New study reveals an unsuspected range of behaviors in the sea squirt Ciona
New research on the sea squirt Ciona intestinalis challenges the view that animals living fixed in place have simple behaviors and limited responses to their environment.
Hovedinnhold
The sea squirt Ciona intestinalis, a small, sessile marine animal closely related to vertebrates, is found along the Norwegian coast where it lives attached to mooring ropes and other hard surfaces. While being a key part of marine ecosystems, sessile animals are often overlooked by researchers and the public. For that reason, researchers Oleg Tolstenkov and Sissel Norland from the Chatzigeorgiou research group at the Michael Sars Centre decided to investigate whether there was more than meets the eye to a sea squirt’s daily routine.
“Although these animals look very simple and immobile, we found that they actually display a surprisingly rich range of behaviors”, Tolstenkov said. “Honestly, we did not expect to find such a rich behavioral repertoire. We definitely underestimated those sea squirts!”
Uncovering patterns of behavior using machine-learning
To record and analyze the movements of adult Ciona, the team used advanced imaging and machine‑learning–based tracking. “A key technique behind this work was the use of elliptic Fourier descriptors, or EFDs, which allowed us to quantitatively describe and track subtle changes in the animal’s body shape over time”, Tolstenkov explained. “The method itself already existed, but to our knowledge we were the first to apply it in this way for behavioral analysis.”
The experimental approach identifies three core “postural engagement” states, ranging from inactive to highly dynamic, that differ in body shape, movement patterns, and duration. Animals transition between these states in structured, predictable ways.
Crucially, the findings show that external sensory cues strongly reshape this behavioral landscape. Mechanical disturbances such as touch triggered rapid, full-body contractions resembling defensive responses. Chemical cues also produced stimulus‑specific motor patterns depending on whether the cues signaled danger or food. These shows that Ciona can discriminate between different types of environmental signals and respond appropriately.
“This suggest that even early chordates may have had a surprisingly complex way of sensing and reacting to their environment.” - Sissel Norland
A full body behavioral map
“In this study, we also show that stimulation of different parts of the body trigger distinct responses, revealing the basic principles of how the nervous system integrates information across the body”, Norland said. “This suggest that even early chordates may have had a surprisingly complex way of sensing and reacting to their environment.”
The mechanosensitivity of the animal’s siphons was previously demonstrated, but other parts of the body had not been systematically tested before. This regional sensitivity allows Ciona to encode both the type and location of stimuli, suggesting a spatially structured sensory system despite a relatively simple nervous system.
The works overturns the traditional view of sessile animals as behaviorally simple, revealing that even immobile organisms can exhibit rich, modular, and context‑dependent behaviors. It provides a new framework for studying behavior in sessile species and opens the door to understanding how simple nervous systems generate complex, adaptive responses.
