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National Geographic

‘Sharktopus' wasn't the first. These animals also hitchhike on other animals.

Cockroaches that catch a ride on ants. Grasshoppers that climb aboard sheep. Hitchhiking animals are more common than you think. A classic example of phoresy, the remora uses a modified fin that latches onto larger animals like a suction cup. Here three remoras (Remora brachyptera) ride a Chilean devil ray (Mobula tarapacana) off the coast of Santa Maria Island, Azores. Photograph By Jordi Chias, Nature Picture Library Spend enough time in nature, and you will see some weird stuff—from orcas swimming around wearing salmon like hats to wombats pooping cubes. But when Rochelle Constantine saw a nine-foot-long mako shark jet past her research vessel in the Hauraki Gulf off New Zealand with a huge, orange octopus globbed onto its head, she knew she was witnessing something new. 'It was just one of those lucky days,' says Constantine, a marine ecologist at the University of Auckland in New Zealand. While sharks and octopuses may seem like animals that live in the same habitat—the ocean—Constantine says that each occupies very different areas. Mako sharks, for instance, are known for spending most of their time in the mid-water, while the New Zealand octopus is a benthic species, living nearly its entire life on the seafloor. 'So it just makes no sense how these two things found each other,' says Constantine. The researchers observed the duo for about 10 minutes and ultimately decided to nickname them the 'sharktopus'. Eventually, the odd couple swam off. Their fate remains unknown. From scuba diving to set-jetting 'We just let them go on their way,' says Constantine. The octopus-shark duo surprised both scientists and laypeople alike, but the behavior of one animal riding another is well known in biology. It's called phoresy, and it mostly occurs in smaller animals, especially insects and other invertebrates, that can't disperse as well on their own. Octopuses aren't known to practice phoresy, though with all those suckers, they seem especially equipped to do so. 'I guess the things that they might hitchhike on—seals, sea lions, dolphins, and sharks—you know, all of those animals eat octopus,' says Constantine. Animals typically don't practice phoresy with their natural predators, making sharktopus even more mysterious. Phoresy occurs all over the animal kingdom While animals occasionally come into contact with each other through happenstance or accident, phoresy serves a purpose. For example, plenty of creatures rely on hitchhiking to find mates or complete part of their life cycle. 'Dispersal is a key trait and affects things like gene flow and population structure,' says Andrew Bartlow, an ecologist at the Los Alamos National Laboratory in New Mexico. That means that getting from one place to another comes with big stakes for the animal and its ecosystem. In 2020, Bartlow published a review study of phoresy in the journal Biological Reviews. After scouring the scientific literature, he and his coauthor, Salvatore Agosta, found observations of phoresy in at least 13 phyla, 25 classes, and 60 orders of animals. All of which suggests that hitchhiking behavior has evolved many different times across the tree of life. Some classic examples include fish known as remoras, which use a fin modified into a sucker to latch onto larger animals, such as whale sharks, and barnacles that can be found securely fastened to the skin of whales or the shells of sea turtles. (Watch a bold little bird snatch a remora off of the world's largest fish.) But there are also cockroaches that ride ants, copepods that ride jellyfish, fairy shrimp that ride trout, water fleas that ride wild boar, millipedes that ride birds, sea anemones that ride crabs, and grasshoppers that ride sheep. Not to mention mites, which make up the vast majority scientific knowledge on phoresy. 'There are so many mites,' says Bartlow. Hitchhiking has its risks Catching a lift on a much larger animal has its tradeoffs. Imagine being a pseudoscorpion—a tiny arachnid the size of a pinhead. Unlike their scorpion cousins, pseudoscorpions lack venomous tails, but they do have similar-looking claws which they sometimes use to cling to larger insects, like the harlequin beetle. By grabbing hold of these beetles, pseudoscorpions don't just get a free ride to a new neighborhood—they receive targeted delivery, as the beetles they associate with are freshly transformed adults, who are themselves in search of newly fallen ficus trees, where they lay their eggs. What's more, each beetle can carry multiple pseudoscorpions while in flight, which means the morning commute is also a great place for pseudoscorpions to find mates. Males will even do battle with each other on the backs of these beetles, knocking rivals off whenever they get a chance. And once one male has monopolized the ride, he can woo any females aboard and even begin the process of fertilizing their eggs. Pseudoscorpions are known to hitch rides on larger flying insects, like harlequin beetles. Photograph By Andrés Better Pseudoscorpions, like this Chthonius ischnocheles from the United Kingdom, are tiny arachnids that have scorpion-esque claws. But for the pseudoscorpions who get knocked off in flight? They'll likely be stranded who knows where. The jungles where these animals live are vast, and full of hungry mouths. 'The risks are quite enormous,' says Bartlow. And the same risks apply to mites riding a migrating bird, or crustaceans known as ostracods riding a frog from bromeliad plant to bromeliad plant—fall off at the wrong time, and you might wind up in a habitat you cannot survive in. 'Using frogs as a mode of transport is beneficial because it allows microorganisms to reach environments where they can survive,' says Alan Pedro de Araújo, an evolutionary ecologist at State Department of Education in Goiás, Brazil. 'Alternatively, they can be d by wind or rainwater, but these methods are highly random and unreliable,' he says. 'Without frogs, the success of their migration would be significantly reduced. The dark side of phoresy Phoresy is extremely common among mites. Here flower mites are visible on the beak of an Ecuadorian hillstar hummingbird (Oreotrochilus chimborazo), as it drinks nectar from a flower. The mites use the hummingbird as transport to move from flower to flower, where they feed on nectar and flower tissues. Photograph By Jim Clare, Nature Picture Library Even with as many examples as Bartlow managed to find, he says there are probably tons more waiting to be discovered. For instance, the ocean seems to be underrepresented in observations of phoresy, likely because it's much more difficult for scientists to study what happens below the waves. Scientists are also still trying to understand how phoresy works. For example, in some species, the relationship between the rider and its host can veer towards the macabre. 'Some barnacles are more parasitic than others,' says Bartlow. Likewise, when freshwater mussel larvae latch onto the gills of the fish who will carry them upstream, the baby mussels also begin to leach nutrients from their hosts, like tiny vampires. 'These things are on a spectrum,' explains Bartlow. But ultimately, scientists suspect phoresy can lead down an evolutionary path toward parasitism. As for the so-called sharktopus, the jury is still out on exactly how or why the cephalopod might have found its onto the Mako shark's noggin and whether this example truly counts as phoresy or a chance encounter. Even after a few weeks of media attention and people coming out of the woodwork to offer similar stories on social media, Constantine says they're no closer to solving the mystery. 'There's been a huge amount of media around us,' says Constantine. 'There is not one single explanation for what we've seen.'