When she spotted the mako shark in the Hauraki Gulf off New Zealand, Rochelle Constantine, a marine ecologist at the University of Auckland, was concerned. The animal had a curious orangey-brown mass perched on top of its head.

“At first, I was like, ‘Is it a buoy?’” Dr. Constantine said. “‘Is it entangled in fishing gear or had a big bite?’”

Wednesday Davis, a technician, sent up a drone to get a closer look at the 10-foot-long shark. As the boat sidled closer, her colleague Esther Stuck dangled a camera overboard to record some underwater footage.

“We could see these tentacles moving,” Dr. Constantine said.

Their eyes weren’t deceiving them. An octopus was riding the shark. They nicknamed it the “sharktopus” and said it was one of the strangest things they had ever seen in the ocean.

The team identified the eight-armed commuter as a Maori octopus. The hefty cephalopods can stretch up to 6.5 feet and weigh around 26 pounds. They are the largest octopus in the Southern Hemisphere. Even riding a huge predator like the shark, a shortfin mako, this hitchhiker occupied a lot of room.

“You can see it takes a fair amount of real estate on the shark’s head,” Dr. Constantine said of the encounter, which the researchers recorded during a field expedition to study marine life and birds in December 2023.

Tucking its arms into a tight ball, the stowaway looked like it was trying to go unnoticed. The octopus wasn’t clinging to the shark “like on a wayward banana boat ride,” Dr. Constantine said. “You could see, every now and then, this little tentacle gets pulled in.”

Although the shark might not have been able to see the crafty cephalopod, it was most likely aware of its passenger. Sharks have sensory organs called lateral lines all over their bodies to help them perceive the world around them.

Sharks and whales sometimes attract suckerfish, which cling on for protection and remove dead skin and parasites from the predator’s body. Makos are known to leap above the water’s surface. Some researchers speculate that when they fling themselves out of the water, they are trying to dislodge these riders when they become irritating. But this shark didn’t seem bothered by its freeloader.

“The shark seemed quite happy, and the octopus seemed quite happy,” Dr. Constantine said. “It was a very calm scene.”

Shortfin makos are the world’s fastest shark, and can swim at top speeds of 46 miles per hour. “Once the shark moves faster, the octopus probably wouldn’t have been able to hold on,” she said.

What was the unlikely duo’s fate? “We have no idea what happened next,” she added. If the octopus slid off, the shark could have eaten it. Equally, in waters this shallow — just 100 to 130 feet deep — there’s a chance the octopus could have landed safely on the seabed.

The biggest mystery is how these animals met. Maori octopuses live on the seafloor. While shortfin makos can swim more than 1,000 feet deep, they’re not usually found on the seabed. “It makes no sense that these two animals should be at the same place and time to encounter each other,” Dr. Constantine said. “We have no idea how they found each other.”

“It is almost impossible to speculate how, or why, this shark and octopus might have come together or what the nature of their connection might be,” said Abigail McQuatters-Gollop, an associate professor in marine conservation at the University of Plymouth in England who was not involved in Dr. Constantine’s observation. “But does that matter?”

While fascinating, Dr. McQuatters-Gollop thinks that what the team observed is ultimately a reminder of how little we know about the ocean, and how important protecting it is.

“The natural environment is a place where special things happen every minute of every day,” she said.