Slime molds have a complicated life cycle..
Amoeba phaseA slimes life begins when it hatches from a spore into a singled celled amoeba. At this stage they have one set of chromosomes - they are haploid. The amoebas feed by engulfing bacteria and digesting them. They are free living and can reproduce by dividing in two. If conditions become to dry the amoeba can dehydrate and form a resistant cyst that will hatch when it gets wet. If conditions are very wet, the amoeba grow tails ( flagella ) and become swimming swarmer cells, they will absorb their tails if they reach a dry area.
Cell fusion and plasmodia
If two compatible amoebae meet - which is not often as physarum has three compatibility genes that have to match up - then they will fuse. Their nucleii will join together making one nucleus with two sets of chromosomes. They've become diploid. This is a very simple form of sexual reproduction.
The newly formed diploid cell continues to feed off bacteria but it stops dividing. As it grows it makes new nucleii but the cell itself just gets bigger and bigger - soon it's visible to the naked eye and starts to feed on debris and waste on the woodland floor. This is the plasmodial stage that we use in our experiments and are handing out at the Big Bang Fair.
Sclerotia and Spores
If things get tough, a plasmodium has two options:
It can tough it out by forming a sclerotium. This is a hardened mass of tissue that dried out completely and can survive for years. If it gets wet, it reactivates and the slime mold resumes its former life.
If the plasmodium is well fed, is exposed to light and has somewhere to climb it might form sporangium - fruiting bodies roughly similar to mushrooms. It will climb to somewhere with a reasonable breeze, form stalks and on the top of those stalks grow small, black sporangia. The spores inside are haploid - they have a single set of chromosomes. When they're dried, any wind will pick up and scatter the spores. Some will land in damp areas, hatch into amoeba and start the life cycle again.
One of the best known abilities of slime molds is that they show chemotaxis - they move towards or away from different chemicals they sense in their environment. When we make mazes or choice chambers for them, we're exploiting that behaviour to test their ability to navigate towards food or test their preference for difference sources of nutrition. In the picture below a slime is attracted towards oats mixed with paprika ( positive chemotaxis ) but repelled by black pepper, chilli pepper and turmeric ( negative chemotaxis ).
It's not vision but physarum responds to light. Normally it just avoids light - particularly blue and UV light which are damaging to it. Experiments have shown that physarum can differentiate between different colours of light and can be trained to respond to them. When it's ready to form spores, the slime will switch from avoiding light to being attracted to it.
The key to physarums success is its memory - which is external. Wherever it goes, a slime mold lays down a chemical trail. When it's exploring something like a maze this trail tells the organism where it's been before and which areas aren't worth looking at again. It's a very similar technique to foraging ants leaving trails of pheromones for other workers to follow.