Thanks to Brilliant for supporting SciShow.

You can check out Brilliant.org/SciShow to check out their Daily Challenges and get 20% off an annual premium subscription. [♪ INTRO]. Usually, you think of “aquatic” and “arboreal” creatures as totally separate animals, as different as octopuses and orangutans.

But evolution doesn’t always result in things that fit into our narrow categories. A number of species have evolved unique physical features and behaviors that helped them make the leap from rivers to redwoods, or oceans to evergreen trees. Like some crabs.

And also… fish. If you think of crabs, you likely picture them scurrying along a shoreline, swimming in a sea, or hanging out in some kind of freshwater habitat. But one crab from Southern India lives exclusively in trees.

It was described in 2017, in a paper published in the Journal of Crustacean Biology, and those authors described this crab as not only a new species, but a new genus! The animal was identified thanks to the expert knowledge of the region’s Kani tribes, who alerted scientists of the crustaceans’ existence. That’s how the crab got its Latin name, Kani maranjandu, which includes the local words for “tree” and “crab”.

As for how these crabs live in trees, they have long, spidery legs that end in a sharp hook, so they can wrap around branches and grab trunks with a firm grip. And although it’s not clear why they evolved this way, they’re making the most of it. They live in hollows as high as 10 meters up, and feed on slugs, worms, insects and also leaves and seeds.

They also take advantage of water trapped on leaves and in tree cavities in order to live without having to travel to a body of water! I mean, who needs rivers and ponds when you have your own plant-puddle paradise to hang out in? Alternatively, you could have your own gigantic tree canopy, if you’re a wandering salamander, that is.

Like many of their relatives, wandering salamanders are commonly found in moist, terrestrial habitats. And a few salamander species migrate up into trees at certain times of the year. But some populations take up residence high in trees, year-round.

And we’re talking way up. Wandering salamanders make their homes in the world’s tallest forest canopy, living on the horizontal branches of giant redwood trees! This seems like it wouldn’t work, since most salamanders need a moist environment to keep from drying out.

But redwood branches can be two meters wide and are often covered in fern mats that develop a thick soil layer over time. Pieces of bark, broken-off twigs and branches and other debris add to this rich organic layer, making these tree limbs mimic the forest floor. And that’s a perfect habitat for moisture-loving salamanders.

But considering some of these salamanders have been found more than 80 meters up, the big question is, how did they get there in the first place? Well, they have prehensile tails that can grab onto branches, much like monkeys and opossums do. They also have long limbs and special toe pads that give them the grip they need to climb sky-high.

Wandering salamanders have been found in redwood canopies during the whole year, so it’s safe to assume they’re not trekking down to the ground for long periods of time. Instead, these salamanders hang out in the canopy, munching on juicy springtails: wingless, soft-bodied creatures that look like insects and are found in high densities in redwood canopies. So if you’re a wandering salamander, there’s just no need to leave your high-rise home.

Now, compared to towering redwoods, mangrove trees might not be very impressive in stature. But for a fish, clambering up out of the water into the lower parts of a mangrove forest, even only once in a while, and then staying there for days is a pretty remarkable feat. Mangrove killifish are found in the salty waters of tropical mangrove forests from Florida to South America.

And they’re actually amphibious, surviving both in the water and on land. In fact, killifish can survive more than two months out of water, as long as it stays moist enough. Now, the problem with most fish is that maybe ironically, they just can’t get enough oxygen on land.

When they’re out of water, their gills collapse, so there’s not enough surface area to take in the oxygen they need. But killifish can adapt. For one, they can expand their blood vessels in their skin to allow more blood to flow around and to allow for more gas exchange.

And long-term, they can even develop new vessels. When they’re exposed to air, they also increase the amount of hemoglobin in their blood, which lets them carry more oxygen around their bodies. Now, you might expect these fish to have unique adaptations like specialized fins to accommodate amphibious locomotion.

But strangely, they don’t have any obvious physical features that make land travel easier. Instead, they seem determined to hurl themselves out of water, using movements that researchers describe as “launches, squiggles, and pounces”. Killifish take to the trees for a number of reasons.

They might be trying to get away from other fish, or to escape poor water conditions, or sometimes to pursue a meal. And it’s not just a few fish here and there. Researchers have found up to 100 fish in a single log!

Breaking the mold from their aquatic relatives, these incredible tree-loving animals have developed unique ways to move up in the world. Whether it’s spidery legs, finger-like tails, or a determined wiggle, these adaptations are examples of just how flexible and unexpected evolution can be. If you like learning about topics like this, you might want to try out today’s Daily Challenges from Brilliant.

There are multiple new challenge questions every day that cover all kinds of science, and every challenge gives you all the context you need to solve it, no prior experience required. Then, if you really like a question, you can learn more about the topic in one of Brilliant’s related courses. If you want to try today’s Daily Challenges for free, go to Brilliant.org/SciShow, and see how you do!

And if you want to sign up for an annual Premium subscription, you can also get 20% off at that URL.