It’s not long after sunrise when I hear the sounds of drums coming from the other end of the beach. I’ve been lying awake for ages, listening to waves gently tugging at the sand and hoping for a signal to get up and go swimming. I unzip my tent, grab my snorkelling gear and join a few other early risers in a small boat.
The journey is short, just a few minutes motoring around to a narrow channel between this and the neighbouring island. My guide, Semmi, has already been in the water this morning, and he’s optimistic for a fruitful swim.
‘When we get in, stay close to me,’ he calls out over the idling engines.
I drop over the side of the boat. Straight away the fast current pushes me along and I fix my gaze into the distance. There’s nothing but blue-green haze. Then I hear a muffled shout, look up and see Semmi with his hand high in the air, palm open – the sign of a manta.
A dark shape glides towards me, just below the surface. The manta ray is swimming upstream but seems unfazed by the current; it is, after all, mostly fin – two pectorals stretched out into triangular wings.
The cluster of humans in the water pauses for a moment, watching, trying to take in its great size. This one is modest for a manta, with perhaps only a two-metre (6.5ft) wingspan, but still, it’s improbably big for a fish. It begins to fade out of sight and a silent whistle goes off, stirring everyone into action. The snorkellers frantically paddle to keep up, trailing after the undulating fish, like kids playing soccer, all trying to get the ball at once. Another, slightly larger manta soon arrives. This one has a neat shark bite taken out of one fin, but the injury doesn’t stop it swimming smoothly and fast, followed by its splashing, kicking human entourage. Prosthetic, plastic fins are no match for the manta’s speed and it’s not long before people begin to fall behind and scramble into the boat to hitch a ride back upstream. Again and again the group takes to the current, like a fairground ride of which nobody grows tired.
Each year, around sixty mantas make repeat visits to this channel, midway along an island chain in northwest Fiji. In a similar way to my Humphead Wrasse, individuals can be identified from black and white markings on their bellies. On average, between April and October, three mantas show up every day. On one occasion, there were 14.
These enormous flattened elasmobranchs come here primarily to feed. Pouring through the channel is a seawater soup laden with zooplankton, animals too small to see with the naked eye but so abundant they make the water thick and cloudy. Mantas swim through their food, their huge mouths wide open. Plankton get lodged on feathery filaments called gill plates or gill rakers that fringe each of their ten gill slits. Every so often the manta closes its mouth, coughs and swallows down the zooplanktonic paste.
Mantas aren’t the only fish that routinely show up in this Fijian channel to eat. While I paddle around, a seething shoal of mackerel hurries past. Silver bodies streak through the water, forming a restless, shape-shifting mass that sweeps up, dives down and performs tight U-turns, swimming back on itself. Most of the time the bodies within the school are lined up, their dark stripes in parallel. But now and then the group fragments into glinting disorder before falling back into synchronised formation. Trailing the mackerel is a hunter’s shadow, a Giant Trevally, attracted to the fizzing activity and waiting for its chance to attack. The mackerel swim on, sifting the water for plankton with their mouths open like a thousand miniature parachutes.
There’s another source of food that comes to the channel with the mantas. Finger-sized fish flit over their bodies, picking off parasites and scraps of dead skin. These are cleaner wrasse; every day mantas spend hours getting themselves groomed and cleaned.
While so many fish gorge themselves, the human swimmers in the channel begin to tire and think of their own breakfast. While the boat zips around picking people up, I hang back and spend a while longer riding the current and watching the water around me.
Once more the two mantas emerge into view. They found a good spot, packed with plankton, and stopped to make the most of it. The two fish are spinning pirouettes through the water. They arch their bodies and swim backward loops, chasing but never quite catching their long tails, all the while their gills filling up with food.
Fish get food for themselves in a huge variety of ways. It’s one of the things that sets them apart from other vertebrates, which in comparison tend to have rather conservative diets. Fish, on the other hand, will eat almost anything, in almost any way, and this has major implications for the rest of the underwater realm.
Many fish are hunters, some legendarily so. Around 30 species of Amazonian piranhas have gained a fearsome reputation as unhinged flesh-eaters, ready to nip your finger off as soon as you trail it in the water. Such stories were stirred by early western explorers, including US President Theodore Roosevelt. In his 1914 book Through the Brazilian Wilderness, Roosevelt describes piranhas mutilating swimmers, and going mad at the mere sniff of blood. More credible are reports that the locals had put on a show for their esteemed visitor. A few days beforehand, they netted a shoal of piranhas and deprived them of food. When Roosevelt arrived a dead cow was thrown in, and the ravenous fish quite understandably had a feast. Piranhas aren’t nearly as dangerous to humans as their reputation would suggest. A lot of the most gruesome cases probably involved fish scavenging from bodies that were already dead. There are, though, concerns that an increase in dam-building across their range in South America is creating ideal spawning grounds for the fish. Along with droughts that are pushing them into deeper waters, it could be that piranhas are coming into more regular contact with human swimmers and as a result more attacks are being reported.
Predatory fish can be extremely adaptable. In southern France, where the River Tarn flows through the historic city of Albi, giant catfish have learned to catch pigeons. Originally from Eastern Europe, these metre-long (3ft) fish were introduced into the river in 1983 for anglers to catch, and since then they’ve settled in well to city life. Known as the Danube Catfish, they lurk in the shallows next to gravel islands in the middle of the river, where pigeons come to drink and clean their feathers. When the birds get too close to the water’s edge, the catfish leap out and deliberately beach themselves (some whales and dolphins employ similar tactics). Researchers from the University of Toulouse took turns to watch this from a nearby bridge, and they saw that roughly one in three hits ended in a catfish getting a pigeon meal. It could be one reason why these fish are such successful invaders, modifying their diet to whatever food is available in new territories. The catfish now live in ponds and rivers from the UK to China.
The world’s waters are filled with many fish chasing after other animals, but there are also plenty of vegetarians. Some piranha species are gentle herbivores that play a crucial role in river ecosystems; with huge, crushing teeth they chew on large seeds, helping to disperse them and promoting germination. Of around a hundred species of parrotfish the majority eat algae, cropping vegetation with teeth fused into parrot-like beaks. They boost coral-reef health by keeping in check seaweeds that can otherwise quickly overwhelm and outcompete the corals. A 2017 study of a 3,000-year fossil record from Panama revealed close ties between the rise and fall of parrotfish and coral reefs. Katie Cramer from Scripps Institution of Oceanography in San Diego excavated parrotfish’s beaky teeth and measured historic rates of coral growth. She found that when the fish were doing well so were the corals; but when the fish declined, including in the last 200 years from overfishing, the reefs stopped growing and seaweed took over. Caribbean reefs are among the most degraded on the planet today. Cramer and her co-authors concluded that the only hope for their recovery is for the ‘significant and immediate reduction of fishing on parrotfish.’
Damselfish take vegetarianism to another level. They’re one of a small clique of animals, besides humans, to shift from being wandering hunter-gatherers to settled farmers. Ants, termites and beetles farm fungi; deep-sea yeti crabs grow bacteria on their hairy arms; and on coral reefs, damselfish carefully tend their seaweed gardens. In territories set up among branches of dead coral, damselfish weed out inedible types of seaweed and cultivate a lush turf containing only the species they prefer to eat, sometimes a monoculture of one species.1 Lacking stomach enzymes for digesting tough seaweed, damsels select the softer, tastier varieties.
A lot of work goes into maintaining these gardens, chiefly in defending them from intruders. Damsels are some of the easiest fish to watch on reefs because they’re unafraid of just about everything else, people included, and they don’t swim away and hide – quite the opposite. Whenever I float above a shallow reef I get accosted by aggressive little fish that are convinced I’ve come to steal their precious seaweed. I’ve watched a large shoal of surgeonfish – wide-ranging herbivores – come stampeding past, stirring up an almighty ruckus among the resident damsels. All at once dozens of angry gardeners, who had been hidden, rose up in the water column, shouting and nipping at the surgeonfish intruders. Damsels will even pick up sea urchins by their spines and carry them off outside their gardens. This may seem paranoid, but studies have shown that when damsels are experimentally removed from their gardens, it only takes a day or two for other animals to come in and chomp through their prized specimens. Some of the most delectable seaweeds, including a red variety called Polysiphonia, have only ever been found inside the damsel’s protected turfs, showing how fish and seaweed have come to rely on each other.
For corals, the damsels’ gardens are not such good news. The fish will bite and kill larger corals to make space for their gardens. Individual damselfish territories range from the size of an A2 sheet of paper to a table-tennis table (the damselfish range in size from a finger-length to a hand span). In places where fishing has depleted bigger, predatory fish – the kinds that people like to eat – damselfish have exploded in numbers, and spread their gardens across the reefs. As well as clearing away corals, the seaweed turfs alter the types of microbes in the ecosystem and seem to promote diseases among the remaining, living coral.2 The loss of bigger, predatory fish shifts the balance towards smaller fish, like damsels, and indirectly corals suffer.
Beyond hunting and farming, there are yet more ways in which fish get their food. To get a good idea of just how versatile fish are in their eating habits, a good place to look is the Great Lakes of East Africa, among the flocks of cichlids. They’ve evolved a staggering range of diets, which it’s thought have assisted their speciation and allow species to coexist in the same place by dividing up resources between them.
There are plankton-eaters, snail-, sponge- and leaf-eaters, dirt-slurpers, eyeball-peckers and cichlids with luscious lips to suck insects from holes in rocks. Others get food by pretending to be dead. They lie motionless on their sides, and their mottled coloration makes them look as if they’ve already begun to decompose. Scavenging fish then come to investigate and get a nasty shock when the apparently dead fish leaps up and takes a bite out of them. And there are predatory cichlids that have learned to dash up and headbutt a female cichlid while she’s brooding young inside her mouth. This forces her to spit out her babies, and the intruder quickly swallows them down.
No matter what they eat, all fish face the same challenges of living and feeding underwater. Water is much stickier than air, meaning that when aquatic animals lunge forwards to catch something, their own body creates a bow wave that pushes their food away from them. The wave also alerts prey to an approaching predator, giving them more time to escape. Mantas, mackerel and all the other filter-feeding fish partially avoid this by opening up a big mouth and letting the water flow through their bodies, rather than around them.3 Filter-feeding is a way of getting food that only really happens underwater, from paddlefish that sift the lakes of North America to the enigmatic Megamouth Sharks that push their giant maws through deep seas.4
The challenges of hunting in sticky water have also led to a uniquely fishy trait. A common feeding strategy for fish (especially teleosts) is to fling their jaws forwards. This creates a smaller bow wave than moving their whole body, and it can also be much faster. It’s how pet goldfish peck at dried food flakes. And if you pull down the lower jaw of a dead salmon or trout, its upper jaw will automatically swing upwards and outwards via a series of articulated bones.
Fish have been thrusting their jaws out for at least 100 million years. In that time they’ve reached further and further forwards. Today, the winner of the jaws-out contest is the Slingjaw Wrasse, a tropical fish that can extrude its snout into a tube that measures 65 per cent of its head length. If I had the same talent, I’d be able to bite a chocolate bar dangling on a string 13cm (5in) in front of my nose, without moving the rest of my head.
A close second is the Goblin Shark. In French it’s known as requin lutin (leprechaun shark) and in Spanish it is tiburón duende (elf shark). When this species was first discovered in the 19th century it was known only from dead, ugly specimens with jutting, snaggletoothed jaws, like badly fitting dentures. Only much later did it become apparent that living Goblin Sharks tuck their jaws away, making them a much sleeker, less terrifying apparition. They only stick out their jaws when they’re eating – or trying to. They were recorded doing this for the first time in 2008, when a team of Japanese scientists netted live Goblin Sharks in a deep canyon in Tokyo Bay. The sharks were carefully brought up alive, then filmed while they swam around in the shallows. A couple of times in front of the camera the sharks tried to bite something, including a diver’s arm (luckily he was wearing a thick wetsuit). The film showed the sharks’ lower jaws dropping open into yawning gapes of 116°,5 then shooting forwards like a sling-shot. Goblin Shark jaws reach half their head’s length and snap shut in under half a second – the furthest, fastest jaws of all the sharks.
Beyond simply coping with the stickiness of water, many fish take advantage of it and have learned how to suck: they shoot their jaws forwards, puff out their cheeks and gulp in a mouthful of water and with it any prey and particles of food that get swept up in the viscous current (try it yourself, slurping a bowl of minestrone soup). In retalliation, prey fish have evolved a speedy escape response; sensing the bow wave of an approaching predator, smaller fish will lurch forwards by a single body-length, just enough to avoid the predator’s short-range suck.
Among the champion suckers are seahorses. Like mantas they eat zooplankton, but rather than filtering seawater, seahorses peck one by one at individual tiny shrimp. A seahorse lines up its snout just below a target, then tightens muscles in its head that stretch tendons, like pulling back an elastic band before you flick it at someone. A trigger muscle then releases the stored elastic energy and the seahorse’s snout suddenly rotates and sucks in a shrimp.
Newborn seahorses are especially good at this, an unusual skill in one so small. Most fish take time to learn how to suck. They have to get used to coordinating their jaws and muscles, and their suction generally improves as they get older. Seahorses, though, spend several weeks growing inside their father’s pouch and they’re born fully developed and ready to go.6 High-speed videos shot in 2009 show that the young seahorses pivot their heads three times faster than adults, faster even than a mantis shrimp when it swings its ferocious claws to crack open a seashell. The foals rotate their snouts at the equivalent of 80,000° per second (mantis shrimp manage a mere 57,000º/sec).
Another group of fish does things in reverse. Rather than sucking in sticky water, they spit it out. Seven species of archerfish use water as a weapon. The earliest known written record of their marksmanship comes from 1764, in a letter sent from Dutch East India7 to a fellow of the Royal Society in London. The letter accompanied a preserved specimen of a ‘jaculator’, as archerfish were then known, donated by Governor Hummel from the hospital in the colonial capital Batavia.8 Hummel had heard about the habits of these curious fish but wanted to see for himself if the stories were true. He ordered several archerfish to be caught and placed in a tub of water, with a stick propped over the side and a fly pinned to its end. How thrilled the governor was when, day after day, he saw his fish shooting the fly and never missing their mark.
Over the years, research has gradually revealed the true extent of the archerfish’s talents. They compensate for the way light bends as it passes between water and air to infallibly hit targets up to three metres (10ft) away, like a male Splash Tetra keeping his eggs wet. Then archerfish position themselves in just the right spot to grab the fallen insect. Not only that, but they shoot their in-built water pistols with power five times greater than any vertebrate muscle can muster. Until recently it was assumed that the secret to this feat was some sort of catapult, perhaps similar to a seahorse’s rotating head. But no matter how closely people looked, they found no trace of such a device in archerfish. It seems that archerfish don’t actually have bows and arrows.
In 2012, Alberto Vailati and colleagues from the University of Milan in Italy finally solved the puzzle. Rather than relying on muscular force, these little fish manipulate the water itself. An archerfish spits out a jet of water by pushing its tongue along a groove in the roof of its mouth. Vailati’s team worked out that the fish push harder towards the end of a water stream, so droplets further back bunch up and collide with others ahead of them, and they merge. So instead of spraying their target with a drizzle of fine droplets, archerfish strike out with a single swift blob, powerful enough to knock a small creature from its perch. When you or I throw a water bomb it may initially sail through the air towards our victim, but it quickly succumbs to the forces of gravity and air resistance, slowing down and plopping to the ground. Archerfish shoot water bullets that actually speed up the closer they get to their target.
Fish that swim through electric dreams
As well as being sticky, water is also very good at conducting electricity – at least a billion times better than dry air. That’s why it’s a bad idea to change a light bulb with wet hands. But rather than worrying about the health and safety risks of mixing water and electricity, there are fish that do this on purpose.
For thousands of years, people have known there are fish that have a certain spark to them. In Ancient Greece, doctors placed electric rays on women during labour, apparently to help them cope with the pain. Ancient Egyptians caught electric catfish from the River Nile and may have used them to treat people who were having epileptic fits. And at the turn of the 19th century, the Prussian explorer and naturalist Alexander von Humboldt saw horses and mules being attacked and overwhelmed by eels in a muddy pond in Venezuela. These and hundreds of other fish species share an unusual ability: they make and control large amounts of electricity.
Every living creature is electrically powered. Charged ions flow in and out of cells, in particular nerve cells; conducting messages, contracting muscles, making thoughts (the electricity that comes out of sockets and powers electronic devices is made of flowing electrons, another form of charged particle). In most living bodies the electric charges are tiny. Various fish, however, have evolved organs that accumulate and amplify electricity and send it out in deliberate bursts. Hunting with electricity is something only fish do.
Years ago, as a zoology student, I met an electric fish, a species of elephantfish (from the same family as the Ancient Egyptian story of Oxyrhynchus). I peered at the one I’d been given in the laboratory class, and saw that its snout was neither as long nor as dextrous as an elephant’s trunk. It bore a closer resemblance to its common name in German, tapirfische, named after the South American Tapir. On closer inspection, I saw that my fish’s proboscis wasn’t in fact its nose at all, but an elongated chin.
My task was to map out the electric field emanating from the elephantfish by measuring the current with an electrode dipped at intervals around the aquarium tank. The wobbly diagram I drew showed the fish was surrounded by a cloak of concentric lines. This was the electric field generated by modified muscle cells at the base of its tail. It gave out a gentle, constant pulse, not strong enough to zap the finger I waggled in the water when no one was watching.
I was recreating an experiment conducted 50 years previously in the same laboratory in Cambridge’s Department of Zoology by the man who uncovered the elephantfish’s hidden talent. Hans Lissmann had seen these fish at the aquarium in London Zoo and noticed that they swim backwards without bumping into things. With eyes focused ahead, they can’t see behind themselves, so he wondered if they used some other sense to find their way around. With similar equipment to the set-up I was using, Lissmann was the first to detect the elephantfish’s weak electric field. He worked out that elephantfish use electricity as bats use sound. This isn’t echolocation, though, but electrolocation.
Just as Lissmann had, the next step in my experiment was to stick a glass rod – an electrical insulator – into the aquarium near the fish and once again map out its electric field. This time the lines were distorted as the electric pulse flowed around and not through the insulated rod. My fish would have known the glass rod was there and not necessarily because it saw it. Until a few years ago, elephantfish were thought to be blind, but recent studies suggest they can probably make out large, moving objects thanks to little crystal-filled cups in their eyes that intensify dim light. Even so, these fish are far more sensitive to electricity than to light. All along its body, my elephantfish had dimples that sensed its own electric charge. Shifts in that field would have told the fish there was something new nearby.
Around 200 elephantfish species live in rivers in Africa, where they emit electric pulses to probe the muddy waters and detect distortions in their personal electric field as it bounces off objects around them. They use their long, sensitive chins to search for food hidden in riverbeds. To process all the information coming in from their tingling senses, elephantfish have enormous brains that use up to 60 per cent of their oxygen supply. Theirs is a similar ratio of brain to body size as humans’, except we use only 20 per cent of our oxygen to power ours.
The gentle elephantfish lie at one end of a spectrum, with the notorious Electric Eel at the other (a fish that goes by the pleasing scientific name of Electrophorus electricus). They’re not true eels but one of many species of South American knifefish, and they can generate 600-volt jolts that can incapacitate and even kill other animals. Kenneth Catania from Vanderbilt University in Tennessee has been getting to know Electric Eels in ways no one has before. Among many new insights, his studies have shown that they don’t fire off their powerful shocks at random, in the hope of hitting something, but they use electricity in much more subtle and smart ways. An eel will often commence its hunt by sending two or three charges into the water. If small fish or crustaceans are hiding, they will give themselves away with involuntary twitches of their muscles triggered by the eel’s exploratory shocks. Their twitches send ripples through the water, which the eel detects with its pressure-sensitive lateral line. It then unleashes a volley of shocks like a Taser gun that overstimulates the victim’s nerves, causing its muscles to contract and putting it temporarily out of action.
Catania has also worked out what may have happened to Alexander von Humboldt’s horses, 200 years ago in the swamps of Venezuela. On his journey around South America, Humboldt asked local fishermen if they could obtain some live Electric Eels for him. Their response was to drive horses into a pond and watch as the eels launched a vicious attack. The men shrieked at the horses and stopped them from running away; two horses drowned and several more staggered off and collapsed. This was, Catania thinks, the eels standing their ground as they do in a situation that naturally arises every year. During the rainy season, water from the Amazon and Orinoco Rivers floods into surrounding rainforests and savannas, and fish move into a temporary wetland. Later, when the rains stop and the waters recede, the fish become stranded in isolated pools; this is when Humboldt was there, during the dry season. Electric Eels are accustomed and well adapted to this – they breathe air and can survive as ponds grow stagnant. But isolation makes them vulnerable, and predators are drawn to these ponds filled with fish that can’t easily escape. Nevertheless, the stranded eels have an effective way of fighting back.
While doing other experiments, Catania saw his captive Electric Eels attack the net he used to scoop them from their tanks. The eels repeatedly charged at the net and leapt from the water, firing electric shocks into the metal handle. To measure the scale of these attacks, Catania lowered a metal pole into the tank that was hooked up to a voltmeter (the eels frequently fired 200-volt shocks). He even set up up a life-sized model of a crocodile head covered in LEDs that lit up whenever an eel shocked it. The eels deliver these powerful shocks by jumping from the water and short-circuiting their electric organ directly through another animal’s body. This is more potent than if the eel simply fired the charges into the water in which the target animal is standing or swimming.
In his latest (2017) study, Catania found out first-hand just how potent the shocks can be. He rigged an experimental set-up to measure the current that flows through a living human arm when an eel attacks, using himself as the test subject. The chosen eel was relatively small, a 40cm (16in) juvenile, but nevertheless it could deliver shocks that peaked at 50 milliamps, ‘greatly exceeding thresholds for nociceptor activation’, as Catania’s paper reports. In other words, it was dreadfully painful. Nevertheless, his arm didn’t go stiff; his muscles weren’t being over-stimulated and locked rigid. Rather than incapacitating a victim, he thinks the purpose of the eels’ leaping attacks is to deter potential predators by giving them a sharp, painful shock.
Catania is confident these leaping eels are not hunting. They don’t bite and chew their food, and they couldn’t swallow anything as big as a crocodile, a horse or a human. Instead, he thinks the small aquarium tanks in his lab may convince the eels they’ve been trapped in a shrinking pond and are in danger from predators, just as they are during the dry season in their native habitat. In this situation, when something big and threatening looms close, the eel’s innate response is to defend itself and make sure the intruder gets a lot more than it bargained for.
Charles Darwin was well aware that waters around the world are inhabited by various electric fish. In his book On the Origin of Species, he pondered how they evolved. ‘... if the electric organs had been inherited from one ancient progenitor ... we might have expected that all electric fishes would have been specially related to each other.’ But, as Darwin knew, the electric fish aren’t all close relatives. Sleeper rays, numbfish, Coffin Rays and torpedo rays are all electric elasmobranchs. Across in distant parts of the fish evolutionary tree, among the teleosts, there are electric stargazers, catfish, elephantfish and knifefish. Darwin regarded them all as important examples of a phenomenon now known as convergent evolution, although he didn’t use that term. This happens when distantly related species evolve to look or behave or somehow operate in a similar way. As Darwin wrote, ‘I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection ... has sometimes modified in very nearly the same manner two parts in two organic beings.’
This is how tree-climbing primates from Madagascar called Aye-ayes and marsupial possums in Australia came to exploit the same food source as woodpeckers. All three groups dig holes in trees and pull out grubs from beneath the bark; the birds use their beaks and long tongues, while Aye-ayes and possums have buckteeth and long fingers.9
Similarly, fish evolved to be electric on at least six separate occasions. Darwin would have been astounded to know how this happened, and how evolution can repeatedly arrive at the same outcomes. Electric organs are packed with electrocytes made from modified muscle fibres, co-opted from different parts of the fish’s bodies. In electric rays these cells are revamped gill muscles, forming two kidney shapes on either side of their round bodies. When hunting, the rays wrap up prey in their wide pectoral fins and zap them with electric shocks. Northern Stargazers live along America’s eastern seaboard, buried in sand and leaving just their eyes poking out. Modified eye muscles generate weak electric shocks that may confuse their prey or deter approaching predators. As for those powerful Electric Eels, more than three-quarters of their bodies are made up of thousands of electrocytes, derived from muscles all along their bodies.
Molecular studies are revealing that all these fish use the same genetic toolkit to become electric. They all follow the same developmental pathways and switch the same sets of genes on and off. The process is complex, involving muscle cells getting bigger, losing their ability to contract and instead pushing lots of ions across cell membranes, creating a flow of charge. Despite evolving millions of years apart, in the oceans and in freshwaters and in different regions of the body, all the fish’s electric organs evolved in essentially the same way.
Mysteries still remain, including how electric fish manage not to electrocute themselves while they’re hunting. It might be that they insulate their vital organs under layers of fat, or they have well-insulated nerve endings, but for now nobody’s quite sure.
What goes in
Bumphead Parrotfish live up to their name very well. They have bumps on their heads and teeth fused together into sharp beaks. They use their stocky foreheads in ritualised head-butting contests that were caught on camera for the first time in 2012. Like Humphead Wrasse (whose humps, as far as we know, are purely for show and not for fighting), these enormous parrotfish congregate to spawn, and it’s then that fully grown males compete for supremacy by charging at each other and banging heads – with a loud crunch. The rest of the time these parrotfish are also quite noisy, taking great rasping bites of coral.
In Palmyra Atoll, in the middle of the Pacific, researchers swam across shallow reefs following Bumphead Parrotfish and watching them for hours on end, carefully noting down every bite they took. This was an impressive feat of fish-watching. The methods noted in the published study specify that only observations lasting 60 minutes or longer were included in the results. The longest the snorkellers swam non-stop alongside a fish was 5 hours 20 minutes. On average, each parrotfish took three bites of coral every minute.
Like elephants on a savanna, these are beasts that leave a clear mark on the world around them. When the parrotfish bite off mouthfuls of coral, they also dislodge smaller coral fragments, some of which survive and stick themselves back to the reef, growing into new colonies. And the parrotfish’s bite-marks clear space for new coral larvae to settle. They excavate vast amounts of living and dead coral, reconstituting and redistributing limestone and sediments, and they play a major role in the dynamic rhythms of the reef.
Adding up their feeding habits over the course of a year, a single, mature parrotfish eats between four and six tonnes of solid limestone reef. They need to eat this much because there’s not much nutrition to be had in this coral-based diet. Food for the fish is the thin film of living tissue that covers the calcium carbonate skeleton, and out of everything they swallow they absorb only around two per cent. A second set of teeth at the back of the throat – known as the pharyngeal mill – grinds the coral down to a powder. Nutrients are absorbed through their long intestines, and whatever’s left over comes out the other end.
Fish faeces and urine provide important nutrients for other organisms. In the early 1980s, Judy Meyer from the University of Georgia watched fish on the Caribbean reefs of St Croix, in the US Virgin Islands. During the day, she saw that some thickets of branching corals had a resident shoal of fish, called grunts, with blue and yellow stripes and large, silvery eyes. At sunset, the grunts would swim off to a nearby seagrass meadow, where they filled their bellies with molluscs and crabs. By sunrise the fish were back on the reef, sheltering among the branches of their coral homes, digesting and defecating. Meyer and her team found that nutrient levels were five times higher in water near corals with fish tenants compared to those without, most likely because of the grunts’ daily evacuations. Over the course of a year, she saw that the corals with fish grew twice as fast as equivalent colonies that didn’t get a regular spattering. It seems these mobile vertebrates forge an important link between the two habitats, sea grasses and coral reefs, from one end of their digestive tracts to the other.
More recently, fish ecologist Jacob Allgeier spent years working out just how much coral reefs rely on fish pee. The practical side of his work, with collaborator Craig Layman at North Carolina State University, involved catching hundreds of fish species and carefully placing them one by one in plastic bags of seawater, for half an hour at a time. By measuring the water’s nutrient content before and after, they calculated how much phosphorus and nitrogen each fish released, chiefly in their urine but also leaking across their gills. Allgeier combined this information with data from his colleagues, Abel Valdivia and Courtney Cox, who had the enviable task of surveying fish populations on hundreds of coral reefs across the Caribbean, some heavily fished and others highly protected where virtually no fishing took place. Crunching all the data, Allgeier estimated that when reefs are depleted of their fish, they’re starved of up to half the nutrients available in healthier, fish-rich, urine-rich habitats.
The nutrient balance on coral reefs is precarious. They can easily have too much or too little. Reefs evolved to thrive in clear, nutrient-poor tropical waters; these ecosystems are highly efficient, recycling and reusing the limited nutrients (in contrast, there are nutrient-hungry ecosystems like kelp forests that require constant feeding, often from nutrient-rich waters welling up from the deep). It’s well known that nutrient pollution is a problem for reefs, when sewage and farmland runoff pour phosphates and nitrates into coastal waters, causing seaweeds to smother corals and take over reefs. But there’s a flip side, too. Reefs are also worse off if they lose their natural source of nutrients.
Studies like Allgeier’s are showing that much of a reef’s crucial nutrient pool is locked up in fish, especially big fish – the biggest produce the most faeces and urine. He didn’t wrestle a metre-long (3ft) Bumphead Parrotfish into a plastic bag. If he had, he would have needed a very large bag, not only for the fish but also for its droppings. Several times when I was diving in Palau, Bumpheads emptied their bowels in front of me, sending a white, chalky plume trickling through the water. Those Bumphead-watching researchers on Palmyra Atoll noted that the adult fish defecate more than 20 times an hour.
Fish make other important contributions to the world around them. Many of the Bumphead’s relatives, like Dusky, Ember and Daisy Parrotfish, are vegetarians that have strong jaws to crop seaweeds from reefs; at the same time they scrape up chunks of limestone rock. All of this passes through the parrotfish and comes out the other end in an altered state. In 2015, Chris Perry from the University of Exeter in the UK led a research team to the Maldives, to analyse the origins of the sands that build these low-lying islands. The team found that parrotfish are responsible for making more than 85 per cent of the sand-grade sediment in and around Vakkru Island; in other words, parrotfish built most of the island with their poo. So should you ever find yourself strolling along a tropical beach, spare a thought for the fish that are industriously feeding, chewing and defecating, down beneath the waves, helping to produce the gleaming white sand between your toes.
Iceland, 16th century
Iceland has very few animals living on land, but the rivers and lakes and surrounding seas are awash with mysterious fish. Some are harmless creatures, and people use them for their own good. Five eels drowned in liquor will protect whoever drinks it from becoming intoxicated; stones extracted from inside a skate’s head can make a person invisible, although only for an hour at a time. But many of the Icelandic fish are dangerous and best avoided. The lodsilungur is a trout that grows a furry white coat to keep warm and has poisonous flesh. The ofug-nggi looks like a trout with coal-black skin, and it swims backwards. Eat one of these fish and the result is instant death. Should you ever go near to a ditch or a pool of stagnant water, watch out for the hrokkull. A wizard made this fish by taking a dead, half-rotten eel and bringing it back to life. Step in water where a hrokkull lives and it will coil around your leg. It has venom so strong it dissolves skin and bones, and unless the fish is quickly unwound, your leg will be cut right off. And the most venomous fish of all in Iceland is the vatnagedda. Shaped like a small flounder and flaming gold in colour, it is very rare indeed and is only ever seen on a foggy night before a violent storm. To catch one, bait your hook with gold and wear gloves made of human skin. Then keep it in a glass bottle, wrapped in layers of horse skin, otherwise it will burn through and sink down into the earth. This fish protects against evil spirits, and can ward off even the most powerful ghosts.
Notes
1 Damsels don’t sow or transplant their favoured seaweed species, but wait for them to naturally settle; it’s thought early human farmers may have done a similar thing, weeding out unwanted varieties from mixed wild vegetation.
2 Diseases can be a big problem for corals; epidemics in the 1980s are another cause of the demise of Caribbean reefs, along with the lack of grazing parrotfish.
3 3D-printed models of whale-shark gills placed in a flume tank have revealed how their gill rakers don’t get clogged up: tiny vortices form, whisking solids off the filter surface and keeping it clean. Engineers are hoping to imitate this to prevent clogging in industrial machines that filter beer and dairy products.
4 Technically speaking orb-web spiders ‘filter-feed’, though I’d say this is more of a trap, as most of their prey isn’t blown in passively.
5 Compare that to an estimated 63.5º for a Tyrannosaurus rex.
6 Seahorses are the only animals we know of in which the males get pregnant and give birth; females deposit eggs inside the male’s pouch, where they hatch and grow, while he nourishes them before giving birth; with sharp contractions of his pouch he squirts baby seahorses out into the world.
7 Now Indonesia.
8 Now Jakarta.
9 Malagasy folk stories tell of Aye-ayes poking fingers into people’s ears while they’re sleeping and picking out their brains.
