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Zebrafish are basically aquatic lab rats. They're a model organism that scientists use to study everything from development to disease, in the carefully controlled environment of a fish tank.

Now, researchers are also looking into how these small, stripy minnows see, and they published a paper about it last week in the journal Current Biology. This study is the first detailed description of how vision works in a vertebrate that has 4 kinds of color photoreceptors, not a measly 3 like humans. And they found that, as babies, these tiny fish have really precise vision that’s more than meets the eye.

In fact, it’s tailored to the different parts of their environment. See, zebrafish are naturally found in clear, slow-moving water in Asia. They rely on their eyes to find their way around, hunt, and avoid getting eaten.

And baby zebrafish are like all eyes — their eyeballs make up a quarter of the total volume of their bodies and contain half of the neurons in their central nervous system. If our babies had similar proportions, their eyes would be like grapefruit sized. This research focused on the retina, which is in the back of the eyeball where light-sensitive photoreceptors are found.

There are two main types of photoreceptor cells: rods detect low levels of light but don't really detect color, and cones work better at bright levels of light but can detect color. In human retinas, rods and cones are pretty evenly distributed, except for a region called the fovea where there are more cones and our vision is a little sharper. But baby zebrafish retinas are more complicated.

While human retinas have 3 types of cones — one sensitive to red wavelengths of light, one to green, and one to blue — zebrafish have a fourth that’s sensitive to UV light. And in their eyes, different photoreceptors are concentrated where they’re needed most. Baby zebrafish hang out in shallow water where death typically comes from above, whether it’s a heron or other predator silhouetted against the bright sky.

So the part of the retina that gets light from above has more rods, because they just need to see if anything’s coming — not what color it is. Most colorful things, like rocks or plants, are in front of or below baby zebrafish. So those areas of the retina have more cones, especially the ones sensitive to red, green, and blue light.

But the single-celled microorganisms that baby zebrafish eat are translucent, which makes them hard to see with those cones. Except they do reflect a good amount of UV light. So the parts of the retina getting light from the strike zone, which is just a cool term researchers use for the horizon of the fish’s vision, have more cones sensitive to UV light.

Basically, all these retinal regions chunk up the world, so different parts of the image have boosted color or contrast like Photoshop manipulation of the world in real time. And this probably plays a big role in helping them survive, otherwise they wouldn’t have evolved to be this way. When baby zebrafish mature, though, they grow into their eyes, move into deeper water, and kind of lose their vision superpowers.

Their retinas change to be more like ours, where different cones are redistributed in a more even pattern. But not all fish have good eyesight! The baby whale fish sounds like a poorly named bathtub toy, and I guess it’s kind of stubby and cute and whale-like.

But unlike zebrafish, it lives in muddy rivers in Central Africa where visibility isn’t so great. So instead of relying on its eyes, it uses electricity to get around. And according to new research published in the journal Current Biology last week, knowing how these fish use electricity may give us insight into some serious human diseases.

Several species of fish use electricity to communicate and sense what’s nearby, kind of like echolocation. And to do that, they have electric organs with specialized cells called electrocytes, which can generate action potentials like muscle cells and nerve cells. Action potentials occur when ions rapidly cross cell membranes and alter the electrical charge.

And when a whole bunch of electrocytes do this simultaneously, they can emit an electrical discharge. But ions can’t cross the cell membrane just anywhere. They have to pass through special proteins called ion channels, which are like carefully controlled doors.

Now, baby whale fish can generate extremely quick electric pulses that are only 2 ten thousandths of a second long. They have to keep it short so they can stay hidden from catfish, which can also sense electricity and will totally eat them. So these researchers wanted to know how baby whale fish make these extremely short action potentials, so they took a closer look at the KCNA7 potassium ion channel.

Humans have this same protein in our heart and muscle tissues, but the baby whale fish has a slightly different version. The researchers discovered a negatively charged patch of amino acids in the fish’s ion channel that makes it more sensitive and able to open more quickly. Understanding how slight differences in these ion channels affect how they work might someday help scientists treat diseases like epilepsy, and certain heart rhythm and muscle disorders.

These conditions are linked with genetic mutations that also change how potassium ion channels work. So maybe understanding this odd little fish will help scientists find a fix. Thanks for watching this fish-filled episode of SciShow News!

If you’d like to stay up to date on the latest research in all fields of science, from aquatic vertebrates to human medicine, we post these News episodes every Friday! Or you can stick around for all of our videos including the hundreds we’ve already uploaded at youtube.com/SciShow where you can subscribe. [ ♪ Outro ].