swiftlet calls booming out of CD players, and stay for the food and climate control via a network of misters.
Here’s t
he problem, though. Unlike the pistol shrimp, all of these sociable weavers are plenty capable of breeding. And while it makes sense to work cooperatively with your family for the chance to pass down your shared genes, first and foremost you need to think about yourself. While a weaver’s relatives are all flying around, busting their butts to build the structure as a whole, it’s mighty tempting for an individual to instead invest time in maintaining its own chamber, boosting the chances of its own survival and that of its young. Or a bird could slack off entirely, focusing on foraging so it can grow big and strong, or mating so it can pass down even more of its genes. Thus we’d expect more of these selfish genes to show up in the population, since there must necessarily be more offspring of self-centered individuals. And indeed, among the sociable weaver colony there are the cheaters. Some sneaky chumps think they can get away with not pitching in, enjoying the benefits of the nest without contributing their fair share.
But maybe they can’t for long. In captivity, it seems enforcers have formed gangs to pursue a slacker, catching it and giving it a good pecking. We should, of course, be somewhat wary of these accounts, though. Animal behavior changes dramatically in captivity, plus the sociable weavers in this particular case were in a small enclosure with little room to flee. But out in the wild, scientists have observed plenty of chases, which could serve to keep the slackers in line. The birds aren’t getting evicted, mind you, just getting a reminder of their place in the society. And after spending some time away, they return and resume work on the nest. The aggression is bad enough that the chance of injury becomes too great, outweighing the costs required for the birds to contribute their fair share. Sure, they may still at times selfishly focus on their own chambers to help guarantee their own survival, and therefore the continuation of their genetic lines, but they’ll also contribute to the nest at large to ensure the success of the society as a whole, and therefore the continuation of their genetic lines by way of their kin.