Ten evolutionary wonders of fish

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Ever stopped for a moment and considered just how wonderful and diverse the aquatic world is? Nathan Hill reveals some of his favourite adaptations by fish to the pressures of everyday life.

We humans are an arrogant crowd. Even for those of us who accept and understand the process of evolution, we seem to have developed the illusion that we are at the top of our game, as if the end – and best – result of billions of years of cat and mouse.

Yeah, we can talk. We make cars and airplanes. We build marvellous devices of communication that we then plaster with images of cats, or use to pick arguments with strangers. We’ve even managed to go way beyond our natural means, harvesting the still living organs of others for transplantation, and using synthetically derived chemicals to kill off our microbial woes.

But evolution is so much more than wearing clothes and developing television talent shows. Life has been throwing up niches within niches within niches over a longer timescale than most of us can even comprehend.

We could talk of symbiosis, animals forming mutual or commensal relationships with others that sit biologically far removed from their own pigeonhole. We can talk about the complex life cycles of parasites, working their way through gaggles of intermediary hosts before finally settling in the gut of some seabird to form a juicy tapeworm.

We could even just sit back and consider how finely tuned some interactions between distinct and fundamentally different organisms are: just like the bees and their beloved flowers. The more nihilistic amongst us would possibly ponder how with the removal of just one species, an entire community of life could collapse.

But here I want to explore the fish. Their hostile environments have turned up some of the most remarkable adaptations to the pressures of daily life. I’ve sat down to pick out ten of my own favourites, though I’m sure that any one of us could think of dozens more. My own efforts here are far from exhaustive, but hopefully they’ll make a few people take a moment out to just comprehend how diverse a world the life aquatic really is…

Mudskippers

When I look at a skipper, I feel like I’m gazing upon a scene from the Devonian era. There’s something antiquated and prehistoric about the shape of these fish, something that makes me wonder exactly what pressures drove those fish to ebb out onto land, looking for a better life.

Mudskippers have two amazing features; three if you count the raised, almost stalked eyes that give them the edge on looking out over a wide area.

The first feature is their adapted fins, with a cleverly shaped girdle and powerful muscles that allow them to lift their own body weight on to land. Not just in some rudimentary way, I add, but with a degree of grace and control. Bearing in mind that much aquatic life, when brought out of water, starts to crush itself under its own bodyweight, these guys have become the hardened reconnaissance troops for the fishy empire.

There’s also their ability to breathe outside of water. Given that these fish have gills, that’s not a skill to be taken lightly. Rather than some fish that have started to breathe atmospheric air, skippers have retained those delicate lamellae, and simply take a big gulp of water, swishing it around over the gills for as long as they need. They’ve even evolved a way of passing their gobfuls of water over an area that allows further uptake of oxygen from the air around them.

Lungfish/breathing fish

It stands to reason that the next step to venturing out on to land involves going beyond just holding your breath and hoping for the best. And that’s where the lungfish, and many others, have stepped up to the mark.

Many fish breathe atmospheric air, in response to stagnant or anoxic environments. Catfish do it, like Corydoras that breathe through the gut, and we’ll all be aware of gouramis that use the labyrinth organ above their gills to soak up oxygen from a small packet of ‘inhaled’ gas.

But it’s the ones that go further, the likes of the Polypterus and the Protopterus that really impress. These fish, with their benthic existence, were burdened with almost vestigial swim bladders. After all, what use is the bladder to a fish that just wallows on the sand and mud?  

Rather than shrink these defunct bladders away, Lungfish put them to use as a respiratory organ, a long, gas filled tube that runs down the body.

And lucky for us they did. If those early fishes of ages past hadn’t come on land, sniffed the air and thought ‘this ain’t too bad’ then none of us would be here today.

Deep-sea angler (Picture by Ted Pietsch)

There’s lots that’s awesome about the deep-sea angler, including that massive, extending gut that allows it to eat prey twice as large as itself.

But I’m more interested in highlighting its unusual approach to breeding. Deep-sea angler males like their women big – ten or twenty times larger than themselves big. And they like to mooch off of their good ladies, to the point where they become the ultimate in lazy, hedonistic men.

The male Deep-sea angler, upon finding a lady of choice, saunters on over and bites her on the rear. It’s his way of getting to know her. Once there, he keeps those fangs sunk in, until eventually he just starts to fuse on to her. The whole mouth grafts itself onto her posterior, and their blood supplies fuse as one. He no longer needs to feed himself, or do anything for that matter. He just lives a near parasitic experience, gorging on her blood, shrivelling in size, until he’s little more than a conveniently attached gonad, which is basically all he’s worth. She lays eggs, he fertilises them. It’s a nice little arrangement, for him at least.

Given the abyssal, dark domain of these fish, and given the likelihood of finding a female or male on demand when you’re ready to spawn, this little trick has ensured the Deep-sea angler a place in our ‘curiosity’ archives.

Bioluminescence

Here I’ll mention the skill above any one particular fish, because this is evolutionary dynamite.

Abyssal dwellers have a tough time in a tactile world. Pretty much everything beyond a certain depth has the kind of dental equipment that is there to damage and maim, and little else. Prey is scarce, and usually comes with teeth of its own, and many fish half your size still consider you fair game. Getting the edge counts for a lot.

When you go deep enough in the sea, you reach a point where light cannot reach. Here, evolution has tossed up a remarkable melding of two very different organisms – fish and bacteria.

Some bacteria, as they do their thing, release a small amount of light energy. Confined to certain parts of the body and kept happy, these bacteria can be used in two ways. The first is for communication, the second for entrapment.

Holding bacteria in pouches that can be opened and closed, some fish can ‘flash’ at each other – a handy function if you happen to be part of a shoal and need to know where your friends and future mates are.

But bioluminescent bacteria really come in to their own when placed on the end of a lure. Suspend some glowing bacteria on an adapted fin ray, right above your cavernous maw, and eventually some little tyke will be dumb enough to wander over and have a look.

Troglodonts 

One of my favourite niches of animal is the cave dweller. Fish that have found their way into some subterranean world, plunged into darkness with no hope of escape, have an imperative to adapt or die. And adapt they do.

Usually the first thing they do is to lose those pesky eyes. There are numerous cave dwelling loaches, barbs and even tetra across the globe, all completely devoid of their peepers. They’re usually light pink in body colour, too.

Troglodont fish live life on the edge. Their populations are often perilously small, as are their natural ranges. However, they persist, and that is in no small part down to their spatial mapping skills.

Cavefish are repeatedly shown to have terrific memories, and are able to draw up a mental image of surprisingly large areas in very little time. They might not be able to see, but in their perpetually pitch-black pools, they know exactly where they are all the time.

Archerfish

With life crawling around on land, it must have been something of a tease to the water-locked fish, watching this abundant source of protein going about its business, happily out of eating range. Unless, of course, something made the mistake of falling in to the water. Then it was toast.

Archerfish level the playing field, at least where cocky, tree dwelling insects are concerned. These fish have evolved the ability to shoot their prey clean out of the sky as soon as it’s above them, and to do this they’ve had to make not one, but two funky adaptations.

The first is the mouth. Some creatures reach out for bugs, like toads that whip out their sticky muscles and grab at a passing meal. But archers have an adaptation in the base of their tongue, a groove that sits somewhat like the barrel of a gun – with a limitless supply of ammunition.

When a spider or fly is daft enough to land on a leaf above an Archer, goading the fish and blowing raspberries out of its little proboscis at them, the fish gets itself underneath, pokes just the tip of its snout out of the water, and then clamps its gills shut, hard, with powerful opercular muscles. The resulting build up of pressure only has one point of exit, and that’s down the tongue groove and out of a tiny notch in the fish’s mouth. The insect gets jetted clean off of its point of refuge, and into the drink where the Archers are waiting.

On top of the spitting, the archers also have to deal with targeting, and refraction of the water. But they’ve had plenty of time to hone this skill, and with some tweaks to their massive eyes, including a crossover on their field of view; they have ended up with remarkable binocular vision. Bad luck, bugs.

Flying fish

Now this is an old trick. The original flying fish – the seaborne ones – were known to be knocking around about 235 million years ago. Our modern flying species evolved the talent again of their own accord about 65 million years back.

But it’s such a good idea that it has come up a few times, and not just in the marine world. We’ll all be aware of the marine flying fish, launching themselves from the ocean and up into the sky with their huge, pectoral ‘wings’ at the first hint of trouble. But there are many in the freshwater world we overlook as well.

Hatchetfish are known freshwater fliers. Spurious reports claim that they take to the wind in order to chase down airborne insects. Other reports ascribe the ability to flap their ‘wings’ as they fly, though hard evidence is lacking. Either way, when startled, these little tykes can launch themselves over a considerable distance.

As can Pantodon buchholzi, the African butterfly fish. These guys even have a hair trigger approach to launching. I wrote in detail about these fish a while back, and one thing that stuck with me was their reaction time. In the event of a fright, they can register the threat, and have muscles already moving to launch themselves in a faster time than it takes we humans to even register a puff of air hitting our eyes. That’s some fast fish, and that requires some pretty finely tuned evolutionary hardwiring to pull off.

Euryhaline fish

Now here’s some talent. We can (sort of) argue about which way fish evolved. Some, though not many, favour the ‘freshwater first’ approach, others think they found their fins in the seas. There are even theories that involve starting in marine, moving in to fresh, and then they came back to the sea again.

Some fish have conquered both worlds, and are able to switch between the two. For creatures where osmoregulation is everything, this is rather an achievement. The overwhelming majority of fish are, as we’re all aware, very set in their water requirements. They enjoy a specific pH value, set levels of hardness, and any straying from this is often enough to eventually kill them. Take your typical freshwater fish and plunge it into the sea, and it’s dead in minutes, and vice versa.

That fish have developed the ability to migrate between the two worlds opens up a whole new world of opportunity. Bull sharks can exploit periodic flooding in regions around the world that allow them to swim inland, gorging on cattle, people and carrion before moving back out to sea.

Others use the opportunity to go the other way, and fatten up in the oceans before moving back inland. Even humble stickleback varieties can do this, getting large on plankton before going back upstream to spawn. Salmon are well known for the behaviour.

Common clownfish

Forget the whole living in an anemone thing, that’s old news. What excites me about the clownfish is its sex life, or how it changes its gender as it goes along.

From an evolutionary point of view, there are plenty of hermaphrodite fish. There are the ‘fixed’ hermaphrodites, like some halfbeaks. Then there are the sequential hermaphrodites that start as one sex and change to another.

The clownfish, Amphiprion percula, is a perfect example of a sequential, protandrous hermaphrodite. Every clownfish starts life as a boy, without fail, and when one reaches a certain size and/or level of dominance over its subordinates, it switches muscle vests for sports bras, and goes girly.

There are many pieces written about the advantages that such evolutionary features as this confer on their host phenotypes, and we only need to look at just how many fish out there have resorted to a single gendered start in life to see that it’s actually a pretty successful way of going about breeding.

Sexual selection and evolution is a deep branch in its own right, and if you ever get the chance, makes for some very insightful reading.

Hammerhead shark

What a great fish to wrap up on. Of the sharks out there, the hammerhead is the most instantly recognisable, and for many years (think 1700’s and 1800’s) had scientists poring over the most elaborate ideas of what use such a broad shovel of a face would have.

We now understand the nature of these sharks well, and have investigated their biology thoroughly, and know the answer. The explanation relates to their detection of prey, and a sense with which they are equipped that none of us will ever physically comprehend: electrodetection.

On the underside of a shark’s snout are tiny, gelatinous pores called the ampullae of Lorenzini that pick up on nothing more than electrical charges.

So sensitive are these pores that they can detect the single heartbeat of a prey fish, or the tiny impulse given off by a muscle contraction.

Such an evolutionary advantage is handy if, like the hammerhead, much of your prey lives beneath the surface of the sand. Hammerheads have a thing for rays, wrasses, inverts and other delicacies that like to secrete themselves into the ocean bed, where they’re hard to sniff out. The hammerhead, in full foraging mode, sweeps that glorious, wide skull in a side-to-side motion over the sand, the underside of its face smothered with electric sensing pores. Once a meal is swum over, its heartbeat gives it away, and the shark can simply scoop it up from where it lays. Think of a man with a metal detector, and how he finds his quarry, and you’ll not be far off.

So those are the fish that stick with me the most. I’m sure you’ll have many other examples of evolutionary features in mind, and I invite you to share them with us.

In the meantime, it just remains for me to ask that we take a moment to try to comprehend just how intricate and fascinating the world of evolution really is. But then, I’m sure you already are.

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