An octopus may not seem too menacing to people. But to a hermit crab, the writhing tentacles signal a lethal threat from a hungry predator and prompt a hasty retreat into its borrowed shell. The bold crab that instead lingers outside its home is just asking for a deadly hug.

Now, marine scientists are wondering whether a dramatic, global shift in seawater chemistry could make some deep-sea hermit crabs bolder — or rather, more foolhardy.

Enter the toy octopus: A team of researchers in California is exploring how the changing ocean chemistry affects a hermit crab’s fight-or-flight response by simulating octopus attacks in the laboratory. A video of one of these ambushes was a big hit with scientists attending the Third International Symposium on the Ocean in a High-CO2 World in Monterey, Calif., last week, sparking laughs and a bit of buzz.

Although seemingly lighthearted, the toy octopus experiment is part of a deadly-serious effort to understand the worrying ecological implications of a process known as ocean acidification.

Over the last few centuries, the ocean has absorbed huge amounts of the carbon dioxide spewed into the atmosphere by human activities, such as burning fossil fuels.

The uptake is helping to slow climate change, but also fueling chemical reactions that are shifting the pH of seawater toward the acid end of the scale.

On average, researchers estimate that surface waters, where key players in the ocean food chain live, have seen a 0.1 decrease in pH since the beginning of the Industrial Revolution; that’s an extraordinarily rapid 30 percent increase in acidity.

Many researchers worry that acidification will make life harder for some shell-building marine organisms such as lobsters, clams, crabs and shrimp; more-acidic water could corrode the creatures’ shells, or make it harder to build them in the first place. But the impact of the changing water chemistry could go even deeper.

Researchers have also been surprised to discover that exposure to these waters can change the behavior of some marine organisms, such as by disrupting brain development in fish. They are particularly worried about acidification’s impact in the deep sea, which could be hit hard by changes in pH.

To see how acidification might affect one deepwater creature, marine biologist Taewon Kim and colleagues at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif., used a robot submarine to vacuum up some deep-sea hermit crabs (Pagurus tanneri) that live off the coast of California at depths of 900 meters. Once the crabs were back in their laboratory, the researchers divided them into two groups: Some lived in tanks filled with seawater with a pH of 7.6, typical of the crab’s deep-sea home; the others lived in seawater with a more acidic pH of 7.1, representing what the deep sea could be like in the future.

Kim’s team then measured a variety of differences between the two groups: how much oxygen they consumed, how quickly they detected prey, and how often they “sniffed” surrounding waters by flicking their antennae.

In general, they found that the crabs in the more acidic water tended to flick their antennae less often, and were slower to sniff out food. But there was a lot of variation among individuals, he told an audience in Monterey, suggesting some crabs have the ability to cope with rising acidity levels.

The researchers also looked at whether acidification altered the “boldness” of the crabs, or how long it took them to withdraw into their shells when attacked by a toy octopus held by a scientist. But there was no statistically significant difference between the two groups, Kim told an audience in Monterey — more acidity didn’t make the crabs become foolishly brave.

That negative result, however, didn’t stop researchers from laughing in delight at the sight of a scientist splashing away with the toy octopus — or sending out numerous tweets and e-mails about Kim’s droll and often hilarious talk.

It ended with Kim suggesting that people might have to start a housing aid group for hermit crabs if a more acidic ocean begins dissolving the abandoned shells they use as their homes. In a nod to Habitat for Humanity, Kim said such an aid group could be called “Habitat for Hermanity.”

 

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