APS News | Research

From Mudskippers to Jellyfish, Aquatic Animals Inspire Robot Designs

For the APS March Meeting, early-career scientists shared their work on waterborne fauna.

Published Apr 12, 2024
An artistic rendering of jellyfish swimming at sea
An artist’s depiction of “biohybrid” jellyfish, each equipped with a small electronic device that enables them to swim faster.
Caltech/Rebecca Konte

Fish, water-hopping bugs, and jellyfish may traditionally be the purview of biologists, but this year’s March Meeting in Minneapolis featured plenty of research on aquatic animals, particularly their locomotion. Interdisciplinary collaborations aim to incorporate these findings into designs for more effective robots. Here are a few projects that piqued our interest.

Jellyfish cyborgs

Timelapse footage of two jellyfish in a tank of water
A jellyfish fitted with the Caltech team’s device — what they call a “forebody” — swims faster than a jellyfish without. This image is rotated 90 degrees.
Caltech

Simon Anuszczyk, an aeronautics engineering graduate student at Caltech, and his adviser, Caltech engineer John Dabiri, have developed an electronic system that affixes to real jellyfish and makes them swim faster. That’s right: They created a jellyfish cyborg.

Jellyfish move by contracting their umbrella-shaped bells, propelling water in the opposite direction. The Caltech team’s device, a dome-shaped “hat,” delivers small electric pulses to the animal’s bell, causing the jellyfish to contract with higher-than-normal frequency. The stimulus allowed their jellyfish to swim 4.5 times faster than their natural speed of about one centimeter per second.

So far, the team has run tests on a previous version of the biohybrid jellyfish in the ocean. For this model, they’ve run tests in a tank six meters deep and plan to study it in the ocean.

The group chose jellyfish because they are “the most efficient animal in the world,” says Anuszczyk, in terms of energy consumption per distance traveled per mass. Jellyfish also live in every ocean and can swim to great depths.

This makes them useful picks for the team’s ultimate goal: to deploy these jellyfish for undersea applications. For example, only a quarter of the ocean floor has been mapped with modern, high-resolution technology. If the team’s devices were equipped with special sensors, jellyfish could collect data in previously unexplored parts of the ocean, or help monitor the effects of climate change.

“We might study the carbon cycle in the ocean or ocean acidification by attaching a pH sensor onto the jellyfish,” says Anuszczyk.

The group worked with bioethicists while designing their cyborg. Jellyfish lack a centralized nervous system and pain receptors, Anuszczyk says. And in their experiments, they found that the jellyfish did not release mucus, a typical stress response, and healed from any damage within 24 hours of the device being removed.

Amphibious fish

A mudskipper jumps up from the ocean sand
Mudskippers can move on land using their pectoral fins and even jump to attract mates.

Once a day, Divya Ramesh and her colleagues drop fish food into the aquariums in their lab, feeding a menagerie of three unusual species: the ropefish, bichir, and mudskipper. All are amphibious fish whose specially adapted ways of breathing allow them to survive on land, from a few hours up to days.

“We study amphibious fishes because they're so capable of moving across the water-land interface,” says Ramesh, a graduate student in mechanical engineering in Chen Li's Terradynamics Lab at Johns Hopkins University. Each species does it differently. The ropefish slithers like a snake, while the bichir assists its slither with its fins, akin to a human army-crawling. The mudskipper propels itself using its pelvic fins like a person using crutches — the only animal known to move like this.

Recently, Ramesh has studied how the animals move on mud of varying wetness. As the mud gets wetter, the mudskipper slows down. But the ropefish and bichir move just as fast. On drier mud, the ropefish and bichir lift their bodies to reduce drag.

But fish aren’t always cooperative. “The animals get tired and don’t want to move,” says Ramesh. To conduct their research more consistently, collaborator Gargi Sadalgekar, also a grad student in mechanical engineering at Hopkins, created a robot that could move like all three species. The team’s goal is to eventually develop robots that can navigate water, land, and the muddy in-between. Such robots could monitor soil properties near rivers or deliver first aid to people injured or stuck in these environments, says Ramesh.

The work has also shaped how Ramesh sees the world. “If I see any animals when I go outside, I start analyzing their movement,” she says, even that of her own dog. “He doesn’t run like other dogs,” she says — his gait includes an unusual gallop. “Why?”

Water striders

A Georgia Tech team found that water striders move in a motion most commonly associated with bacteria.
Credit: Ishant Tiwari/Georgia Tech

Ishant Tiwari, a postdoc at Georgia Institute of Technology, studies Rhagovelia, or water striders, which exploit high surface tension to walk on water. The bugs first captured Tiwari’s attention shortly after he moved to Georgia, when he saw the insects on a hike. Wanting to study how the insects moved, he and his colleagues captured a few in a net and brought them back to the lab in a box full of dried leaves.

Analyzing their behavior in a tank, Tiwari and his colleagues found that the insects move with a characteristic “run-and-tumble” motion — runs of straight lines followed by random changes in direction. Curiously, scientists most commonly associate “run-and-tumble” movement with the bacteria E. coli. Tiwari says this is the first instance he’s seen of a macroscopic animal moving this way.

Researchers think this movement pattern helps bacteria explore a space to find food, but the specifics aren’t clear. “We are studying how this could be an efficient way of searching,” says Tiwari.

These studies could inspire more efficient search-and rescue algorithms, like those used by the Coast Guard to quickly find people lost at sea. “These algorithms would eventually be translated to patterns of movement for robots,” he says. This research is still in its early stages, but “that’s the dream.”

Sophia Chen

Sophia Chen is a writer based in Columbus, Ohio.

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