A recent anatomical study of the barred mudskipper (Periophthalmus argentilineatus) has revealed new information about how these boggle-eyed fish have adapted to walk on land.
One of the most important discoveries was the presence of fascia tissue in the mudskipper's pectoral fins. The tissue's main purpose is to provide internal structure – the scientists believe it's this that gives the mudskippers the strength to move on land.
Barred mudskippers are curious creatures. Unlike other fish, their eyes sit on top of their heads rather than on the side. They have no lungs but can breathe air, absorbing oxygen through their skin or mouth. "They even swallow a big mouthful of air and hold it there to bring it to their eggs,“ says Dr. Fabienne Ziadi-Künzli from the Okinawa Institute of Science and Technology in Japan, and lead author of the study.
Yet most astounding of all is the mudskipper's ability to walk on land.
“They are fish, but they can walk and live partly on land,” says Dr. Ziadi-Künzli.
"Our ancestors developed limbs and digits before they left the water, but we don't see this in these fish. Mudskippers are amphibious and still have fins that function for both swimming and walking,”
“Mudskippers have a unique way of moving, which has not been seen in any other living amphibious fish species. It is called crutching,” explains Dr. Ziadi-Künzli. Unlike humans, who use their legs alternately to walk, mudskippers swing their pectoral fins forward simultaneously (imagine using crutches to take the weight off an injured leg).
"We started wondering what morphological adaptations to movement on land we might see when studying the fins of the amphibious mudskipper.”
Dr. Ziadi-Künzli and her colleagues were surprised to learn that the last in-depth anatomical study on mudskipper fins was done in the 1960s. As a result, information on how these fish – and in particular their muscles and soft tissues – adapted to life on land was scarce. So, they decided to investigate the subject themselves.
Using micro-computed tomography (µCT) – a 3D imaging technique that uses X-rays to see inside an object – the team took internal images of the mudskipper, as well as some of its close and not-so-close relatives.
Interpreting the images was one of the trickiest and most tedious parts of the study, says Dr. Ziadi-Künzli, who had been working on analysis since 2019. But their efforts were rewarded with the discovery of some unique adaptations to life on land.
“We found that their muscles in the pectoral fins are bigger, and the same is true for the shoulder girdle to which they attach,” says Dr. Ziadi-Künzli.
The researchers were even more astounded to discover that the bone-connecting tendons in the mudskipper's pectoral fins (which they use to walk) were replaced by fascia tissue. “We think this is an adaptation that helps the mudskippers to push themselves forward during walking because the fascia tissue gives more stability and might help to create the strength needed to move their mass on land,” explains Dr. Ziadi-Künzli.
The team also found that the mudskippers have a joint that connects the shoulder and the pelvic fin – this wasn't seen in the other fish scanned – an observation that hints at how strong the evolutionary pressure might be when organisms make the move from water to land.
When mudskippers crawl out of the water and onto land, their fins are suddenly put under a lot more pressure as the weight of the body increases. This has led to adaptations in the animal's bones, explains Dr. Ziadi-Künzli. “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. We think this might give the fin more mechanical stability.”
Similar shapes of fin ray bones have been described in the fossils of extinct fish that were ancestors to land-walking animals.
Spurred on by their discovers, the team are keen to find out more about the unique fish. “When the mudskippers are in their larval state, they look no different from many other goby fish larvae, but during metamorphosis, they change their body and fin anatomy rapidly. We want to look at this development from larvae to adults to understand this transition better,” says Dr. Ziadi-Künzli.
Read more about the study: Anatomical insights into fish terrestrial locomotion: A study of barred mudskipper (Periophthalmus argentilineatus) fins based on μCT 3D reconstructions.
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