Insect ‘submariners’ don’t implode at depth, leaving oceanic mystery unsolved

Deep-diving insect larvae are challenging a long-held idea about why there are no insects in the ocean, thanks to adaptable air sacs.

Lake fly larvae can withstand pressures equivalent to twice the depths they usually dive, demonstrating insects can indeed survive in the oceanic deeps.

The findings, published today in Science, bring into question a long-standing theory that the ocean is devoid of insect life due to their respiratory systems being unable to withstand the pressure.

“This was a big surprise,” says senior author Dr. Philip Matthews, professor of zoology at UBC. “It tells us pressure isn’t the barrier we thought it was to insect life colonizing the ocean.”

Daily dive to dodge death  

In Lake Malawi in East Africa, billions of lake fly larvae (Chaoborus edulis), make an unusual commute: By day, they sink more than 200 metres into the lake’s dark oxygen-starved depths to hide from predators where they respire anaerobically. By night, they rise to safely feed – avoiding a horde of hungry fish.

In the ocean, zooplankton use the same strategy to avoid predators, but it was thought that insects’ air-filled respiratory system would implode under intense water pressure before they could reach the deep, dark waters where they can safely hide from fish.

Deploying a sonar system at the bottom of the lake, Drs. Matthews, McKenzie and their team mapped the extent of Chaoborus’ full dive for the first time, watching “clouds” of larvae descend successfully into the fish-free depths.

“Examining the larvae, we found they had turned part of their respiratory system into two pairs of tiny pressure-resistant air sacs that function like adjustable floats to control their buoyancy,” said Dr. McKenzie, who conducted the research for his PhD. “While the existence of the air sacs was long-known, how they worked, and their capacity to function at extreme depth was a mystery - until now.”

Under pressure

The secret to the larvae’s air sacs lies in a stretchy protein called resilin. Found in insects, it gives flexible durability to moving parts like wing hinges or joints. In Chaoborus edulis’ air sacs, rings of resilin are encased in firm cuticle, expanding or contracting like an accordion as the insect changes the pH of the air sacs’ wall. To test how strong the sacs were, the researchers also placed them in tiny pressure chambers to simulate deep-water conditions. They proved remarkably tough, holding out to almost half a kilometre; far beyond the larvae’s daily dives.

Evolutionary trajectory

Chaoborus species can be found all over the world, and the researchers originally began studying the larvae closer to home: in ponds at UBC’s Vancouver campus. Living in shallow, fish-free waters, this local species can only dive around ten metres deep and have air sacs that are a different shape from Lake Malawi’s edulis larvae, which are uniquely curved, like a banana – a potential adaptation that allowed edulis to flourish asLake Malawi deepened to its current state, about 75,000 years ago.

So if pressure isn’t stopping insects from living in the ocean, what is?

Matthews says it could be caused by challenges in dealing with salt water, or from a lack of available ecological niches – the search for new food and habitats that caused insects to evolve on land.

“Crustaceans, which are distant cousins of insects, still fill those niches, so the insects can't just move back into an empty apartment – the tenants are still there.”

pH-powered engines

Beyond biology, the study may help inspire new technologies. Resilin has been studied in other insects as a nearly perfect biological rubber resistant to wear-and-tear. The ability of Chaoborus resilin to actively change air sac properties could be of interest to material science, McKenzie said.

“It could be developed as a so-called “smart material” that morphs its shape in response to changes in its chemical environment.”

“The resilin in Chaoborus air sacs doesn’t function as a passive elastomer, but through its ability to swell and shrink in response to changes in pH, it generates force of its own.”

For instance, such properties could potentially be harnessed to create artificial muscles or valves activated on chemical command.

For now, the discovery offers a simple but powerful insight: even the smallest creatures can solve seemingly impossible challenges.