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Hank Green: What you're about to see in this SciShow Dose may shock you.  There are about 8600 animals in the world that are electroreceptive.  They can feel electric fields.  Like the sense of touch, eletroreceptivity carries information about the shape, size, and texture of things in the environment, and like hearing or vision, it works at a distance, but what it actually feels like is something that we will never be able to even imagine.  

For electroreceptive animals, electric currents and electric fields are as real and immediate as color or music are for us.  Most electroreceptive animals are amphibians or fish, though there are rare exceptions, including the duck-billed platypus, because everything that could possibly be weird about the platypus is weird.  Electroreceptivity has evolved over and over again, totally independently, in a global act of convergent evolution.  Animals that live and hunt in murky water where they might not be able to see or hear very well gain an advantage if they can feel their surroundings through special electroreceptive cells in their skin.  And a few animals take it even further.  There are 716 known species of fish capable of electrogenesis, the creation of electric fields and even strong electric shocks.  And these fish don't  have a common ancestor either.  Biologists believe that electrogenesis has evolved separately at least 11 different times in places as isolated from each other as the African interior, the Amazon basin, and the coral-rich waters of Australia.  

The most famous electric fish is the electric eel, which is electric but is not an eel--it's actually a kind of knifefish that lives in South America and can produce an electric shock up to 600 volts -- enough to hurt, but not seriously injure, something as big as a human, but if you're an itty bitty tiny fish trying to go about your daily business of swimming and eating and not dying, yeah, 600 volts is the last problem you'll ever have.  

All cells in living things produce an electric charge; it's a normal part of cell biology.  They do this by pumping positive ions of metals like sodium, potassium, and calcium outside the cell membrane.  So the outside of your cells are slightly positive compared to the insides.  Every cell in your body has a resting voltage of about 0.085 volts.  Of course, my cells aren't organized in a way that would let me add those voltages together, but these fish have special cells called electrocytes, they're stacked in long chains like the batteries inside a flashlight.  And each one of those cells is connected on one side to a nerve fiber.  When an electric eel charges up, its brain sends a signal down those nerve fibers.  When the signal hits the electrocytes, little pores open up in their cell membranes, allowing those positive ions to rush into the cell.  Each cell becomes like a little tiny battery.  Instead of having a negative inside and a positive outside, the cell now has a negative left side and a positive right side.  And just like in a battery, electrons flow between the two ends to equalize the charge.  Since the cells are stacked in chains, positive to negative to positive and so on, the voltage of each cell adds up with every other cell in the chain.  String enough of those cells together and fire them all at once, and bleaughh, underwater barbecue.  

But this arrangement of electrocytes doesn't have to be used offensively, and in most electric fish, it isn't.  Instead, they create a constant electric field around themselves, usually with the power of only a few millivolts.  They use this to sense their immediate environments so they know when food or predators are nearby, even in total darkness.  And some species of fish use these electric fields to like, communicate with each other. Using distinct patterns of discharges, they can signal aggression, submission, alarm, even courtship.  When they're ready to mate, the males and females of many kinds of electric fish will perform electric duets.  Electric boogie!  

So if you feel like you're missing out, you might be wondering, why didn't we evolve to create awesome electric fields that we can control with our minds?  Well, because we can see and hear.  Seeing and hearing may seem kind of boring compared to being like, electric fish Magneto, but they use a lot less energy than electrogenesis.  Plus, one of the most common questions people ask about electric eels is 'why don't they electrocute themselves?' The answer being that they do.  When an electric eel fires, you can actually see it flinch. I don't know about you, but I think I'll stick with my senses since they don't actually cause physical pain to use.

Thanks for watching this electrifying SciShow Dose, and if you'd like to help us keep sharing natural wonders like this, go to Subbable.com/SciShow to find out how you can help us keep our batteries charged, and don't forget to go to YouTube.com/SciShow and subscribe.  

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