“It is Practically Hovering on Air”

Sea skater dodges water downpour. Reproduced under Creative Commons Attribution License​ 4.0 @ Mafadik et al. 2020​.

Tiny sea skaters may hold the secret to developing improved water-repellent materials. A study by researchers at Saudi Arabia’s King Abdullah University of Science and Technology (KAUST) provided an unprecedented look at the insect’s physical features, including the hairs and waxy coating that cover its body, and its movement to evade the sea’s dangers.

The KAUST team relied on the expertise of Lanna Cheng, a marine biologist at Scripps Institution of Oceanography at UC San Diego who is the world’s leading authority on marine insects. 

“Our multidisciplinary study is the first of its kind to investigate two marine skater species, the ocean-dwelling Halobates germanus, and a coastal relative, H. hayanus,” said Gauri Mahadik at the Red Sea Research Center, who worked on the study with colleagues under the supervision of Himanshu Mishra, Carlos Duarte, and Sigurdur Thoroddsen. “We wanted to understand how these insects had evolved to survive in harsh marine environments where others failed.”

The ocean insect Halobates was first discovered during a Russian circumnavigation voyage around the world and described in 1822. Although it has been collected on almost all subsequent oceanographic expeditions, it was largely neglected by marine biologists until Cheng joined Scripps Oceanography in 1970. She collected specimens during an expedition on the Scripps research vessel Thomas Washington in 1973 and succeeded in bringing them back to campus alive. It was not possible, however, to keep them alive in the laboratory for any length of time.  

All subsequent work had to be carried out at sea or based on specimens collected and preserved. Cheng was the first to discover the unique body covering of Halobates which consisted of densely packed tiny hairs measuring 1.5 microns in length. This hair layer acts like an air bubble surrounding the insect. She hypothesized that it not only protects the insect from drowning but also enables it to breathe underwater during accidental submergence — possibly the most important adaptation for Halobates to survive in the open ocean. 

When scientists at KAUST became interested in studying Halobates, they discovered Cheng’s long history of research on sea skaters and reached out to her for assistance. 

Researchers captured the two Halobates species from the Red Sea and coastal mangrove lagoons at KAUST and acclimatized them to an aquarium environment.

“It is difficult to keep marine Halobates in the lab, and there was considerable trial and error before we got it right,” said Mahadik. “These insects are cannibalistic, so it was important to keep them well-fed. We spent hours trying to capture their natural behaviors on film because they jump around a lot.”

The researchers used high-resolution imaging equipment, including electron microscopy and ultrafast videography, to study the insects’ varied body hairs, grooming behavior, and movements as they evaded simulated rain drops and predators. The insect’s body is covered in hairs of different shapes, lengths and diameters, and it secretes a highly water-repellent waxy cocktail that it uses to groom itself.

“The tiniest hairs are shaped like golf clubs and are packed tightly to prevent water from entering between them. This hairy layer, if the insect is submerged accidentally, encases it in an air bubble, helping it to breathe and resurface quickly,” said Cheng.

“In its resting state, not even five percent of the insect’s total leg surface is in contact with the water; so it is practically hovering on air,” said Mishra.

If water droplets land on the creature, they roll off or, as the KAUST team caught on camera, the insect jumps and somersaults to shed the drops. The researchers were surprised by how fast it moved to evade predators and incoming waves.

“While taking off from the water surface, we observed H. germanus accelerate at around 400 meters per second,” said Thoroddsen. “Compare this with a cheetah or Usain Bolt, whose top accelerations taper off at 13 m/s2 and 3 m/s2, respectively. This extraordinary acceleration is due to the insect’s tiny size and the way it presses down on the water surface, rather like using a trampoline, to boost its jump.”

The wax secreted by the insect is of great interest to the team’s materials scientists, who are exploring new approaches for liquid repellent technologies. The insect’s hair structures are also informing the design of new materials.

“Inspired by the mushroom-shaped hairs of Halobates, my group is developing greener and low-cost technologies for reducing frictional drag and membrane fouling,” said Mishra.

• Adapted from KAUST