Living beneath the waves is all well and good: Breathing is easy, food is generally plentiful and it's not hard to get around. But what if you need to survive on land too?
What we mean is, what happens when your circumstances change? What do you do when your food gets scarce, your tide goes out or your river runs dry? Well, if you're an adventurous young fish with a never-say-die attitude, you pick up your fins and go.
And where better to go than where it's all happening: dry land? We took a look at seven aquatic adventurers who would never imagine that dry's the limit.
1. Epaulette Sharks(Hemiscyllium ocellatum)
In the southern seas between the coasts of Australia, Indonesia and New Guinea lie long coral-reef systems where the undulating tidal flow creates isolated pools in the shallow waters.
In these puddles, small fish, crustaceans and other sea creatures can quickly find themselves trapped, cut off from the open ocean until the tide returns. And, for them, that's a problem. Because their natural predator, the epaulette shark, doesn't even need water — it can walk between pools, sneakily using its pectoral and pelvic fins as feet and picking off its prey at will.
Not only does it have these "cheat feet," the epaulette shark can also survive on land for prolonged periods without oxygen by slowing its breathing and heart rate and by reducing blood flow to its brain.
Mudskippers are, as their name suggests, just as happy on land as they are in water — sometimes even happier. Typically, they live on and around tropical coasts of western Africa, Samoa and Tonga, in steamy regions of tidal mudflats and mangroves.
In this amphibious environment, mudskippers have developed two methods of breathing: through their gills like normal fish and through cutaneous respiration.
Simply put, they have a complex set of capillaries under the skin inside their mouths that work rather like a lung, allowing them to breathe on short trips to new habitats and temporarily survive on land.
These adaptations have left the mudskipper with the inability to efficiently repay the oxygen debt to its body — meaning, while it's underwater, the fish is actually short of breath.
3. Walking Catfish(Clarias batrachus)
Walking catfish are both a huge success and a potential disaster for Mother Nature. They have evolved a high tolerance for harsh living conditions where oxygen levels in the water may be severely depleted and are able to maneuver their way to new pools using stiff, spiny pectoral fins. This gives them a much greater chance of survival over their purely water-bound cousins.
Sadly, this fish's hardiness also means it's a hazard. Firstly, because it can survive out of water for longer, and therefore remain fresh when caught, the walking catfish is much prized as a food product.
Secondly, partly thanks to humankind, it has quickly become an invasive species, spreading from south east Asia to the Phillipines, India and even southern Florida since its introduction by aquaculture farmers in the 1960s.
Paying harshly for its success, the catfish is now "blacklisted" by the federal government and illegal to keep without a permit.
4. Lungfish(Protopterus annectens)
The lungfish is a true survivor — and may well teach us how to be the same. They've been living, practically unchanged, in the rivers and freshwater swamps of western and southern Africa for (wait for it) about 400 million years.
When its environment dries up, the lungfish fetchingly covers itself in a layer of mucus, which keeps it moist. Like the mudskipper, it breathes through lungs until the rains return. And it can survive on land this way for about a year, if necessary.
Crucially for us, the lungfish also hibernates. Burying itself at the bottom of its pool, slowing its metabolic rate and absorbing the necessary nutrients from muscle tissue, astonishingly, the lungfish can survive, almost inert, for up to four years.
Scientists hope the lungfish can show us how one day we might use these skills ourselves to prolong life during surgery or to sustain us on long space trips.
5. Brownbanded Bamboo Sharks(Chiloscyllium punctatum)
Like epaulette sharks, bamboo sharks live on the coral reefs of northern Australia, as well as around Japan. Waving their whisker-like facial barbels, these fish — otherwise known as "cat sharks" — come out at night to prowl the pools for shrimp and squid and can survive on land for up to 12 hours.
They use their pectoral fins to clamber across rocks and reefs in order to find new sources of food. And, though they are no threat to humans, at around 3 feet long, a hungry bamboo shark is probably not what you want to find marching toward you with a determined look in its eye.
6. Climbing Gourami(Anabantidae)
Also known as the climbing perch, the southeast Asian climbing gourami can also hibernate in river beds for up to six months. And when it's not waiting for the river's return, this wandering creature might be seen using its fins and tail to "walk" to the next nearest pool.
There are even reports of them climbing (or should we say "scaling?") low-hanging trees. This is all made possible by a "labyrinthine" or suprabranchial chamber, a structure over the gill that lets them absorb oxygen from the air and means they can spend a little time onshore — or maybe up a tree.
7. Pacific Leaping Blenny(Alticus arnoldorum)
The blenny (or leaping rockskipper) is also no stranger to dry land — in fact, far from it. Although it does enjoy partaking in low-lying algae from the shallow ocean's bottom, this fish actually spends the majority of its life using its pectoral fins and tail to cling to and climb the rocks of Pacific islands like Guam.
But, in addition to the usual pedestrian clambering you might expect, blennies are also capable of using their tails to fling themselves up rock faces using amazing athletic leaps and bounds or plunge into holes to find something (or someone) to nibble on — because eating is not all they get up to when the tide has its back turned.
Blennies so enjoy living out of the water, they're quite relaxed about finding and frolicking with a blenny mate when the appropriate season comes around. It's certainly a curious way to take the plunge.