A fish out of water
Discover the Japanese mudskipper, which lives life on the edge
Life is tough in a halfway world between land and sea, especially for the little fish that inhabit tropical mudflats. Mudskippers have to endure extreme heat and the sticky, salty mud, plus daily flooding and exposure as the tides rise and fall. Not only must they find ways to survive as fish out of water but, to make matters worse, this is a crowded place, filled with rivals to fight and predators to avoid. It’s every walking, jumping fish for itself.
“The big expanses of mud seem monotonous at first,’’ says wildlife photographer Rémi Masson, who has
spent hours lying on scorching mudflats on Kyushu Island, in southern Japan, immersing himself in this inhospitable realm. “My biggest surprise was discovering the incredible diversity of species living there. It’s a fascinating spectacle.”
As the water retreats each day, egrets and other wading birds fly in and stalk across the mud searching for prey. Fiddler crabs skitter around and a territorial species of mudskipper, known in Japan as matsugoro, emerge from their borrows in the mud. The fish are a handspan in length and defend a territory 2m across, roughly the area of a pool table. When a crab wanders too close, it receives a stern warning from the resident mudskipper, which raises two blue, spotty fins along its back and opens its big, gaping mouth – a sign that the crab should back off.
Things get more heated when the intruder is another mudskipper. At first, the quarrelling pair put on an intimidating display, dorsal fins raised and mouths open wide. If neither retreat, then a short, sharp skirmish breaks out and the fish leap at each other, wrestling and biting. “The fight scenes are really difficult to photograph,” says Rémi. “They happen very quickly.”
When not posing or fighting, matsugoro mudskippers can be seen scraping up mouthfuls of mud, before strutting over to shallow pools and vigorously shaking their heads from side to side. Their main food are diatoms, microscopic algae in the mud, which they sift through their teeth like prospectors panning for gold.
Around the world, over 30 species of amphibious mudskippers live between the
Some species scramble up vertical rocks and the roots of trees to bask in the sun.
tides on mudflats, mangrove forests and swamps. All of them are types of goby and, for a portion of their daily lives, they are fully terrestrial. To survive on dry land, or sticky mud, they have evolved a suite of specialised characteristics.
Like frogs, toads and salamanders, mudskippers breathe through their damp skin with a rich blood supply to absorb oxygen from the air. An outer covering of slime stops them from drying out. Mudskippers also gulp air into their big, puffed-out cheeks and exchange gases across the skin lining their mouth and throat. Giant mudskippers, which can be as much as 27cm long, live for extended periods beyond the high-tide mark and actually breathe more efficiently on land. Their cheeks make up almost a quarter of their entire body volume.
Keeping an eye out
Another frog-like characteristic of mudskippers is their bulging eyes, which perch high on top of their heads and let them peep out of water and look around for predators and food. Their eyes swivel independently in different directions, giving them nearly 360-degree vision. To stop their eyes from drying out, mudskippers dip them into water-filled cups on their heads.
“They have beautiful eyes with heartshaped pupils,” says Rémi, who has observed many up close. It’s not known for sure why they evolved that way, but the curious shape of the iris may help mudskippers focus on both near and distant objects when they are prowling the mudflats.
Mudskippers walk using their stiff pectoral fins like crutches. Their powerful tails also help. Some species of mudskippers are especially agile. Slender examples from Indonesia scramble up
vertical rocks and shimmy up the trunks and roots of mangrove trees to bask in the sun and hunt for insects and crustaceans. Important for their tree-climbing abilities are their pelvic fins which, located underneath their bodies, are flexible and prevent them slipping backwards. Nonclimbing species have pelvic fins that, fused together, act like a suction cup. This holds them firmly in place on rocks and firm patches of mud at the tideline. Mudskippers also produce an especially sticky slime that makes them less likely to get swept away by a wave or fall off while they’re climbing.
The missing link?
It was sometime around 400 million years ago that a group of ancient fish abandoned the water for good and gave rise to all the vertebrates that live on land — birds, amphibians, reptiles and mammals, collectively known as tetrapods. Mudskippers are not the missing link between sea and land, but they are one of several dozen groups of fish that have independently tested their fins out of water and subsequently lived a partially terrestrial life. They evolved much more recently, from fish that stayed behind in the oceans, but they can offer clues as to how their ancestors may have made the permanent transition to land.
Scientists have sequenced the genomes of four species of mudskipper, including the Japanese matsugoro. In the species that spend most time out of water, they found that changes in genes related to key functions including vision and the immune system, giving mudskippers better defence against diseases they might catch on leaving water. It’s possible that similar genetic changes took place in the earliest tetrapods as they made themselves at home on land.
Historically, people have not been fond of the intertidal realm where mudskippers roam. Mudflats and their adjacent mangrove forests are commonly seen as a smelly, mosquito-ridden inconvenience and they’re dredged and cleared to make way for hotels and marinas, shrimp aquaculture ponds and golf courses. Fortunately for mudskippers, their robust nature and ability to cope with the challenges of life between the tides helps to make them remarkably resilient to the presence and impacts of humans. They can tolerate high levels of pollutants that pour off land and they are often common in areas close to urban settlements.
In certain locations – including mainland China, Taiwan and the Philippines – mudskippers are considered a delicacy, They have also been used as an ingredient in traditional medicines. On Kyushu Island in Japan, along the beaches of the Ariake Sea, mudskippers are traditionally served raw, fried, grilled or marinated in sake. Several decades ago, there were concerns that overfishing and habitat degradation were causing a decline in the Kyushu Island mudskippers.
With their blue spots shining, the males arch their backs and fling themselves into the air.
However, in 2013, The Japan Times reported that, with the introduction of conservation measures by the environment ministry, numbers had rebounded and the matsugoro were out of trouble.
Generally, mudskippers are abundant and widespread, so they’re not currently at risk of extinction. But you might want to think twice about eating them – it’s known that toxins and pollutants accumulate in the tissues of living mudskippers. As a consequence, they are often used as bioindicator species, to track pollution levels and monitor impacts on ecosystems.
Diligent dads
Back on the mudflats of Japan, an impressive display begins to take place each June. This is when matsugoro mating season gets in full swing and the males engage in jumping contests to seduce the females. With their blue spots shining especially brightly, the males arch their backs and fling themselves into the air over and over. Presumably it’s the most impressive jumps that females find the most alluring.
Once a female has picked a partner, she will follow him to his burrow. Inside, the male has already dug a side chamber, which is where the female lays her eggs, sticking them to the smooth, vaulted ceiling. Then she leaves and it’s up to the male to look after the developing eggs and guard the nest.
Compared to their exuberant displays on the mudflats, much less is known about what happens inside the males’ secretive burrows. A long-standing puzzle was how the eggs survive in burrows without suffocating. When water trickles in at high tide, it soon becomes depleted of oxygen and stagnant in the anaerobic mud. Recently, biologists realised that male mudskippers gulp mouthfuls of air at low tide and bring them down into their burrows. It turns out that a clutch of eggs doesn’t grow in water but in a bubble of air, frequently replenished by the male.
A week or so after they’re laid, the time comes for the mudskipper eggs to hatch. At night, when the tide is rising, the male empties his burrow of air, sucking in mouthfuls and exhaling outside. This floods the chamber and triggers the eggs to hatch. The fish larvae then have to make a quick escape out of the burrow, before they succumb to the stagnant water inside. Once they make it to the open water, it’s up to the tiny mudskippers to swim off and find their own piece of mudflat to call home.
Compared to their exuberant displays on the mudflats, much less is known about what happens in males’ burrows.