A strange sea creature known as the salp reproduces asexually, building long chains of clones. Researchers have now decoded the way these long salp chains propel themselves, a finding that could lead to better propulsion systems for underwater vehicles.
Salps are clear gelatinous creatures that resemble jellyfish but are actually a lot closer to humans thanks to their advanced nervous systems complete with brain, and their complex digestive and circulatory systems. Salps are technically a microplankton that can exist as solo creatures in the sea, but more often link up to form long chains that can stretch to be up to 15 ft (about 4.5 m) long. They have two phases of reproduction: one asexual during which clones are created that get added to the chain and one sexual as they break free as hermaphrodites and carry a baby salp inside. This dual-phase reproductive ability makes them "one of the fastest growing multicellular organisms on Earth," according to the Woods Hole Oceanographic Institute.
“Salps are really weird animals,” said Alejandro Damian-Serrano, an adjunct professor in biology at the University of Oregon's (UO) Institute of Marine Biology. “While their common ancestor with us probably looked like a little boneless fish, their lineage lost a lot of those features and magnified others. The solitary individuals behave like this mothership that asexually breeds a chain of individual clones, co-joined together to produce a colony.”
Nightly marathon
While the reproductive prowess of salps is impressive on its own, the recent UO-led research on the creatures was more concerned with the way salp colonies locomote.
“The largest migration on the planet happens every single night: the vertical migration of planktonic organisms from the deep sea to the surface,” said lead researcher Kelly Sutherland, an associate professor in biology at UO. “They’re running a marathon every day using novel fluid mechanics. These organisms can be platforms for inspiration on how to build robots that efficiently traverse the deep sea.”
So Sutherland and her team used specialized cameras to track the nightly undersea salp odyssey and found two forms of locomotion. While all salps use a jet propulsion system in which they take in, filter, and then expel water, the team found that smaller chains of the creatures moved in spiral patterns the way in which a well-thrown football sails through the air. Larger salp chains, however, moved in a corkscrew pattern known as helical swimming.
Wonder and awe
The means of locomotion are not new findings. In fact, earlier last year the propulsion patterns of salps were used as inspiration to create a series of linkable soft robots. But what is new here is that the UO researchers figured out that different salps in a linked chain activate their jets at slightly different moments, which creates an incredibly smooth movement pattern for the whole colony. They also took note of the angled orientation of each salp in a chain to understand how the colony's corkscrew motion was created.
“My initial reaction was really one of wonder and awe,” said Sutherland. “I would describe their motion as snake-like and graceful. They have multiple units pulsing at different times, creating a whole chain that moves very smoothly. It’s a really beautiful way of moving.”
While we've seen plenty of smaller underwater robots inspired by sea creatures, the findings in this study, says Sutherland, could lead to the design of larger undersea vehicles that are more efficient and create less turbulence.
“It's a study that opens up more questions than provides answers,” she said. “There's this new way of swimming that hadn't been described before, and when we started the study we sought to explain how it works. But we found that there are a lot more open questions, like what are the advantages of swimming this way? How many different organisms spin or corkscrew?”
You can learn a little more about sea salps in the following video from the Woods Hole Oceanographic Institute.
The research has been published in the journal Science Advances.
Source: University of Oregon