The research vessel Aurora Australis heaves through a lumpy Southern Ocean, albatrosses and prions wheeling in its wake. On board is marine ecologist Stephen Smith, on his way to subantarctic Macquarie Island to study the effects of an oil spill on the rocky shores there.
While others in the team pass the time spotting seabirds from the bridge, Smith’s gaze is instead attracted to hunks of bull kelp roiling off the side of the vessel. Even here, hundreds of kilometres from land, they are surprisingly common. With two days at sea remaining, he gives in to his researcher’s instinct. He starts counting.
Several years later, after two more voyages and some demanding mathematics, Smith arrives at a number: 70 million. At any one time, he predicts, that’s how many chunks of bull kelp are afloat on the Southern Ocean.
His finding has huge implications for our understanding of how life on Earth travels from one place to another. That’s because these aren’t dead chunks of seaweed, bound for oblivion. At the base of many dwell little ecosystems. Eventually, most of them wash up somewhere.
Bull kelp is found all around the colder reaches of the Southern Hemisphere—on the shores of South America, New Zealand, and the wild islands that stud the Southern Ocean. It clings to rocky surfaces with a fleshy fist called a holdfast, a tough structure strong enough to withstand the waves that pound it.
As the kelp grows, it becomes encrusted with barnacles and bryozoans, which form colonies on the holdfast, blades and stem. Worms and tiny, shrimp-like crustaceans—amphipods and isopods—eat holes in the base of the kelp, and these are later colonised by slugs, limpets and heavily armoured chitons.
Some of these creatures scavenge for food on and around the kelp, while others are filter feeders, sifting their meals from the constantly swirling water. The deep recesses between the holdfast and the rock provide these small animals with protection—from both the tug of the surf and from predators, such as sea stars, that prowl the kelp forest.
When kelp is ripped off the coastline by storms, so strong is the holdfast’s grip that it usually takes a chunk of rock with it. Some of these fragments weigh up to 100 kilograms.
“The kelp is just honeycombed with gas sacs,” says University of Otago evolutionary geneticist Hamish Spencer. “So it’s amazingly buoyant and tough. A whole community goes with it. If you’re under there, you’re on a trip somewhere and you don’t know where you’re going.”
Where you’re most likely to be going is the Antarctic Circumpolar Current, which flows eastwards in a loop around the southern latitudes. It’s the biggest current on the planet, transporting 150 times as much water as all the world’s rivers combined.
Numerous smaller currents eddy and swirl around it, bathing the land masses of the Southern Hemisphere.
Kelp riding these underwater rivers can end up just about anywhere. Some of this castaway material finds its way to the shores of southern New Zealand, where in the past decade it has caught the attention of a group of University of Otago scientists tackling the big question of how life gets around on the high seas.
How animals and plants came to live across the scattered land masses of Earth is a question scientists have pondered for hundreds
Why, for example, are there lemurs on Madagascar, 400 kilometres across the ocean from their nearest primate relatives in Africa? How is it that some Pacific islands have iguanas, and some don’t? When was New Zealand populated by flightless birds?
Charles Darwin proposed that species ‘rafted’ from one place to another—floating here and there on vegetation or icebergs—but this idea has long been considered far-fetched. After the theory of continental drift came to the fore in the mid-20th century, plants and animals were thought to have become separated when land masses split up, and over millions of years, evolved into
Now, scientists can measure genetic differences between closely related species and, using ‘molecular clock’ techniques, work out how long it has been since they diverged from one another. Then they compare this with geological evidence about how long land masses have been separated. Often, geology and biology just don’t match up.
Nothofagus beech forest, for example, clothes the Southern Alps, but closely related species are also found in Australia, New Guinea and South America. These beech trees were thought to have been separated 80 million years ago, in the breakup of the ancient supercontinent Gondwana, but molecular analysis shows that our beech trees actually split up with the Australian ones just 30 million years ago.
Monkeys are found on both the African and South American continents, despite those land masses breaking apart 110 million years ago, long before monkeys existed.
Did beeches and monkeys float across the ocean instead? Solid evidence for rafting is hard to come by—so University of Otago biologist Jon Waters began looking around his own neck of the woods for answers. He was fed up with theories—he wanted to prove something.
“I wanted to get away from storytelling,” he says, “and come up with some decent mechanisms for how things could possibly get around.”
Waters and his colleagues started looking for evidence of rafting happening right in front of them, on a regular basis. Stephen Smith’s 70 million kelp rafts, and the communities of tiny animals that live on them, were an obvious place to start.
Most kelp dwellers are small, sedentary creatures, content to live their lives on a single kelp plant. In other words, not obvious candidates for epic voyages of thousands of kilometres.
“Many of these things are terrible dispersers,” says Hamish Spencer. “You wouldn’t expect them to have a broad distribution. But then they do. So how do you explain it?”
Some species lay eggs that release larvae into the ocean currents, but these usually don’t survive long enough to cross oceans. Kelp residents whose eggs hatch into fully formed offspring, rather than larvae, spread more widely around the world than those with a free-living larval stage.
By studying the genetic signatures of species that wash in on the tide, and matching these with samples from elsewhere, Waters and Spencer’s team can usually pinpoint where they have come from. They began by looking at the genes of kelp washed up on Dunedin beaches and found that much of it had arrived from islands in the subantarctic.
Then they turned their attention to the little creatures they were finding on the kelp, comparing their DNA with specimens from all around the Southern Hemisphere. This revealed something astonishing—not only were these animals crossing vast ocean barriers and colonising new land masses, but they were doing so all the time.
“There’s a constant rain of things,” says Spencer.
Rebecca Cumming, another member of the team, studied a species of marine slug in the Onchidella genus, for example, that is found in New Zealand, South America and the Falkland Islands. Genetically, they’re no different from each other, which suggests the gene flow across the ocean is essentially uninterrupted. In a genetic sense, it is as though the Pacific Ocean didn’t exist.
The team has also studied chitons, limpets, sea stars and topshells, and found similar patterns.
Even Bluff’s treasured oyster, Ostrea chilensis, is an intercontinental hitchhiker, having at some stage crossed the 7500-kilometre gulf between New Zealand and South America—one of the longest stretches of open water on the planet.
“It’s really satisfying when you cross another one off the list,” says Waters. “It’s not just telling a story; now we’ve got evidence.”
What about the kelp itself? Unlike the roots of a plant, a kelp’s holdfast provides no nutrition to the plant. It’s simply an anchor. Once torn from the coast, kelp can carry on living in cold water for many months, meaning that when it arrives on a new coast, it can release spores and colonise bare rock.
“It’s happening every day,” says University of Otago marine biologist Crid Fraser, who has been testing bull kelp around the Southern Ocean. “And it’s very effective—when there’s new territory to colonise. Interestingly, we don’t see any evidence of it having an impact when there’s already established kelp and invertebrates.”
If shores are already crowded with kelp forest, says Fraser, there’s just no room at
“So the kelp that’s washing up at St Clair Beach all the time, some of it’s coming from the subantarctic, but it’s not getting its foot in the door. It washes up, those animals die and the kelp doesn’t get established.”
What’s needed for a successful rafting event, says Fraser, is disturbance. On an earth-shattering scale.
The 2016 Kaikōura earthquake presented Jon Waters and University of Otago geologist Dave Craw with a once-in-a-lifetime opportunity to observe the process of recolonisation. The quake lifted a long stretch of coastline out of the water by as much as five metres.
Over the past year, Waters and Craw have been monitoring the uplifted area to record how, and from where, species are arriving. So far, kelp rafts from Otago and the subantarctic islands have been found washed up on the uplifted shore.
“It’s a really interesting way of seeing how ecosystems get assembled—how things start and which things get in first,” says Waters. “Some of the species that live inside the kelp are already back here—snails, chitons and burrowing crustaceans. The things that can get around well are getting back within a year.”
As warming waters affect species’ travel around the planet, you might expect these changes to be more obvious in the Far North. Warm currents from Australia and the western Pacific bathe the tail of Māui’s fish, regularly bringing species from places such as New Caledonia and Australia to our shores.
Massey University PhD student and New Zealand Geographic contributing photographer Irene Middleton has spent countless hours diving in Northland’s waters, looking for these unusual visitors.
Her research has recorded a swathe of tropical species: batfish, mahimahi and sargassum fish, plus barnacle crabs and fireworms from the Indian Ocean.
While the tiny larvae of many fish species are capable of drifting long distances on ocean currents, the gap between their places of origin and New Zealand is too great for them to have made it all the way here alone.
“New Zealand is pretty isolated,” says Middleton, “so there has to be some sort of stepping-stone habitat for things to be actually turning up here.”
Middleton thinks adult fish may be making the long journey in association with flotsam.
Objects that get caught in ocean currents quickly accumulate residents. Goose barnacles sprout from exposed surfaces, creating little forests of life where molluscs and other sea creatures thrive. In the emptiness of the open ocean, flotsam provides shelter and food for fish. The longer an object is at sea, the more species it accumulates.
In particular, plastic is helping many species cross the high seas, says Middleton.
A trigger fish, native to the tropics and never before seen in New Zealand, was recently found hiding out under a floating bottle in the Kermadec Islands.
“There are much more long-lived debris ending up in the ocean now. Previously it might have been a raft of weed that would break apart in a couple of weeks’ time, so a species might not have turned up here, but now it might be a big sheet of plastic or a crate that could extend some things’ dispersal potential well beyond where it would have been.”
Unlike species that arrive in ships, in their bilge water or attached to hulls, species that float have time to acclimatise to changing water temperatures.
“By the time that debris gets here, there could be a population of these organisms growing on it that are acclimatised and ready to settle,” says Middleton.
Warming ocean temperatures may make it easier for tropical species to successfully reproduce and form viable populations.
University of Otago PhD student Kerry Walton is working alongside Middleton to form a clearer picture.
Despite the constant flow of tropical species into New Zealand waters, Walton believes the chances of most becoming established here are slim, at least under ‘normal’ circumstances.
“A flotsam raft bringing just a handful of individuals and species is unlikely to result in those species establishing,” he says. “Generally speaking, if you’re a population in low numbers, you have a very low chance of long-term survival, but having a slightly larger founding population will greatly increase chances.”
In the case of these warmer-water visitors, it would take something dramatic for species to get established here, believes Walton: a natural disaster, a major storm, or a very large object that has been at sea a long time arriving on our beaches.
Semi-sunken hulks of boats, for example, can drift for months or years, accumulating large communities.
In March 2019, a recreational fishing boat washed up in Henderson Bay in Northland. It had been abandoned off the coast of Australia 18 months earlier. It was carpeted in foreign life, including dozens of species of molluscs.
Numerous such wrecks have appeared on Northland shores in recent decades, many from Southeast Asia.
“There were hundreds or even thousands of individuals of many of those species on those wrecks,” says Walton. “The high numbers mean the chances of successful breeding is much greater.”
During the weeks after the Henderson Bay wreck was discovered, an Australian scallop species was frequently seen swimming hundreds of metres from the wreck. It remains to be seen if the scallops have managed to establish here.
Tsunamis and storms dislodge large amounts of material, both natural and manmade, into the ocean. Following the 2011 earthquake and tsunami on the east coast of Japan, debris is still washing up on North America’s western shores. Almost 300 species of Japanese origin have been recorded living on and around these objects.
One piece of a floating dock, which landed in Oregon, carried nearly two tonnes of living matter, much of it foreign to the United States. It is not yet known if any species have become established there, but keeping tabs on arrivals along such a long stretch of coast is virtually impossible.
Among the Japanese fish believed to have made the 7000-kilometre crossing of the Pacific are barred knifefish, which have been seen swimming in Californian waters since 2014. These fish have also recently been found in New Zealand. How they arrived here is not clear, but Middleton says rafting is the most likely scenario.
Huge rafts of floating rock pumice, such as that that spewed from an undersea volcano in the Tongan group earlier this year, may also provide opportunities for the large number of organisms to establish on foreign shores.
A 2006 Tongan eruption produced a raft comprising an estimated 100 trillion pieces of pumice. Scientists found more than 80 species living on and around the pumice—an enormous biological consignment which travelled more than 5000 kilometres before making landfall in Australia.
As we deepen our understanding of the way species move across the ocean, the potential of rafting as a means of getting around becomes more and more plausible, and with it our view of New Zealand’s biological fixity. Perhaps not so much a land in splendid isolation, but, as biogeographer Matt McGlone memorably phrased it in 2005, the “fly paper of the Pacific”.
And as our oceans change, and the amount of junk within them increases, so may the swarm of new arrivals.