It is testament to the number of spectacles packed into Blue Planet II that a giant wrasse’s strategic change of gender is – scientifically speaking, at least – one of the least remarkable. Changing gender, or sequential hermaphroditism, is a fact of life for more than 400 species of fish, and has already been widely studied.

But many of the programme’s marvels are new not just to television but to science itself. Some have only been published within the past half-decade; others existed only anecdotally until now. Here we track some of the most astonishing findings of the series.

An orange-dotted tuskfish holds a clam in its formidable jaws on the Great Barrier Reef, Australia. Photograph: Alex Vail/BBC

Fish may be smarter than we once thought (episode one, One Ocean)

The tenacious tuskfish filmed cracking open a clam on coral – “persistent Percy”, as he was named by those who shot the sequence on the Great Barrier Reef – could advance the case for the intelligence of fish.

The behaviour was first recorded in the Coral Reefs journal in 2011, with photographs showing a tuskfish grasping a cockle in its jaws and striking it repeatedly on a rock. The paper’s authors argued that it constituted tool use, considered a sign of intelligence in many species. That this clam-cracking behaviour has been demonstrated repeatedly, and by wild animals, may help shore up the case for more advanced cognitive abilities of fish than previously thought.

Rachel Butler, an assistant producer on the first episode, told the series’s accompanying podcast that the behaviour could not be purely instinctual as the tuskfish seemed able to adapt to circumstances. “When he couldn’t crack open a clam on one anvil site, he’d go to another ... and he was finding those anvil sites.”

Anecdotal reports of fish using tools date back decades, but what little evidence of it exists – a 2009 paper documenting captive stingrays using water jets to flush out trapped food, for instance – has been tempered by debate over what tool use actually is. Some scientists argue that it is a flawed metric by which to assess intelligence when many species are precluded from demonstrating their smarts by their lack of physical dexterity, or their environment.

Butler pointed to the “referential gestures” – a rudimentary form of sign language – of coral grouper in episode three as indicative of an intelligence on a par with chimpanzees. One of the biggest leaps in the 16 years since the first Blue Planet was broadcast was in comprehension of the cognitive abilities of fish, she said: “Gone is the notion of the goldfish and the seconds-long memory.”

The deepest ocean is nearly 11km down. Photograph: Luis Lamar 2017

Life on Earth may have begun in the deep (episode two, The Deep)

More people have been to the moon than have been to the deepest ocean, nearly 11km down, where many scientists believe life on Earth began four billion years ago.

More than 75% of all volcanic activity on earth occurs in the deepest ocean, mostly along the mid-ocean ridge: an underwater mountain range spanning nearly 50,000 miles. Along it, primordial hydrothermal vents spew plumes of chemicals so hot they could melt lead. In one unnerving Blue Planet II scene shot from a manned submersible, a cut-throat eel, scavenging in a pool of brine formed from erupting methane gas, succumbs to toxic shock. “It started to tie itself up in knots – our eyes were on stalks,” said producer Orla Doherty. “That was completely new. I don’t know that anyone has ever seen that happen before.”

That life can exist at all within these highly acidic conditions has shaped thinking on early origins of life on Earth since the first “black smokers” (hydrothermal vents formed from deposits of iron sulfide) were discovered in 1979. But thinking continues to evolve.

The so-called Lost City of episode two was discovered about 2,300 miles east of Florida only in 2000; the National Oceanic and Atmospheric Administration (NOAA) led a 10-day mission there in 2005. It is a hydrothermal field like no other seen on Earth: the vent fluids are alkaline, not acidic, and microbes live off methane and hydrogen instead of the carbon dioxide that is the key energy source for life at black smokers.

That hydrocarbons – the molecules that are the building blocks of all living things – are being created spontaneously within the Lost City’s chimneys has led many to believe that the life on Earth began at such a vent four billion years ago. A report assembling decades of research to make the case was published in the Astrobiology journal in 2014.

However, a paper published last year by researchers at the Tokyo Institute of Technology in Scientific Reports expressed scepticism, finding that “estimates of the ubiquity of suitable environments for the origin of life in and beyond our solar system may be somewhat overestimated”.

The Coral Reefs episode shows how an octopus and a grouper work together to catch prey. Photograph: BBC NHU 2017

Fish like to work together (episode three, Coral Reefs)

The unlikely cooperation between a reef octopus and a coral grouper – two predators working together to maximise their chances of capturing prey – had never been seen on film before the Blue Planet II team captured it: the grouper, spying fish hidden in crevices in the reef, signals to the octopus by tipping on to its head, flashing white and wiggling. The octopus – an animal that itself is thought of as solitary – then reaches in and flushes them out.

“They operate as a pair,” Sir David Attenborough told the podcast. “Sometimes the octopus wins, and sometimes the grouper does. It’s just extraordinary.”



Research on this cooperative hunting behaviour between groupers and not only octopuses but moray eels and wrasses was first published in the Nature journal in 2013. The lead author on the paper, Dr Alexander Vail, also shot the sequence over a month on the Great Barrier Reef.

Cooperation between different species tends to be fairly rare, given the likelihood for conflict over prey. “There’s potential for fights to break out, which means it’s just evolutionarily not generally selected for,” said Yoland Bosiger, the series researcher, on the same podcast. In this case, the octopus and the grouper take turns at reaping the rewards. In later research published in 2014, Dr Vail found evidence that coral grouper “choose appropriately when and with whom to collaborate” – opting to hunt with moray eels over other possible partners, or alone, when that added to their chance of success.





A giant trevally patrols the shallows of a lagoon. Photograph: BBC





Everything is a predator (episode four, Big Blue)

When yellow-fin tuna charge the lanternfish at more than 40 miles an hour, the sea churns and turns white. According to a maritime legend dating back centuries, this savage feeding frenzy is referred to as the “boiling seas” – but it hadn’t been captured on camera until the Blue Planet II team shot it from a helicopter off the Pacific coast of Costa Rica.

The real surprise, however, was that mobula rays – thought to subsist on plankton alone – joined in the frenzy alongside predators such as sailfish and spinner dolphins. According to Sarah Conner, the episode’s assistant producer, small fish found in the stomachs of dead rays in the past were thought to have been eaten by mistake.



The BBC footage was shared with Josh Stewart of the Manta Trust and the Scripps Research Institute, whose analysis found it to prove for the first time that mobula rays were deliberately and actively hunting fish. A scientific paper is forthcoming – “just one of many examples where Blue Planet II have collaborated with the scientific community to better our understanding of life beneath the waves,” said Conner.

Many predatory behaviours captured on Blue Planet II have never been seen before. One of the most jaw-dropping spectacles of the series so far was of giant trevally propelling themselves out of the water to catch birds mid-air. When the film crew arrived in the Seychelles to film it in September 2015, they had only reports of local fishermen to go by. Not even smartphone footage appeared to exist.



The remarkable final sequence, which took two years to film, showed not only the startling accuracy of the giant trevally’s aim, but its ferocity. The crew observed a Coke can with teeth marks in it, while a local fisherman had “a chunk taken out of his ankle”.

Life partner ... the mantis shrimp can form relationships that last up to 20 years Photograph: Yoland Bosiger/BBC NHU 2017

Monogamy has its advantages (episode five, Green Seas)



Cephalopods stole the show in episode five, which filmed octopuses attempting to smother a pyjama shark and covering themselves with shells to avoid detection. Elsewhere, on an outcrop of reef in South Australia – home to the world’s largest gathering of giant cuttlefish – a small male distracted the competition by pretending to be female.

In an episode that could have accurately been subtitled Octopus v Shark, the giant or zebra mantis shrimp, filmed in a seagrass mangrove in northern Australia, was never going to stand out. Yet its monogamous practice is rare in nature, and has been poorly studied in animals other than mammals and birds.

The male shrimp was shown hunting for food to share with its partner of up to 20 years. Monogamy may seem counterintuitive when mating with many different partners increases chances of reproductive success. But research published in 2015 suggests it may have evolved in shrimp as a means to avoid predation – just like the octopus’s shellsuit.

Dr Molly Wright at the University of California, Berkeley, concluded that mantis shrimp were most vulnerable to being eaten when hunting for new mates. Committing to one partner allowed them to live a sedentary lifestyle, away from predators.

In fact, mantis shrimps’ eyes are better developed than cephalopods’ – and those of humans. The male’s are particularly large, a characteristic thought to have evolved from the fact they spend more time hunting.

Sadly, the Blue Planet II footage poured cold water over reports of the mantis shrimp’s “marital bliss”, showing the male leaving his lifelong mate in her burrow – to venture down a larger one, home to another female. She had lost her mate, and was seemingly sending out distress signals to lure others’ away, the programme found.

In this case, it seems the base imperative to reproduce did trump monogamy – but “infidelity comes at a price”, as Sir David Attenborough noted. A larger female will produce more eggs, but she will also demand more food.

Evolving like a human being ... a Pacific leaping blenny in its nest hole Photograph: Chase Weir

Before blennies walked on land, we did (episode six, Coasts)

The coastline is not always the most enticing location for Blue Planet II viewers. As the producer Miles Barton explained on the Blue Planet II podcast, viewers have a tendency to favour settings full of “big, charismatic animals” such as dolphins and sharks. It’s harder work to get them as excited about more familiar locations, even if it’s ultimately “more rewarding to get people to realise the beauty in a rock pool”.

One small but significant scene from this episode was that of the Pacific leaping blenny, which throws itself out of the water to reach its miniature caves above the tideline. To capture the slow-motion sequence of 3in fish leaping, a cameraman sat for hours on a short fisherman’s stool, waist-deep in the tide.

The blenny is the most terrestrial fish on the planet, as evidenced by its apparent feet – stiff fins that serve to propel it out the water. What probably evolved as a means of escaping predators is now their preferred state of being, with blennies spending most of their lives above water.

Dr Jon Copley, an associate professor at the University of Southampton’s National Oceanography Centre, says that more than 30 species of fish have evolved to crawl out of the ocean over the last 400m years. Human beings’ ancestors would have been among them. We’re simply the result of an extremely long process that blennies are currently in the throes of.

“The blennies haven’t stopped evolving,” Copley told the Blue Planet II podcast. A paper published in May this year explored in detail their “colonising land … [and] terrestrial activity”, with a view to establishing what their transition from water on to land might have taught us about our own.

We are a pestilence on this Earth (episode seven, Our Blue Planet)

In the 16 years since the original Blue Planet broadcast, it’s not only our technological ability to capture marine life in all its majesty that has advanced: it’s our understanding of the many ways in which we’re destroying it. Every episode addressed humans’ devastating impacts on oceans and the life within them, whether through over-fishing, plastic, or noise pollution.

One of the most haunting sequences of the series was in episode four, a pilot whale carrying its dead calf thought to have been poisoned by its mother’s milk contaminated by plastic pollution. Criticism of the sequence as “fake news” and speculation seemed rather beside the point: one calf’s cause of death aside, it is impossible to overestimate the harm humanity is doing to marine ecosystems as a whole.

The finale of Blue Planet 2, which aired on Sunday, was an earnest attempt to communicate just that. It served as a kind of epilogue, pouring cold water on the jaw-dropping marvels of science and cinematography of earlier episodes by warning that, if we do nothing, they will exist only on DVD.

Orcas, hunting near herring fishing activity, get caught in nets. Albatrosses unwittingly feed their chicks plastic. Dolphins’ deaths are linked to plastic pollutants. Clownfish can’t hear each other communicate. Rare turtles’ breeding sites are being lost. Whale sharks are struggling to procreate in unprotected waters.

Despite the dire warnings, people around the world have devoted their lives to improving the state of our oceans, and not without success. Norwegian spring spawning herring populations have recovered. Sperm whales have returned to the seas off Sri Lanka. The intended takeaway is clear: we must all do our bit, and it does make a difference. But the biggest challenge, that posed by climate change, will take a global effort. For Blue Planet 2, for the first time in history, a manned submersible makes a historic dive of 1000m to reach the Antarctic seabed and record data on rising sea temperatures.

“What shocks me is how fast things are changing here,” says Dr Jon Copley, an associate professor at the University of Southampton. “We’re headed into unchartered territory. ... What’s happening here right now affects all of us.” As Sir David Attenborough makes clear in his addresses to camera (one delivered on a beach at night as he reclines alongside a turtle), those of us at home cannot sit by and watch it happen.

• This article was amended on 11 December 2017 because an earlier version referred to the National Oceanograpahy Centre as part of the University of Southampton.