How jet propelled gelatinous marine organisms could inspire underwater vehicle design

Nanomia bijug is a marine animal related to jellyfish that swims with the use of jet propulsion. But unlike octopuses and squids, which use a single jet to propel themselves through the water, ituses a multi-jet approach.

A dozen or more structures on its body, called nectophores, pump water backwards to push it forwards. But it isn’t an all or nothing approach, the animal can control the jets individually – syncing them up or pulsing them in sequence.

According to a new study published in PNAS, the two different swimming styles actually allow Nanomia to prioritise speed or energy efficiency, in the same way many fish have distinct swimming modes that differ between steady, routine swimming and the speed needed to escape from a predator.

This “multi-jet” swimming strategy has only evolved in two taxa of marine swimmers: the siphonophores –which Nanomia is a part of – and the distant salps. This strategy could be used to inform the design of underwater vehicles that can change their propulsion to fit different needs.

Nanomia bijuga, jet-propulsion
Nanomia bijuga, a marine animal related to jellyfish, swims via jet propulsion. Credit: the Sutherland Lab.

Nanomia are technically a colony of individual animals called zooids – like corals are each a colony of thousands of individual organisms called polyps.

Each of Nanomia’s jets is produced by an individual unit called a nectophore, which are clustered on a stalk-like structure at the front of the animal and form the nectosome. They often have ten to twenty nectophores, though the exact number varies from colony to colony.

Wispy tentacles trail behind, carrying structures specialized for feeding, reproduction, and protection.

“Most animals can either move quickly or in a way that’s energetically efficient, but not both,” says senior author Kelly Sutherland, Associate Professor of Biology at the University of Oregon in the US.

“Having many distributed propulsion units allows Nanomia to be both fast and efficient. And, remarkably, they do this without having a centralised nervous system to control the different behaviours.”

The team used video recordings and computer models to analyse the swimming patterns, to understand how the different patters of activation of those nectophores would impact the animals’ swimming style.

“We’re interested in why multi-jet swimming is useful, and what we were really interested in here was the timing,” explains first author Dr Kevin Du Clos, a postdoctoral researcher at the Oregon Institute of Marine Biology, University of Oregon.

They found that by pulsing all nectophores in synchrony, Nanomia is sent forward very quickly, a tactic useful for evading predators. Asynchronous pulsing, on the other hand, moves the animal more slowly, but more steadily and their computer modelling experiments suggest that it’s a more energy-efficient way to swim.

Nanomia inhabits depths from the sea surface to at least 800 metres, sometimes migrating hundreds of metres vertically per day, so asynchronous pumping might be better suited for use then.

The researchers suggest that the general principles identified in this study could be useful for underwater vehicle research and design.

 “It gives a framework for developing a robot that has a range of capabilities,” says Du Clos.

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