Creationist's nightmare - part 1

Credit: HK wetland park

  The first time I saw these fish they were climbing onto rocks. Jerky bogle-eyed mudskippers crawled, hopped and flipped at the water's edge in southern Lantau, their thin tubular bodies glistening in the sun. Close-up they have bulging hamster cheeks, or sagging fat-man chops, and eyes that sit on top of their heads. They walk on their front fins, which are jointed at the "elbow" in nature's imitation of cartoon fish.

    I say walk but I admit that may be stretching it a bit. Their arms are used more like crutches, with a mermaid's tail dragged behind, sometimes with an arched push to assist the labour. Like other fish of the goby family that they belong to, they have a fin on their underside that has been redesigned as a sucker. It gives them a hold on surfaces while they bring their fins around for the next thrust forward, in a lazy locomotion described by some as “crutching.”

    But the skipper label works better when these land hopping fish shift up the gears to fight, hunt, or escape. Imagine a human swimmer doing a vigorous butterfly as the tide goes out. He finds himself in the soft mud after the water has sucked away, but he's so absorbed in his task that he hasn't noticed the water has gone. He continues all the way up the beach to his towel, in a lumbering, laboured skip. That's the mudskipper in second or third gear. It would be a good idea to rename the swimming stroke to the "mudskipper." 

    But the true skipping only kicks in in fourth gear, usually triggered by terror, for example by the sight of a curious human, like me, or a hungry stork. For this, think of a world class triple jumper executing a perfect hop-skip-and-jump. The rapid motion will see them shoot off the mud flats, across the water’s surface, to then sink with a plop when the momentum has fizzled.

    If you stay still long enough, beady eyes will rise back out of the water, like a pair of periscopes. These amphibious fish can see better out of water than in. Once it feels safe enough it will crutch-walk out of the water, gain momentum with its butterfly flips, and get back to business in the mud.

    Note the adjective “amphibious,” not the noun. Mudskippers are fish, they just happen to spend more time outside of water, than in, which admittedly is strange behaviour for fish. But that makes them of great interest to evolutionary scientists.

Credit: HK wetland park

     Mudskippers are specialists of tropical inter-tidal mud flats and humid mangrove forests, both of which are little known facets of Hong Kong. They make their living in the zone where tides creep up into shallow estuaries twice a day, only to draw back long distances leaving an inhospitable but nutritious shimmering surface of organic sludge. While other fish float in and out of that space with the tide, mudskippers stay put, exploiting resources the others are forced to give up twice a day.  Contrary to their aquatic evolutionary heritage, mudskippers live out the most active parts of their lives outside of water, during low tide, including feeding, fighting and finding mates.

    In Hong Kong we have three species of these truly strange fish, and they are easy to spot as long as you go to the right place. You won’t find them along the Victoria harbour waterfront because there is no mudflat for them to ply their trade, but their existence elsewhere is proof that not all the coast has been tamed, blasted and caste in concrete. The common mudskipper is carnivorous, feeding on little crustaceans and insects, while the blue spotted and the blue eat the microbes and algae that fill the inter-tidal shore. You’ll find the three species dotted variously on coastal marshlands and mangrove swamps from Mai po to Sai Kung, and around the remote shores of Lantau island.

HK mangroves. Photo: HKU

  

    Around the world there are 29 other recognised species, mostly circling the tropics and sub-tropical belt. They all share a similar morphology: funny looking slimy tubes with cartoon flipper-arms, decorated with exaggerated flag-like dorsal fins, with Dizzy Gillespie cheeks and alien bauble eyes sticking up above their heads. There’s good reason for the shared characteristics, they’re closely related, plus they’re equipped for similar environments.

    The arm-like jointed fins are an obvious adaptation to walking on muddy land.  And with so much time spent out of the water their exaggerated dorsal fins are mostly redundant as swimming aids. Un-needed for locomotion, they have grown into decoration for attracting mates and scaring enemies, like the peacock’s showy curtain of tail feathers. 

   The bulging cheeks are filled with water to help breathing, in a deceptively simple adaptation often described as a reverse aqualung. Like any other fish, mudskippers use gills to extract oxygen from water. But unlike the others they have developed pouches around their gills to carry water on their expeditions out of water. Inside the water-filled pouches, the gills carry on working as if they were submerged. You’ll see the skippers coming out of the water with swollen cheeks that gradually deflate until another dip is required. 

    But they also breathe through their skin, and through their mouths and throats, which are lined with tiny capillaries for exchanging air and carbon dioxide. They have to keep moist for skin diffusion to work, which means frequent rolls in muddy puddles and quick dives into pools they can never stray too far from. This activity keeps them very busy all the time.

    Their mad periscope eyes can be dipped completely into their heads for a swirl in the water-filled gill chambers. It is the mudskipper’s equivalent to a blink, partly serving the same function a blink serves mammals, to moisten the eye and remove dust. They’re the only fish that can blink, but if you think about it they are the only fish that need to blink. But the eye-ball dip serves another purpose. Pulled into the gill chambers they swirl the carried water around, helping to circulate oxygen. In other words, they blink for air. 

    Another acquired skill for inter-tidal survival is burrowing. Male mudskippers dig holes to create security bunkers, love-nests, and egg hatcheries. In low-tide they need to duck and dive from shorebirds, and other predators, like humans, that learn to negotiate the treacherous mud. In high-tide bigger fish come exploring for snacks. The burrow works for both occasions, as a bolt-hole from danger.

    Anyone watching mudskippers for the first time will easily see how territorial these fish are. They are constantly squabbling with aggressive flag-waving fin displays and muscular wrestling bouts. Some species build a muddy wall around their burrows, like a fortress against raiders. They chase off male rivals that come too close, and wait for passing females, eager for an opportunity to impress with their prowess.

    They do that by jumping up and down. Their fins unfurl in handsome display, and they flip themselves into the air, as high as 60 centi-metres at a time. Women say men are simple, but at least humans try to add conversation or wine to the mix to express the biologically-driven desire to mate. 

Credit: info@gov.co.hk

    But surprising for us, jumping does the trick for mudskippers. A suitably impressed female will follow the male into the burrow and lay her eggs in a specially designed chamber. The romance ends there, as the now sated male will sometimes chase the female out of the burrow and start duties himself as “mother.” 

    One of the main jobs of the primary care-giving male, is to regularly supply oxygen to the incubating eggs. The eggs are laid on the ceiling of a chamber that holds a small air pocket below the waterline. The only way to maintain air circulation is for the adult fish to swim above water, gulp air into its mouth, carry it to the bottom of the burrow and release it in the chamber. The male does this repeatedly as the eggs develop. When they are finally ready to hatch, he floods the chamber at high tide, for the new born to disperse like floating microbes in the ocean.

    These surprising traits are enough for any fish-appreciating person to award mudskippers an array of worthy titles such as coolest fish on the planet, or top water-beasts. But there’s one title I’ve come across that really beats them all: creationists’ nightmare.