Thanks to Brilliant for supporting  this episode of SciShow.

Go to Brilliant.org/SciShow to see if  you can solve today’s Daily Challenges. [♩INTRO]. We often think of the ocean as, like,  a very serene, lovely place to relax.

But that’s only because we’re oblivious  to the racket that is going on underwater. From sea urchins rasping on  rocks to whales bellowing, there’s a cacophony of sound in  the ocean — and for good reason. Water’s densely-packed molecules  make it a great sound conductor.

In fact, sounds travel more than four times  faster and farther in water than in air. But, don’t expect to hear that from the beach. When sound waves from underwater  hit air at the surface, they reflect back into the depths.

And even when we’re swimming, our ears  aren’t adapted for underwater hearing. That said, the incredible sound  transmission underwater makes the ocean a great place for marine animals to communicate, or even use sound to defend themselves. And while most people are familiar  with whale songs and dolphin chatter, there are some unlikely  culprits, including these six.

First up are, maybe unexpectedly, seahorses. You might not think of them as loud, but they make two kinds of  sound that are hard to ignore. The first is a growl.

It’s only been recorded in captive seahorses, but researchers think this warning sound  is likely used in the wild as well. It comes from the animals using muscle  contractions to vibrate their bodies particularly, their cheeks. And at 115 decibels, the growl  is as loud as an ambulance siren.

Researchers think the point of the  sound might be to scare away danger and give the seahorses a chance of escape. I would be afraid of a 115 decibel growl, even if the thing that made it was like this big. Specifically, if a predator has captured  a seahorse, these low, long pulses, along with the body quivers,  might make them let go.

That’s only half the story  with these animals, though because when seahorses are courting, they also make another distinctive  noise: loud clicking sounds. They’re not as loud as their  growling, but at 105 decibels, the clicking still rivals  a car stereo at max volume. The seahorses make the sounds with their skulls.

As an amorous seahorse rubs  its own skull bones together, it generates the clicks each one a low-frequency sliding sound  followed by a higher-frequency knocking. Males and females even make  duets of headbone clicks during their complex array  of courtship behaviors. And while it’s not clear  how this behavior evolved, a study found that female seahorses  click more often than males.

So it may have something to  do with sexual selection, where males choose a female  based on how “clicky” she is. Next, many ocean fishes can make  some impressive sounds of their own, using their swim bladders: gas-filled  organs that help a fish maintain buoyancy. A swim bladder is like a  personal balloon that expands and contracts as it lets air in and out.

But some fish also use these big  balloons for their acoustic properties like a group called toadfishes. The male toadfish guard their  nests and, during the night, they use muscles around their  swim bladders to vibrate them, creating a call to lure in females. [toadfish grunts]. This call involves a grunt like a foghorn  followed by higher frequency boops.

And it can reach 138 decibels, which  is louder than a jet taking off. Each male toadfish has a  signature style and voice, which is a key part of using  sound for communication. Like, their grunts vary based on the  number of boops, length of boops, and pauses between them.

And these signals might convey  information that helps females decide whether he’s worthy of her attention. Researchers hypothesize that the  males might be signaling things like their age, health condition,  or level of motivation to mate. Or maybe even their size!

For instance, a 2010 study on Lusitanian  toadfish found that larger males vibrated their swim bladders faster,  thanks to bigger and healthier muscles. Now, one toadfish is loud enough on its own. But when fishes with sonic swim bladders group up, the volume goes through the roof.

Just ask the Gulf corvina fish. Every spring, all the adults  congregate in one spot: the Colorado River Delta  in the Gulf of California. Unlike the toadfish, who spread  out into different burrows, the corvinas cluster into a  huge swimming, spawning group.

Spawning aggregations like this are  thought to increase breeding success by concentrating males and females  in a known place at a known time, based on natural cues. Like, in this case, Gulf corvina might  be taking advantage of the extreme tides in the area at certain parts of the lunar cycle, which could wash their eggs out to sea. Guided by the tides and moon cycles, adults arrive within a few days of each other.

And that’s when the shouting starts. [gulf corvina grunts]. Biologists compare the gathering  to a cocktail conversation, with males trying to be heard  by females over the din. In a school of thousands or millions  of fish, the male corvinas generate 177 decibels of sound —  something louder than a shotgun.

In fact, these sounds are so loud  that they lead to overharvesting. They actually make fishing boat hulls buzz, sometimes like a massive swarm of bees. Which while it might be super startling  at first, does eventually provide a really great way for commercial  fishers to hone in on their targets.

Now, communication is a huge  part of underwater sounds. But lots of ocean animals also use  sound for defense — like spiny lobsters. Lobsters are arthropods, in the  same large group as insects.

And many arthropods make sounds by rubbing parts of their hard exoskeletons together,  including grasshoppers and locusts. But, spiny lobsters have taken a novel approach. They rub a piece of soft  tissue called the plectrum at the base of their antennas  against an area by their eyes.

As the plectrum sticks and  then slips on this area, it works kind of like a sticky  violin bow and makes a rasping sound. The ability to use soft tissues  might be especially helpful to a recently-molted lobster that’s just  outgrown and shed its old exoskeleton. During this time, the animal  is especially vulnerable since its new body covering hasn’t hardened yet.

But even though it’s lacking that defense, it can still make the same  warning sound as everyone else. Also, if you’re thinking that soft  tissue can’t generate that much noise, you’d be surprised. At 150 decibels, this lobster’s  warning is as loud as a fighter jet.

And like many animal warning sounds, it’s abrasive enough to help drive  off predators that get too close, along with other defense behaviors  like lunging or flipping their tails. Also, as a nice bonus, this sound fades into the loud  background noise pretty easily, too so it can deter predators, but also  doesn’t draw much attention to the lobster. Next, even an ocean worm can make itself heard.

Snapping worms live inside sponges  more than 100 meters underwater, off the coast of Japan. They have see-through bodies, with  tentacles surrounding a circular mouth. And when it comes time for the worms to defend the narrow sponge chambers they call  home, their mouths come in handy.

The worms will run into each other, mouth-first, and then will burst away with  a startling popping sound. It’s hard to figure out  exactly why the sound evolved. Like, it might have been  to startle the competitor, or could just be a side effect of the attack.

But either way, it’s definitely noticeable:. At 157 decibels, the sound is louder than the fans at the loudest sports stadiums in the world. Scientists recording these interactions  are also still figuring out how the worms make such a loud sound, but one idea  is that it has to do with part of their digestive tract, and a process called cavitation.

Cavitation happens when part of a  liquid experiences a big temperature or velocity change, which causes  a dramatic change in pressure. It’s so dramatic that the liquid can  temporarily go from a liquid to a gas, forming a bubble. Then, when that bubble encounters  the rest of the liquid, the gas turns back into a  liquid with an intense pop.

In the case of the worms, scientists  think they use their muscles to block off a piece of their digestive  tract — one that’s full of water. And then, they use other muscles  to expand that little chamber, changing the pressure in there so  fast that the water becomes a gas. And finally, they open up that  part of their digestive tract and let the chamber collapse.

When they do, that gas  converts back into a liquid, and the water shoots out of  their mouths at the other worm creating the pop, and also  sending them flying backward. Now, cavitation isn’t unique to these worms. If you’ve ever been around a loud boat  propeller, you’ve probably experienced it.

But the fact that these little  worms make so much sound, and probably with their little  internal water guns, is pretty special. Still, when it comes to cavitation, they can’t beat the pinky-sized snapping shrimps. These animals make repetitive cracking sounds that punctuate the tranquility of coral reefs.

In fact, the cracking is so  constant that in World War II, submarines took advantage of it to hide from sonar a technique that uses sound  waves to identify objects. [shrimp crackling]. The shrimp make this racket  by snapping their claws shut, forcing out a jet of water so fast, it  forms a low-pressure zone behind it. The water in that zone changes state into a gas, and then it’s basic cavitation all over again.

When that gas runs into the water around it, it converts into a liquid  again, and you get a pop. Except this time, it releases so much  energy that it creates a flash of light, along with a shockwave that kills nearby fish. And at 220 decibels, the collapsing  cavitation bubbles are even louder than the takeoff of the Saturn V rocket,  which carried astronauts to the Moon.

Researchers think this mechanism  may have evolved for feeding, but today, it’s also used to spar with rivals. Because if evolution gives you a weapon  like that, you might as well use it. The diversity of underwater sound is  incredible, and speaks to how much animals have evolved to take the water they  live in and use it to their advantage.

But also, be thankful human ears  don’t work so well down there! These sounds are so loud, that if our  ears were evolved to actually hear them, they could easily damage  nerves or even bust eardrums. So if you can, it’s best to put on a  snorkeling mask and just enjoy the view or the feel of the water without  eavesdropping on anyone’s conversations.

If you liked this episode of  SciShow, and are into learning more about the world around you, you might want to try your hand at  today’s Daily Challenges from Brilliant. Brilliant puts out multiple new  challenge questions every day about all kinds of topics,  from math to computer science. But the cool thing is that you don’t  need to be an expert in those fields to answer them; Brilliant gives  you all the context you need.

That makes them a great way  to brush up on your skills, or discover something new. And if you like a question, there’s a  related course to teach you even more. If you want to try out today’s questions,  head over to Brilliant.org/SciShow.

And if you decide to sign up for  an annual Premium subscription, you’ll get 20% off. [♩OUTRO].