Flexible Fins Allow Fish to Climb Trees

June 23, 2016 | Erica Tennenhouse

mudskipper in a tree
Photo credit: Klaus Steifel/flickr (CC BY-NC 2.0)

Fish out of water… and up in a tree!

The sight of a fish climbing a tree may seem peculiar. But for some fish, this behavior is nothing out of the ordinary.

All mudskippers spend most of their lives out of water and can walk on land, but only a few species of this fish are known to climb. The slender mudskipper, for example, is able to climb very steep inclines of trees and rocks to find food or sunbathe.

To figure out whether the climbing mudskippers possessed certain physical adaptations that were absent in the non-climbing species, a team of researchers collected slender mudskippers and blue-spotted mudskippers, a species that does not climb, from the coastal area of Central Java.

A key difference between the two species is that the slender mudskipper’s pelvic fins (the fins on the lower surface of the body) are split, giving the appearance of fin-like legs protruding from either side of the fish. In the blue-spotted mudskipper, these fins are fused together under the body.

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The researchers first tested the amount of suction and friction that each species was able to exert on a surface. They attached gauges to their pelvic fins and then either lifted the fish off a surface (to measure suction) or pulled them along the surface (to measure friction).

Slender mudskippers exerted greater friction, which the researchers proposed might reflect their need for stronger attachment parallel to the direction of movement when climbing trees vertically. Blue-spotted mudskippers, on the other hand, had better suction, perhaps because they commonly attach to rock faces where they must resist to the push of strong waves.

Next, the team examined the mucus that the mudskippers secrete from their undersides to see if it might help the climbing species stick to surfaces. The slime was found to be highly adhesive for both species, meaning it was likely not the key to the slender mudskipper’s propensity for scaling up trees.

Finally, the researchers analyzed how each species’ pelvic fins move along a surface. Slender mudskipper fins are considerably more flexible than those of the blue-spotted mudskipper, allowing them to make more intimate contact with their climbing surfaces. The researchers note that this superior contact serves to bolster the adhesion and friction exerted by slender mudskippers, which enable them to climb with ease.

Bambang Retnoaji and Parvez Alam, co-authors of the study that will soon appear in the journal Zoology, told The Science Explorer, “It is most interesting that not one, but several adaptations are necessary in the development of a fin that can be used relatively proficiently for tree and rock climbing.”

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Editor's note (June 24): The authors' names were mispelled in the original version of this article. We appologise for any confusion this may have caused.

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