Brainless slime mould has an external memory

Slime mould Physarum polycephalumThe "remarkably intelligent" slime mould

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Slime moulds use a form of spatial "memory" to navigate, despite not having a brain, a study has found.

Scientists in Australia studied the organisms in an experiment normally used to test robots.

They found that the slime mould could navigate around a U-shaped maze to a food source, using their slimy deposits.

Researchers compare its path-finding method to Hansel and Gretel's breadcrumb trail.

Amazing feats of memory

Their full findings are published in the journal Proceedings of the National Academy of Sciences.

"A slime mould is not a fungus or mould, but is in fact a protist, which is really the odds and ends of the natural world that don't fit in with the rest of our taxonomic grouping system," said PhD student Christopher Reid who led the study.

"They are truly alien creatures and yet they are all around us: all over the world, preying on yeast, bacteria and fungi, out of sight in the undergrowth."

Following on from earlier research into the behaviour of the bright yellow slime mould, Physarum polycephalum, Mr Reid and colleagues form the University of Sydney, Australia, set out to test its navigational abilities.

They placed the organism, which Mr Reid described as a "large, blob-like cell", on one side of a dish of agar gel.

On the other side, researchers placed a sugary food source that they knew the slime mould would be attracted to.

In a test more commonly used to analyse the artificial intelligence of robots, they placed a U-shaped trap between the slime mould and its food source to understand how the organism moved around its environment.

"The whole organism is made up of bits of pulsating tissue, which are constantly expanding and contracting, using a similar mechanism to our own muscle cells," explained Mr Reid.

Slime mould Physarum polycephalumThe mould at the top was attracted to the sugar solution below the maze

"Each part changes the speed at which it pulsates according to what it can sense in the environment around it - for example food, light or heat - which are detected by chemical receptors on the cell's surface."

"The pulsating parts are also influenced by the throbbing of their neighbours within the cell, which means that they can communicate with each other, to pass information through the organism about what is happening in the environment outside. The different speeds of contraction directly influence which direction the cell will then move in."

In their experiment, Mr Reid and colleagues observed the slime mould exploring the dish, leaving a trail of slime behind wherever it went.

According to the scientists, this slime trail was key to the organism's path-finding because it acted like a trail-marker, comparable to Hansel and Gretel's trail of breadcrumbs.

They found that the slime mould did not revisit areas it had already investigated.

"In essence, the slime mould is memorising where it has been - storing this memory in the external environment and recalling the information when it later touches the slime-coated area," said Mr Reid.

Of the moulds tested, 96% successfully found their way to the sugary substance, taking on average 57 hours to do so.

But when researchers covered the whole dish in slime only 33% were able to reach their goal within the 120-hour time limit.

Slime mould Physarum polycephalumThe cell moves by expanding a network of pulsating tissues

"Without the benefit of memory, the slime moulds spent almost 10 times longer pointlessly re-exploring areas they had been before," said Mr Reid.

The findings are the first to identify 'memory' in an organism without a brain or central nervous system.

Although, according to the scientists, the internal memory of higher, multi-cellular organisms probably did not directly evolve from this externalised system, the findings offer insight into how ancient organisms behaved.

Mr Reid said that further research will now focus on what other information the slime trail may hold.

"For a single-celled organism, it has continually surprised researchers with its abilities, such as solving mazes, anticipating periodic events, and even making irrational decisions like we do," he told BBC Nature.

"It is truly a remarkable creature that is redefining our notions of 'intelligence'."

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