Devoid of 'brain', jellyfish can still learn from past experiences: Study

The ability to learn from past experiences could be independent of possessing a "central brain" like the one that humans, mice and flies possess, according to latest findings of a study in jellyfish.

Instead, the jellies saw obstacles and employed associated learning, that involves making mental connections between sensory stimulations and behaviours, to guide themselves through murky waters and around underwater tree roots to snare prey, the scientists from Germany and Denmark demonstrated in Caribbean box jellyfish (Tripedalia cystophora).

These "seemingly simple" jellies, no bigger than a fingernail and living in mangrove swamps, have a complex visual system with 24 eyes embedded in their bell-like body, they said.

Shedding light on the evolutionary roots of learning and memory, the study findings challenge previous notions that advanced learning, including learning from prior experiences, requires a centralised brain, they said in their study published in the journal Current Biology.

"Learning is the pinnacle performance for nervous systems," said first author Jan Bielecki of Kiel University, Germany.

"Looking at these relatively simple nervous systems in jellyfish, we have a much higher chance of understanding all the details and how it comes together to perform behaviours," said senior author Anders Garm of the University of Copenhagen, Denmark.

The researchers observed the jellyfish for 7.5 minutes in a tank dressed in gray and white stripes to mimic a mangrove environment.

They saw that while initially the jelly bumped into these seemingly far stripes frequently, by the end of the experiment, it had on average distanced itself from the wall more by about a half and quadrupled the number of successful pivots to avoid collision.

The findings suggest that jellyfish can learn from experience through visual and mechanical stimuli, the researchers said.

Investigating the roots of associated learning in jellyfish, the scientists found that the animal's visual sensory centres, rhopalia, serves as a learning centre.

Using weak electric stimulations that mimicked the mechanical stimuli of a collision, the researchers trained the rhopalium to "recognise" the tank's light gray bars as mangrove roots and found that the learning structure generated obstacle-dodging signals in response to the light gray bars.

Each rhopalium houses six eyes and generates pacemaker signals that govern the jellyfish's pulsing motion, which spikes in frequency when the animal swerves from obstacles, the researchers said.

"Even the simplest nervous system seems to be able to do advanced learning, and this might turn out to be an extremely fundamental cellular mechanism invented at the dawn of the evolution nervous system," said Garm.