Nature News: Slime molds are beautiful and smart

I probably shouldn’t get quite so excited about the life I find in my wood pile. I was all set to write about baby turtles this week when, while neatening up the debris from last year’s wood pile, I found a beautiful slime mold on one of the partially buried, partially rotten logs. Baby turtles were immediately forgotten - here was an awesome life form that most of us have seen but have trouble comprehending because it is so weird and strange looking. This slime mold was a chocolate tube slime mold, member of the genus Stemonitis, a distinctive genus of slime molds that are characterized by upright, tubular brown projections (sporangia - that part that holds the spores) held up by slender stalks. They grow in clusters on rotting wood and often have common names that reflect their hairy appearance: chocolate tube slime, tree hair, pipe-cleaner slime and so on.

I was first introduced to slime molds by my mushroom identification book - they are often lumped in with fungi and used to be considered fungi. It is now known that they are not at all related. One big difference, they eat more like an amoeba, engulfing bacteria and digesting inside their bodies (instead of digesting food externally like a fungus).

There are three main types of slime molds - plasmodial, cellular and slime nets. The plasmodial slime molds are, I think, the most charismatic. These spend most of their lives as one huge cell with thousands of nuclei - the result of individual flagellated cells swarming together and fusing into one cell. Scientists and biology teachers like plasmodial slime molds because you can observe and study cytoplasmic streaming (the movement of the contents of the cell) under relatively low magnification and because of their large size. Big plasmodial slime molds can get as large as a couple meters across!

The chocolate tube slime mold (and most plasmodial slime molds) spends most of its life as a glistening cobwebby material oozing around wood piles and the forest floor - yes, these are motile life forms! The plasmodium can creep at speeds as great as one inch per hour! At some point (usually when resources run out or environmental conditions change) the slime mold crawls up out of the litter and starts to form sexual spores in structures called fruiting bodies. (This term is also used for the spore-bearing structures of mushrooms and lichens, for example, a toadstool is a fruiting body). The fruiting bodies of Stemonitis species start out as little yellow dots that elongate and turn black as they mature. The deep chocolate brown color comes from the spores - as the sporangia release their spores to the wind they also coat any nearby surfaces. This is what I found in my woodpile — the fruiting bodies of a mature Stemonitis slime mold.

Slime molds also have their own kind of intelligence. In 2010, Japanese researcher Toshioyuki Nakagaki, of Hokkaido University, challenged a slime mold to work its way through a maze. And it did much more than that. Within 24 hours it had found the shortest way through the maze. Nakagaki then challenged the slime mold with a map of Tokyo, arranging small piles of oats (slime molds love oats) on all the suburbs and railway stations. The slime mold created a pattern from oat pile to oat pile that was almost identical to the Tokyo metro system map, finding the shortest distance between any two points. What’s more, slime molds have repeated these feats of highway design for the United States and Canada; complicated engineering, even by human standards! Engineers are now looking to use slime molds to help design more efficient transport systems.

There are many types of plasmodial slime molds oozing around our woods with equally colorful names - dog vomit, pretzel, and wolf’s milk slime. Look around wood piles and leaf litter in the forest - that’s where they find the necessary moisture and bacteria for survival.

Susan Pike, a researcher and an environmental sciences and biology teacher at St. Thomas Aquinas High School, welcomes your ideas for future column topics. She may be reached at spike3116@gmail.com. Read more of her Nature News columns online.