Evolution: fish on land; mudskippers (Evolution)

by David Turell , Sunday, May 21, 2023, 16:38 (550 days ago) @ David Turell

Very much a fish adapted to land trvel:

https://www.discovermagazine.com/planet-earth/meet-the-mudskipper-the-fish-that-walks-o...

"Mudskippers — of which there are 25 species, inhabiting mudflats, swamps, and mangrove forests from Africa to South Asia to South America — spend more than half of their life on land, doing everything from eating to mating out of the water.

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"If fish have gills to extrapolate oxygen from the water even at depth, how do mudskippers breathe in the open air?

"There are two ways that they can go about this. In the first, they breathe through cutaneous respiration, so the amphibious fish are constantly capturing oxygen from the air through their soft skin — as well as the lining in their mouth and throat — which is filled with capillaries that can absorb oxygen.

"In order for this to work, however, they need to be wet, — and that’s why they spend most of their time out of the water. Still, in moist environments like mudflats, mudskippers can be seen rolling around in the mud to keep a sheen on their skin, flipping side to side in puddles to cool down and freshen up.

"The second method is more direct: A mudskipper’s gills trap water in their cavities and store it for longer-term oxygen retrieval. This is why some species can often be observed making their way to a body of water, opening their mouth and taking gigantic gulps of it.

“'They will put their jaws in the water and you can see them pumping water or sucking water in," says Gordon. "And then they end up puffing out their gill covers, and they have a mouth full of water that they walk around with,” says Gordon.

"To trudge around on land, mudskippers use their pectoral fins like two tiny arms. Their fins, in fact, have mini-joints similar to the elbow and shoulder, allowing them to fold their flippers for pushups off the land, propelling them forward one step at a time.

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"As the name suggests, mudskippers also literally skip on mud. But that’s not thanks to their pectorals as much as it is thanks to their tails. By bending their tail rapidly, they're able to use it as a lever and launch themselves into the air, a behavior called a C-start. This, too, evolved in the water.

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"When they’re not out hunting for food — snails, insects, small plants, anything the detritus of the midlands has to offer — mudskippers, especially the smaller species, build long, complicated burrows underground in between tides. To excavate their hideaways, they shovel long tube-like chunks of mud into their mouth, regurgitating them outside of their tunnel once they’ve reached the open air. These are great for hiding from predators, resting, or aiding their breathing and thermoregulating abilities.

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"Unlike most fish, the mudskipper’s eyes are on top of their heads like periscopes for greater visibility, they can move independently of each other, and they can… blink.

"Blinking is typically reserved for limbed vertebrates. But because mudskippers evolved to thrive outside of water, they also developed this attribute. Blinking helps mudskippers moisturize and keep their eyeballs wet, clean them from anything that might obstruct their vision, like mud, and reflexively protect them from, say, objects flying their way, according to a paper published in 2023 in the Proceedings of the National Academy of Sciences. Their blinking method is also highly sophisticated, as they retract their eyeballs inwards into a skull cavity replete with water.

"But mudfish didn't require a lot of changes to their eye apparatus to evolve this behavior, says Brett Aiello, a biologist at Seton Hill University and one of the study's authors. They're relying on the same six extra-ocular eye muscles to retract the eye into the skull, so they took the structures they already had and they started using them in a completely different way.

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"Hypothetically, we can look at walking fish like mudskippers as nature’s example of an ongoing evolutionary experiment, says Gordon." (my bold)

Comment: the bold says it all. It shows uu how the aquatic beginning of life ended up on land.