Australasian Science: Australia's authority on science since 1938

Slime Mould Has a Memory

By Stephen Luntz

The slime mould Physarum can remember where it’s been even if it doesn’t have a brain.

The capacity of slime moulds to find their way through mazes belies their nature as a combination of single-celled organisms (AS, June 2010, pp.44–45), but the discovery of them having a memory is new.

Its common name not withstanding, Physarum is not a mould; it is an amoeboid protozoa that feeds on bacteria and fungi.

PhD student Chris Reid of the University of Sydney’s School of Biological Sciences adapted a U-shaped trap used to test robot navigation and then placed Physarum on an agar surface with sugared water on the other side of the trap.

“The glucose dissolves through the agar so the Physarum can taste it through receptors and follow the gradient towards the source,” Reid says. The slime mould does not always take a direct path, which Reid attributes to variations in the sugar diffusion, or unreliability in detection. However, the Physarum eventually finds itself blocked in the U-shaped maze, and seeks a path around the obstacle.

Although the mould moves slowly, stop-motion video shows a relatively efficient search to circumvent the trap rather than repeatedly traversing the same territory.

Reid says the slime mould leaves a slime trail as it travels. Upon encountering its own slime, the Physarum recognises it has passed this way before and tries alternative routes.

In a paper published in PNAS, Reid records that the Physarum reached its goal within 120 hours in 96% of cases with slime trails to guide it. However, when Reid pre-coated the agar with slime, the success rate fell to 33%. The time devoted to searching previously explored areas increased almost tenfold.

The slime mould easily crossed a trail when no unslimed directions were available, but preferred to avoid doing so, apparently to avoid fruitless searches.

The slime protects slime moulds from drying out and against bacterial invasion. “It may also be a lubricant to help it move and assist it to ingest food,” Reid says.

Species of slime moulds have varying reactions if they encounter the slime of other species, or even different strains of their own species, and Reid says there is much more to explore here.

“We think slime mould has the same morphology as very early life forms before multicellularity,” Reid adds, “so there is no reason early life could not have used this capacity to solve its problems.”