Bizarre fish can walk and thrive on land

Imagine coming across a fish, one that looks uniquely out of place with its eyes sitting atop its head, breathing not underwater, but in air. Guess what? You don’t need to take a dive into the ocean to catch a glimpse of such a creature; the mudskipper already walks and roams our land.

Mudskippers among the mangroves

Nestled in the lush backdrop of Okinawa’s mangrove forests, where crabs scuttle and kingfishers swoop, resides a peculiar resident: Minami-Tobihaze, famously known as the barred mudskipper.

“They are fish, but they can walk and live partly on land,” explains Dr. Fabienne Ziadi-Künzli, part of the Nonlinear and Non-equilibrium Physics Unit, who led the recent mudskipper study.

Unique fish walking on land

Periophthalmusargentilineatus, the barred mudskipper, certainly is an oddball in the world of aquatic life. With the ability to absorb oxygen through their moist skin or their mouth, mudskippers don’t rely on lungs like terrestrial creatures do.

They sometimes take in a big mouthful of air and hold it there to ensure their eggs receive ample oxygen.

However, their fascinating adaptability doesn’t end there. They walk – yes, walk. Without limbs or digits, these amphibious fish have developed a way to use their fins, specifically their pectoral fins, for both swimming and walking.

Mudskippers’ movement, known as “crutching,” is as captivating as it is unexpected. “Instead of moving their pectoral fins alternately… the mudskippers swing their pectoral fins forward simultaneously,” says Dr. Ziadi-Künzli.

It’s much like how we would advance our crutches together when easing an injured leg.

Mystery within the fins

Following a project on the stability offered by the human foot’s shape, Professor Mahesh Bandi and colleagues brought their curiosity to the unique locomotion of the amphibious mudskipper.

The last comprehensive anatomical investigation of mudskipper fins dates back to the 1960s. With the scarce information on the adaptation of the mudskipper’s muscles and other soft tissues to terrestrial life, Dr. Ziadi-Künzli decided to take the plunge herself.

Thanks to the micro-computed tomography (µCT) at the OIST Core Facilities, she and her team took a closer look at these adaptations.

Studying mudskipper’s ability to walk

The µCT uses x-ray sourcing and microscopic detection to study soft tissue with better contrast – thanks to iodine. The team studied various fish, starting with the mudskipper and its close evolutionary relatives and the zebrafish for comparison.

Analyzing thousands of images from the µCT was no small feat. “We had to manually sort through all those images to identify each tissue,” recounts Dr. Ziadi-Künzli. However, their painstaking work bore fruit.

The images reveal…

The 3D images eventually unveiled several adaptations. For one, the pectoral fin muscles and the shoulder girdle they attach to were significantly larger in the mudskippers.

Additionally, some tendons connecting the bones in the mudskipper’s pectoral fins – their ‘walking’ fins – were replaced by fascia tissue. “We think this is an adaptation that helps the mudskippers to push themselves forward during walking,” explains Dr. Ziadi-Künzli.

The gravity on land also led to another modification in the mudskipper’s bodies. “There is a connection between the shoulder and the pelvic fin through a kind of joint that we don’t see in any other fish we scanned,” notes Dr. Ziadi-Künzli.

Changes were also noticed in their bones. “Usually pectoral fin rays are crescent-shaped if you look at a cross-section but, in the mudskipper, they were round near the fin ray base and then changed to crescent shape towards the tip of the fin ray,” says Dr. Ziadi-Künzli.

Significance of the walking fish

The revelations from this study have further sparked the researchers’ interest in deepening their understanding of mudskipper evolution. They are particularly intrigued about the rapid body and fin anatomy shift that takes place during the transition from larvae to adult stage.

In a collaboration for future research, Dr. Ziadi-Künzli and Dr. Ken Maeda from the Marine Eco-Evo-Devo Unit aim to study this intriguing metamorphosis.

“Using these virtual dissection tools gives us a whole new perspective on the anatomy of animals. After all, how shall we understand an organism and its evolutionary adaptations if we don’t know how they are built?” concludes Dr. Ziadi-Künzli.

The study is published in the Journal of Anatomy.

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