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It's Valentine's Day! Today, people all around the world are celebrating love and relationships, which means that this episode is all about bacteria.

Specifically, the beneficial relationships, called mutualistic symbioses, between bacteria and animals. In nature, you can see lots of symbioses with the naked eye, like clownfish and sea anemones or bees and flowering plants. But there are also plenty between animals and bacteria that you would need a microscope to observe.

And like, no offense to Beyonce and Jay-Z or whoever, but they might be some of the coolest relationships out there. Because in exchange for a nice place to live, many bacteria give their animal companions superpowers, like immunity to poison, or invisibility. So, in honor of Valentine's Day, here are five of the coolest animal-bacteria partnerships.

Unless you grow plants, you probably haven't thought a lot about aphids lately. They're the little insects that suck the fluids out of vegetation and transmit plant viruses, and although they're major pests, they're actually pretty impressive. There are more than 4000 known species throughout the world, and they reproduce ridiculously fast.

An aphid can have about 80 babies in just 10 days. In fact, while they can mate, most are essentially born pregnant through a kind of asexual reproduction called parthenogenesis. This makes aphids great at multiplying quickly and damaging lots of vegetation.

But their diet also isn't that great for them:. The juices they eat lack many of the essential amino acids they need to survive. You'd think that the response to this would be to eat a more varied diet, but the aphids found a more creative, and lazier, solution: Many have teamed up with bacteria that make amino acids for them.

They're from the genus Buchnera, and they're passed down from generation to generation while the insects are still embryos. These bacteria produce some of the missing amino acids themselves, like tryptophan. And in other cases, it appears that molecules from both the aphid and the bacteria work together to synthesize the molecules.

Scientists estimate that this partnership began up to 250 million years ago, possibly after bacteria living in aphids' guts made themselves indispensable. And now, neither organism can live without the other. The bacteria allow aphids to survive on meals that would be impossible for other insects.

And the aphids give the microbes food and a safe place to live:. They have special cells called bacteriocytes that they use to house their microscopic buddies. This relationship is so successful that the bacteria have even jettisoned a large chunk of their genome, including genes for responding to environmental changes and for building strong cell walls.

Cuz, like, if you already have a nice house and a stable environment, those genes are useless. Now, these bacteria are down to around 500 genes, while bacteria that live independently usually have at least 1500. So the next time you hear about aphids going to town on crops or your friend's houseplants, or my cherry tree, know that it is not the insects: It's a whole power couple.

Competition for food can be fierce in the animal kingdom, especially in harsh environments like deserts. There generally isn't a lot of food to go around, and the stuff that does exist, like plants, doesn't really wanna be eaten. Desert plants often have defenses like spikes or poison that keep most would-be diners away.

One example is the creosote bush. It contains a toxic resin that can cause severe liver and kidney damage. Except, some animals seem to be more or less immune:.

For decades, scientists have observed desert woodrats voluntarily eating the plant. By itself, this isn't that weird. Some animals can resist toxins.

That's, like, normal evolution stuff. What was weird was the discovery, published in 2000, that not all desert woodrats, even within the same species, had this immunity. Woodrats living in Southern California's Mojave Desert, where the creosote bush grows naturally, were able to eat a lot of the plant without getting sick.

But woodrats living farther north in the Great Basin Desert, where the bush doesn't naturally grow, got sick and lost weight after being fed the plant in experiments. It took more than a decade before researchers figured out why. In a paper published in 2014 in the journal Ecology Letters, scientists revealed that it wasn't the rats themselves that were different, it was their gut bacteria.

When Mojave woodrats were treated with antibiotics, their superpowers disappeared, and they could no longer eat the creosote bush without getting sick. On the flip side, when the Mojave gut microbes were transferred to the Great Basin rats, the northern woodrats gained the ability to eat the plant. Scientists still don't know exactly what type, or types, of microbes are necessary for this immunity.

Given that creosote bush resin contains several toxic compounds, it's likely that many microbes work together to protect desert woodrats, and that they're passed around through coprophagy, a fancy word for eating poop. But whatever they are, they're doing a great job. And in exchange for helping the woodrat stay alive, the gut bacteria are rewarded with a steady stream of nutrients whenever that rat eats food.

Life somehow manages to exist in all kinds of bizarre places, including the bottom of the ocean. When scientists first observed deep-sea hydrothermal vents in the 1970s, they were amazed at the density and variety of life clustered around them. In particular, the deep-sea clams they found, now called vesicomyid clams, were much larger and more numerous than they had expected.

After all, most clams are filter feeders, getting their nutrition by straining tiny organisms like plankton out of the water. And down at the bottom of the ocean, clam food is in short supply. Today, we know that vesicomyid clams manage to live down there, as deep as 6800 meters, with the help of symbiotic bacteria.

The clams have extra-large gills containing lots of bacteriocytes, which house microbes that oxidize the sulfur pouring out of the hydrothermal vents. The bacteria harness the sulfur's energy to support both themselves and the clams. And in return, they get a safe place to live.

Kind of like with the aphids' bacteria, these microbes ditched the genes related to cell structure and movement, and they haven't been found anywhere outside the clams. Also like aphids, these clams acquire their bacteria before they're born. Bacteria from a mother clam is housed inside her eggs, so that when the offspring hatch, they're already equipped with the partners they need to survive.

Most clams, and most other living things, could never survive at the bottom of the ocean, so far from sunlight. But with the help of these special bacteria, this clam has carved out a niche in an incredibly competitive ocean. Another superpowered ocean-dweller is the blue-ringed octopus.

This animal is super cute. It weighs less than thirty grams and would probably fit in your hand, but I wouldn't recommend picking one up, because it is also one of the most venomous creatures on the planet. Its venom contains a potent neurotoxin called textrodotoxin, or TTX.

It's about a thousand times more toxic than cyanide, and it works by blocking the sodium channels on nerve cells. This stops the cells from firing, which means they can't tell muscles to move. That means, like, the muscles that move your lungs, and that would lead to respiratory paralysis and death within minutes.

Which seems kind of like overkill when you consider that the blue-ringed octopus mostly uses this venom on tiny crabs and mollusks. But that's beside the point. What's especially interesting is that the blue-ringed octopus doesn't produce its own venom.

Instead, some researchers think it's produced by symbiotic bacteria in the octopuses' salivary glands. That being said, there is some controversy about this, and not every study has been able to grow TTX-producing bacteria from those glands. But many scientists are on board with the idea.

Scientists know of several bacteria that produce TTX, and from what we can tell, the blue-ringed octopus seems to have established a partnership with several of them. A paper published in the journal Marine Biology in 1989 identified at least six different strains from several different genuses, including Alteromonas, Bacillus, Pseudomonas and Vibrio. Scientists aren't sure how exactly this partnership began or the exact benefits that the bacteria get.

At the very least, living inside an octopus likely gives them protection from predators, which is a definite plus. As for the octopus, having a toxin is no good if you get poisoned yourself. So like other animals that use TTX, including pufferfish, the blue-ringed octopus also has evolved resistance to the toxin.

Among other things, its nerves have a different type of sodium channel that TTX doesn't disrupt. So these little animals are able to go about their lives paralyzing others while staying, themselves, quite safe. Finally, life in the ocean can be rough, especially if you're a Hawaiian bobtail squid.

These critters are only 3 centimeters long when fully grown, so they're a perfect snack for larger predators. They also search for food at night, which is risky. Light from the stars and moon filter down through the ocean water, which means the squid's silhouette is easily visible to predators swimming below it.

At least, it would be, if the squid couldn't become invisible. It's all thanks to the help of Vibrio fischeri, a type of bioluminescent bacteria. The squid keeps these microbes in a special light organ in its mantle, the top part of the body that kind of looks like a hat.

Then, when night falls, the luminous bacteria mimic the starlight and moonlight shining down from above, making the squid invisible from below. By simulating different amounts of moonlight in experiments, scientists have even shown that the squid uses special tissues to regulate the amount of light it gives off. That way, it can mimic what's coming from the sky.

So anyone passing underneath would have no idea that they're missing out on their next meal. Bobtail squid aren't born with these bacteria, but Vibrio fischeri are found in abundance in their habitat. And as the squid draws bacteria-laden ocean water into its body, the microbes get stuck in a special type of mucus in its light organ.

Unlike some of our other examples, this relationship isn't totally harmonious, though. To prevent overgrowth of the bacteria, the squid has to expel up to 90% of them every day at dawn. But the lucky ones who remain get to hide from predators.

And there's even some evidence that the squid provides them with nutrients so their numbers are replenished by nightfall. This partnership is obviously really useful to the squid, but it's also proving helpful to researchers, because Vibrio fischeri is closely related to other bacteria that can cause diseases like cholera. Scientists are trying to learn how Vibrio fischeri has developed such a friendly relationship with the bobtail squid.

Ideally, they'll be able to use that knowledge to answer questions about what makes a bacterium beneficial or dangerous. So, for squid, bacteria, and human scientists, this one is a win-win-win. Bacteria are best known for causing diseases, and even when they're useful, they don't always seem that cool.

But for a lot of animals, teaming up with microbes allows them to survive in places they'd never be able to live alone. And although they don't necessarily give you what you would call superpowers, you can thank bacteria for a lot, too. The microbes throughout your body help you produce vitamins, digest food, and regulate your immune system.

So this Valentine's Day, maybe send a little love to your bacterial buddies. And speaking of partnerships, if you want another way to team up with us and support free educational content online, you could become a SciShow channel member! We could get into that relationship.

It's Valentine's Day. Click the little join button, that's how we know you love us. You'd be helping us make more episodes like this.

You'd also get a bunch of cool perks. Things like exclusive emojis for live chats, a public badge that appears next to your name when you comment, and access to anything we post in our community tab. That's a symbiotic relationship if I've ever seen it!

SciShow will always be free to watch, but if you want another way to support the channel, you can click on that "Join" button below this video or on our channel page. And to all of our current channel members and Patreon supporters, thank you! [♪ OUTRO].