While it may seem impossible, the fact is that some species of fish can climb trees. These species have a range of unusual adaptations that allow them to breathe outside the water and use their fins to grasp or even climb tree roots.
The BBC labels the mangrove killifish (Rivulus marmoratus), found among the mangroves in Florida, Latin America, and the Caribbean, "The most extreme fish on Earth." For starters, this is the only vertebrate animal that is known to fertilize its own eggs. There are males and females in the species, but most of these little fish are hermaphrodites. These fish are short-lived; some survive for only three months, making them the world's shortest-lived vertebrates.
Mangrove killifish are able to alter their gills to be able to live out of the water. When the water around the mangroves dries up, these fish climb up into the trees and hide in logs until the water returns. While on land, they're able to waddle around by lying flat, then lifting their heads and straightening their backs, pushing their tails down. They push so hard that they actually propel themselves up and forward, "flying" through the air. Once it's safe, they change their gills back and venture back to the water.
The family of Anabantidae, also called the climbing gouramis or climbing perch, is a great addition to any home aquarium. They hail from Africa and Southern Asia, and while they look a bit like perch, they are not close relatives.
All of the 34 types of climbing gourami are labyrinth fish, meaning that they possess a special labyrinth organ adapted to breathing air. Ordinarily, climbing gourami live in freshwater—but if the water they live in dries out, they will climb out and travel in search of a new home. The climbing gourami's gills are spiny, and the climbing perch can use them (as well as its anal fin) to even climb up trees.
The most recent discovery by scientists is the Lithogenes wahari or climbing catfish. This member of the catfish family can actually grasp with its pelvic fin. Specimens have been found clinging to rocks, but it's not a stretch to think that they could climb trees too. This fish also has a sort of bony armor that protects its head and tail.
L. wahari was discovered in 2009. According to Science Daily, "Lithogenes wahari shares traits with two different families of fish: the bony armor that protects its head and tail, and a grasping pelvic fin that allows it to climb vertical surfaces."