The Cenozoic also saw a huge proliferation of fish species, in particular the group known as the spiny-rayed fishes because they have sharp bony spines in their fins.^j Flatfish, tuna, swordfish, rockfish, mackerel, and cod are just some of the many abundant spiny-rays in the ocean today. Coral reefs are brimming with spiny-rayed fishes, among them the butterflyfish and angelfish, parrotfish, pufferfish, and groupers. Seahorses evolved in the Cenozoic when seagrass meadows spread between Australia and Indonesia. Their unusual upright posture, horses’ heads, and curling, gripping tails were all adaptations seahorses evolved that camouflage them and help them navigate through the grass blades.

While bony fishes with spiny rays were doing very well in the Cenozoic, another group of fishes, with bendy, cartilaginous skeletons, were not so lucky. Sharks had been swimming through the ocean for hundreds of millions of years but almost went extinct nineteen million years ago. This near-extinction was discovered only in 2021, from a study of their toothlike scales, called denticles, which dropped down onto the deep seabed and became trapped in sediments. Close to three-quarters of shark species disappeared, and it’s not obvious why. Fossils suggest that sharks have never been as abundant as they once were, but within a few million years they began to regain their diversity and to get a lot larger. The megatooth shark lineage was made up of species that gradually got bigger through the Cenozoic, with the unrivalled giants evolving around sixteen million years ago. Megalodons^k had seven-inch-long teeth and ten-foot-wide jaws and from head to tail were between fifty and sixty feet long, roughly two full-size school buses parked bumper to bumper. The species went extinct 3.6 million years ago, shortly after the arrival of a newcomer, the great white shark. It’s possible great whites pushed their competitors towards extinction by hunting the same prey as young megalodons.

At odds with the popular idea that sharks are prehistoric creatures that haven’t changed much in millions of years, new species have evolved relatively recently in the Cenozoic. The youngest belong to a group known as the walking sharks, which live in shallow seas of Papua New Guinea, Indonesia, and northern Australia. These three-foot-long sharks spend their days resting on the seabed, then set off at night to hunt, using their pectoral and pelvic fins as rudimentary legs, strutting along in a gecko-like gait. With their leisurely strolling and reluctance to swim, these sharks never move too far under their own steam. The most westerly species arrived in its current location in Indonesia not by swimming but by riding on its home island. That’s how sedentary these sharks are—they use the geological forces of tectonic drift to get around. This means walking-shark populations are easily cut off from one another, between one island and the next, increasing the chances of splitting into separate species. All nine known species of walking sharks evolved in the past nine million years, and the youngest species split apart only two million years ago.

The view we now have back through deep time tells us that life on earth and in the ocean has always been turbulent and ever changing. Life has been sculpted by life itself and steered by powerful external forces that shift currents and continents and alter the climate. Now, in the present moment we’ve reached in the Cenozoic era, a single global force has come into effect. The Anthropocene, the age of humans, is underway.

Homo sapiens has been modifying the earth for millennia, by hunting other animals, felling forests, burning vegetation, and domesticating animals and crops. But only in recent times have humans been altering the planet on such vast scales and in so many ways. The word Anthropocene is intended to embrace the accumulating and accelerating ways human activities have become dominant agents of global change, from chemical pollution to climate change, from the mass rearing of livestock to the rampant destruction of biodiversity.

When exactly the Anthropocene began is a rather esoteric matter, one that a team of geologists, the Anthropocene Working Group, has been considering since 2009. In 2023, they proposed that a lake in Canada serve as the marker for the start of the Anthropocene, with the idea it will be identifiable thousands of years in the future. Crawford Lake, near Toronto, is six acres in area and almost eighty feet deep, and contains layers of undisturbed sediment that gently sink down into it, including key molecules that indicate when human activities shifted gear to a world-altering scale. A thin layer laid down in 1952 contains plutonium particles from the hydrogen bomb tests that drifted around the world. There are also spherical particles of carbon released by the widespread burning of fossil fuels, and nitrates released by the flood of chemical fertilisers. For Crawford Lake to be officially adopted will require three more committees of geologists to vote in favour, but already the idea of the Anthropocene has taken hold.^l

A critical part of anthropogenic change is the endangerment of thousands of species that is driving the earth’s sixth mass extinction. Humans have already caused countless species to disappear, ranging from documented extirpations, like that of Steller’s sea cow, to the hidden extinctions as species slip away when entire ecosystems are demolished. One way to get a sense of the scale of what’s happening now is to compare current extinction rates against the background rate at which species have steadily been lost from the fossil record in aeons gone by when there wasn’t a mass extinction raging. Whichever way you slice the data, the rate of extinction is now far higher, perhaps a hundred times, than during those peaceful intermissions of the past. A contemporary mass extinction is in full swing.

Messages from deep time warn that there will be no creative destruction, no wiping clean of the slate to make way for a flourishing new era of biodiversity. New radiations of species will happen in time, but they’re unlikely to be the direct result of Anthropocene extinctions. There will be no swift recovery of lost species, and certainly not on timescales relevant to human lives. Following even the milder mass extinctions of the past, biodiversity has taken hundreds of thousands of years to recover. If our collective impact comes close to the worst extinctions, life on earth will be diminished for millions of years to come. Decisions made now in the Anthropocene will set changes in motion that will alter the path of life far beyond the likely lifespan of humanity itself.

There’s no telling who the survivors in the long term will be and what kind of new living worlds they will build together in the ocean. If the past is anything to go by, probably predators and prey will continue to exist, as will animals swimming around with hydrodynamic bodies and quite possibly flippers and tails. There’s no telling which group of animals might take on those roles, any more than we could have guessed that an ocean filled with trilobites scurrying and paddling around would be replaced by a realm ruled by giant swimming reptiles, then much later by a similar-looking crowd of mammals.

Perhaps another group of vertebrates will fill niches vacated by the sixth mass extinction; maybe birds will become the next underwater giants, or perhaps an age of oceanic amphibians will dawn and frogs, toads, and salamanders will follow their ancestors back to the seas. Or it could be the turn of invertebrates to become the ocean’s dominant predators once again. Octopuses or squid could rise up and diversify into blood-chilling apex predators or gentle filter feeders. Reefs too could keep growing along the edges of continents and islands, built by new kinds of builders; maybe a different group of corals, shells, or seaweeds will take on that role, or maybe sea stars or sea urchins will press their hard skeletons together in great colonies and shoulder the ramparts of new reefs. Until the ocean enters a post-Anthropocene era, we can only guess what will come next.

What we can do is meaningfully consider the changes that will likely happen within our own lifetimes and that people in future generations will witness. We can contemplate who the likely winners and losers will be, human and nonhuman, and look for effective ways to overcome the challenges facing the ocean. And we can recognise the fundamental difference between what’s happening in the Anthropocene and everything else that came before. Past changes have had multiple, often unidentifiable triggers. This time a single species is to blame, one that’s aware, at least to some extent, of the trouble being caused. Humans are in the unique position of having a chance to make conscious decisions about the future of the earth’s biodiversity and with it our own future.

a The five mass extinctions of times gone by were in the Ordovician (445 mya [million years ago]), Devonian (375 mya), Permian (252 mya), Triassic (200 mya), and Cretaceous (66 mya).

b Strictly speaking, these are the non-avian dinosaurs, because birds are living dinosaurs.

c Look at the enormous, fleshy siphon of a living geoduck clam to see where that development led.

d The Cenozoic era is divided into seven or eight epochs: the Palaeocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Holocene, plus arguably the Anthropocene.

e Carbon dioxide is sucked out of the atmosphere when natural mild acid in rainwater reacts with some rock types, breaking them into compounds such as bicarbonate that wash into the ocean and are used by marine organisms such as phytoplankton to make shells and are eventually deposited as layers of limestone on the seafloor.

f The Delphinida includes the oceanic dolphins (Delphinidae), the Amazonian dolphins (Iniidae), the South Asian river dolphins (Platanistidae), and the narwhals and belugas (Monodontidae).

g The phocids, otariids, and odobenids.

h Aotearoa is the name the Māori people use for New Zealand, their native islands. I have included aboriginal and dual place names and spellings throughout the book.

i Adélie penguins (Pygoscelis adeliae) were named after Adèle Dumont d’Urville, who was married to a nineteenth-century Antarctic explorer from France, Adélie being an alternative way to spell Adèle.

j Also called acanthomorphs, these fishes have sharp bony spines on their dorsal and anal fins, which they can use as protection from predators; they can also protrude their jaws to snap up food.

k Otodus megalodon was previously thought to belong to the Lamnidae, the same family as great white sharks, but is now placed within a separate sister group, the Otodontidae, or megatooth sharks.

l An alternative suggestion is the Capitalocene, a term that emphasises the global change associated with the history of capitalism and colonialism and recognises the uneven distribution of costs and benefits in this age of humans.

Chapter 2 Remixing Seas

Ten years ago or so, I met a lionfish that I couldn’t bring myself to kill. Hunkered under a coral overhang on a reef in the Bahamas, it was one foot long, striped like a zebra in white and red, its fans of long fins folded back against its body. I’d heard about how lionfish had been set free from aquarium tanks and were spreading across the Caribbean and western Atlantic. And yet, coming face to face with a species I had last seen half a world away, I felt more unsettled and conflicted than I’d expected. In that moment, the so-called lionfish invasion shifted in my mind from a textbook example of how people are meddling with the ocean into this one beautiful yet misplaced animal. This fish wasn’t to blame for the troubles going on; it was not an individual animal to victimise but simply one caught up in our human ways. Here was unavoidable proof of the changing seas, gazing right back at me.

Some years had passed since I had scuba dived in the Caribbean. Last time I had been there, I wouldn’t have seen a lionfish no matter how hard I might have searched. Most of my research had taken me to the Indian and Pacific Oceans, the native range of lionfish. I’ve always liked seeing them underwater, and only once have I encountered their darker side. In Madagascar, on a small sand island off the west coast, I met a fisherman who a few days earlier had caught a lionfish and been stung by one of its venom-tipped fins. He held out a horribly swollen, discoloured hand. We helped him into our boat and gave him a ride to the mainland to get medical attention.

Now, here I was on a Bahamian reef, staring at a fearless lionfish that wasn’t about to swim away from me. I had never tried my hand at spearfishing before, but I’d been informed that shooting lionfish is easy. Generally, they are slow, fluttering fish that aren’t easily scared off, relying on their bright colours to warn intruders away from their venomous spines. I was working as part of a research team studying the effects of lionfish on Caribbean coral reefs. We weren’t there to catch as many lionfish as possible; we were gathering samples to unpick the contents of their stomachs. As I clutched the spear, held taut with a giant rubber band, I found myself quite unable to let go and send it flying through the water to put an end to that fish.

The order of the ocean’s fauna and flora used to be shaped just by climate, ecology, and geography. Now it’s being directly shaped by people too. Not so long ago, there were places that were home to only certain combinations of species, with well-traversed routes and highways where they roamed. Now, a turbulent remixing is underway, making it a daunting prospect to work out what the future ocean will look like.

One way this happens is when people deliberately move species, usually with some economic goal in mind. Land-based bids to do this have often gone badly wrong and spiralled out of control, like when giant African land snails were introduced to oceanic islands in the South Pacific. The idea was for people to farm and consume these massive molluscs, which can reach twelve inches long, but instead the snails escaped and began demolishing wild vegetation. So, people released another foreign snail species, the carnivorous Florida rosy wolfsnail, which they hoped would eat the giant snails. But that plan didn’t work out either. Instead, the wolfsnails turned their attention to the islands’ endemic tree snails, which live nowhere else on earth and soon were being hunted to extinction. The story was playing out like the children’s rhyme about the old lady who swallowed a fly, then a spider to catch the fly, and we know how that went in the end. Conservationists arrived in French Polynesia just in time to gather up the remaining tree snails and whisk them away to safety in zoos around the world. Still, this and other catastrophic cases from land ecosystems haven’t been enough to put people off from deliberately moving species around the planet, including in the seas.

Red king crabs are formidable, spiny crustaceans with legs that can stretch almost six feet across and ochre-coloured shells that brighten to scarlet when they’re cooked. Their native range stretches across the North Pacific, from the coasts of Korea and Japan across the Bering Sea to Alaska and Vancouver Island. For the past few decades, they’ve also been living much farther away. In the 1960s, Russians gathered up red king crabs from the Sea of Okhotsk, off Kamchatka, and moved them more than three thousand miles to the Barents Sea, in the far northwest of Russia. They translocated a total of 1.5 million larvae, ten thousand young crabs, and close to three thousand adults. The aim was to establish a commercial crab fishery in waters that are more easily accessible to Russian fishing fleets—and it worked. But of course the crabs didn’t stay where they were put, and soon they were breeding and spreading.

Thirty years later, fishers in Norway, to the west of Russia, began pulling huge red crabs from their nets and traps. At first, Norwegians feared the invading crabs would ruin their livelihoods and tear apart their fishing gear, until many of them realised they were looking at a lucrative new source of income. This was before Deadliest Catch became a hit television series about the Alaskan fishery of the red king crab in its native range in the Bering Sea, but people still knew about the high prices these crustaceans could command, especially when kept alive and shipped to high-end restaurants around the world. So that’s what Norwegian fishers started doing.

Wrestling king crabs from icy seas is not an easy way to make a living, but it pays. The new red king crab fisheries have transformed many people’s lives in Norway, and the export fishery is now worth tens of millions of euros every year. But the crabs are also a major cause for concern. These are large, omnivorous generalists that make a mark on their surroundings. Adult king crabs grab prey in their powerful pincers and tear it to shreds. When the crabs arrived in the Barents Sea, many other species dramatically declined. In some places, the newcomers completely wiped out mussels and starfish. King crabs also eat a lot of worms, which are important for digging and stirring up sediments. When there are fewer worms, less oxygen is mixed into the seabed, altering the ecosystem and making it harder for other animals to survive.

There are also concerns for other fisheries. King crabs may compete for food with commercially fished species like Atlantic cod, plaice, and haddock, and they eat the eggs of capelin, a key fish species in the Barents Sea food web. What’s more, leeches that are hosts to a type of parasite that infects cod like to fix themselves onto the king crabs’ spiny shells.

The red king crabs have already reached the Norwegian city of Tromsø, and they could continue along the Scandinavian peninsula and offshore to the Lofoten Islands and there threaten major seasonal cod fisheries. And so, in an attempt to halt this westerly march of the king crabs, Norwegian fisheries officers have drawn a line. The twenty-sixth meridian east runs through Magerøya Island near Knivskjellodden, the northernmost part of mainland Norway, 150 miles from the Russian border. On each side of that line, different stories about king crabs are playing out.

To the east, the fishery is run with the aim of operating for years to come. Quotas are set for the total weight of king crabs a fisher can take each year, and fishers are permitted to keep only those with shells bigger than five inches across. Crabs any smaller go back in the sea to carry on growing and reproducing. Fishers commonly fill their quotas within a few weeks, then some spend the rest of the year running tours, bringing tourists to the fjords to see and taste the king crabs.

West of that line of longitude, fishers can keep all the king crabs they catch. Here, no crabs go back in the sea. This westerly free-for-all is technically known as an eradication fishery, the aim of which is to remove as many crabs as possible as quickly as possible.

It would be far-fetched to assume the eradication fishery will entirely halt the king crabs in their tracks. Their larvae will keep drifting, and adults will keep walking, and not all of them will end up in nets and traps. Fishers farther west are anticipating their arrival. In January 2022, fishers in the north of England pulled up their crab pots and found inside spiny red giants. The news that invading Russian crabs had made it all the way across the North Sea sparked shock and excitement in the UK fishing industry; some fishers worried about local ecosystems and their regular catches of brown crabs, while others began dreaming of export markets. However, experts from London’s Natural History Museum later confirmed these were not in fact red king crabs but a similar species, the Norway king crab, a North Sea native, although rarely caught in British fisheries. Still, there’s a good chance the red kings will make their way over.

On occasion, ocean species simply show up out of the blue and with no obvious human help. Solitary animals arrive in places where they’re not normally expected, such as a huge male walrus that recently left the Arctic and toured the coasts of Europe, gaining an entourage of fans who spotted him in Ireland, England, France, and Spain, then back up north in Iceland. They nicknamed him Wally.^a However, when a single unusual animal is followed by another and another until they can no longer be considered occasional visitors, then humans are usually involved.

Ships are a major source of long-distance travellers, their hulls encrusted with hangers-on and their ballast water tanks twitching with hitchhiking larvae and microbes, which get picked up in one place and dumped elsewhere. Not all the transported species survive the journey or thrive when they arrive, but many that do end up causing trouble. This was how, in the 1980s, sea walnuts, a species of comb jelly, moved from North American waters to the Black Sea. So many were stowed away in ballast tanks and released thousands of miles from their native waters that their numbers soon exploded. Superabundant sea walnuts ate so much plankton that many fish went hungry, triggering the collapse of local fisheries.

Adding to the drip-feed of species that people are accidentally moving great distances, a wave of climate migrants is making its own way across the ocean. With temperatures rising, much ocean life is on the move.

Along both the east and west coasts of North America, large shark species are swimming to places where they were not normally seen in the past. Historically, the waters off the northeast coast were too cold for tiger sharks, cold-blooded apex predators that need to stay in warm waters. Back in the summers of the 1980s, they would ride the Gulf Stream from the Caribbean and spend time off the coast of Florida. Now the ocean is warmer, and tiger sharks are venturing hundreds of miles farther north. Large congregations now form off the coast of North Carolina, and tigers swim as far north as Cape Cod, Massachusetts. This northerly shift is making tiger sharks more vulnerable to fishing, because they’re now swimming outside of marine protected areas and coming within range of longline fisheries.

In the west, on the central California coast, great white sharks have started showing up. In 2014, sightings in Monterey Bay rose dramatically. Drone cameras photographed a half dozen at a time cruising together. Strangely, these were all relatively small sharks, around eight feet long, making them juveniles between three and four years old, which don’t normally swim this far north. Even though this shark species is warm-blooded, young great whites are usually confined to warmer waters because they are less chunky than adults and easily get chilled. The arrival of young sharks in Monterey Bay coincided with a huge marine heatwave that struck the Pacific coast.

Around the world, a cavalcade of ocean migrants are escaping to higher latitudes, from lobsters to flounders, pipefish, and starfish. In the Pacific, the Humboldt squid is expanding its range on either side of the equator, to both the north and south. Collectively, ocean species are moving towards the poles, travelling at a global average pace of forty-five miles per decade. This is a coarse figure, obscuring details gathered from masses of tracking studies that follow individual animals and species moving this way and that, but it speaks to the immensity of the changes already taking place.

On land, the pace of climate migration is an order of magnitude slower than in the sea. The ocean offers far fewer obvious boundaries and obstacles for migrating species to overcome. And the seas are full of organisms that during some portion of their lives—from egg to larva to mobile, swimming adult—make use of their free-flowing liquid realm to find mates and new territories to settle and grow. In the sea, it’s common for species to end up living a long way from where they were born, giving them an inbuilt capacity for shifting to cooler climes as the ocean around them heats.

Ocean dwellers in a warming world also need to move more urgently than land dwellers because they tend to live within narrow thermal safety margins. They’re not used to getting either much hotter or colder, as they evolved to live at temperatures that fluctuate very little on a daily or seasonal basis. Air temperatures can swing by tens of degrees between day and night, while the thermal inertia of water keeps conditions in the sea much more stable. Consequently, ocean species are highly sensitive to warming. Even small changes in their surrounding temperature can be lethal.

To make matters worse, there’s nowhere easy to hide from heat in the ocean. Terrestrial animals can stay more or less where they are and adapt their behaviour to hot conditions—for example, by lurking somewhere cool during the day and coming out only at night. More elaborate adaptations include those of fogstand beetles in Africa’s Namib Desert, which survive on sands that reach 140 degrees Fahrenheit by climbing to the tops of dunes in the cool hours of the morning and raising their bodies so that fog blowing off the sea condenses on their bumpy exoskeletons. In the sea, there’s no point in sweating or panting, digging burrows or retreating to the shade when the water warms. The only real option is to move somewhere cooler, by either travelling towards the poles, tracking cold currents, or retreating to greater depths.

It’s simple enough to foresee that ocean species will generally tend to strike out for chillier waters, but a range of factors influences who moves, when, how far, and how fast. Ocean mixing is happening in a disorderly fashion. There’s no mass relocation with entire assemblages moving together in one go. Some species move quickly, and some stay behind; some spread to new territories while also maintaining old haunts; some move on and abandon their former ranges; some slide down deeper, and others hold their depth.

In the Mediterranean, hound sharks and spiny dogfish, unihorn^b and musky octopuses, and jewel and flying squid are among the species that in recent years have been increasing their maximum depth. In contrast, bony fishes and crustaceans are staying nearer the surface. It could be that the sharks and octopuses are more sensitive to rising temperatures and have greater need to sink, or they are better equipped to cope in deeper waters, where sunlight dwindles, less food is available, and the pressure ramps up. But just because those animals are the first and fastest to move downwards, doesn’t mean they will survive in the longer term. Once they reach the bottom of the sea, they’ll have nowhere deeper to go.

For many sea-dwelling animals, climate change not only brings the direct stress of rising temperatures but also forces them to shift their range in order to keep breathing. Since the middle of the twentieth century, average oxygen levels throughout the ocean have fallen by around 2 per cent; by the end of the twenty-first century, it’s expected the ocean will have deoxygenated by between 3 and 4 per cent. This is happening, in part, because of the simple fact that warmer water can hold less dissolved oxygen. Other factors also make oxygen loss far worse in certain areas. In coastal waters, fertilisers and sewage washing off land in rainfall runoff and rivers stimulate great blooms of phytoplankton, often painting the sea livid green or red. When these dense patches of microscopic algae die and decompose, the water is stripped of oxygen. Pollution from land also explains why enclosed seas, such as the Black Sea and the Baltic Sea, are notoriously oxygen poor, and now they’re joined by regions of open ocean much farther from shore. Low-oxygen zones are expanding and intensifying in the eastern Pacific, off the west coast of Africa, and in the Arabian Sea and Bay of Bengal. This is happening because climate change is altering wind patterns and currents, reducing the amount of oxygen circulating through the ocean.

Hit hardest by deoxygenation are the fast-swimming animals with racing metabolic rates. Tuna, marlin, and sailfish are already losing part of their vertical habitat because of declining oxygen levels in deeper waters. They are forced to stay closer to the surface, where oxygen levels in the water are higher. Bluefin and yellowfin tuna are expected to shift their ranges polewards in the coming years to seek out enough oxygen.