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An example can be seen in this Youtube video, where the slime mold pulsates as it engulfs a rock in order to form its fruiting body.

Why does this pulsing happen?

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What you have observed is called shuttle streaming. This is how Physarum polycephalum (I assume this is the species of slime mold in the video) and many other slime molds move.

I found this here;

The movement of P. polycephalum is termed shuttle streaming. Shuttle streaming is characterized by the rhythmic back-and-forth flow of the protoplasm; the time interval is approximately two minutes. The forces of the streaming vary for each type of microplasmodium.

The force in amoeboid microplasmodia is generated by contraction and relaxation of a membranous layer probably consisting of actin (type of filament associated with contraction). The filament layer creates a pressure gradient, over which the protoplasm flows within limits of the cell periphery.

The force behind streaming in the dumbbell-shaped microplasmodia is generated by volume changes in both the periphery of the cell and in the invagination system of the cell membrane.

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  • $\begingroup$ +1 for identifying the name of the behaviour. But why does this happen (as in, what benefit does the fungus gain from having its protoplasm flow back and forth, instead of directly oozing along?) $\endgroup$
    – March Ho
    May 11 '15 at 18:48
  • $\begingroup$ @MarchHo Its not a fungus. $\endgroup$
    – Rover Eye
    May 11 '15 at 18:56
  • $\begingroup$ @MarchHo This behavior is very similar to our own circulatory system. Even when this organism is not in the act of moving, this shuttle streaming is still going on, only now it is acting more like cytoplasmic streaming, which is what most cells use to move nutrition around the cell. I.e. the main reason for this behavior is to transport nutrition, waste material, etc. wherever it needs to be. $\endgroup$
    – CDB
    May 11 '15 at 19:25
  • $\begingroup$ @RoverEye Thanks for clearing that up, you got here a little sooner than I did. $\endgroup$
    – CDB
    May 11 '15 at 19:27
  • $\begingroup$ @RoverEye Thanks for the correction, corrected the tag. $\endgroup$
    – March Ho
    May 11 '15 at 20:06
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Adding to @CDB's answer:

...Saigusa speculates that it instead depends on an internal mechanism of some kind, perhaps involving the perpetually pulsating gelatinous contents of its one cell, known as cytoplasm. The slime mold's membrane rhythmically constricts and relaxes, keeping the cytoplasm within flowing. When the amoeba's membrane encounters food, it pulsates more quickly and expands, allowing more cytoplasm to flow into that region; when it stumbles onto something aversive—such as bright light—its palpitations slow down and cytoplasm moves elsewhere. Somehow, the slime mold may be keeping track of its own rhythmic pulsing, creating a kind of simple clock that would allow it to anticipate future events.

from: http://www.scientificamerican.com/article/brainless-slime-molds/ In the paper itself he says:

The model is based on physiological observations. We assume that multiple chemical oscillators of a series of periods underlie the multirhythmicity of locomotion, as multiple rhythms were observed in cellular activities in a Physarum plasmodium. This means that there are continuous frequencies of oscillation....

...Oscillators with the same biochemical identity can interact over a distance by chemical diffusion and active advection of protoplasmic streaming. These direct interactions will tend to synchronize phase, but it may not be enough to make a strong synchrony. In fact, macroscopic cellular behaviors often show fluctuating oscillatory variations rather than clear oscillations with a large amplitude, and the fluctuating oscillatory variations of different chemical identities display frequent switching between in-phase and out-of-phase relationships, although each biochemical component still shows the slightly oscillatory behavior. This consideration implies that the noise ξ should be small so that desynchronization effects are relatively weak.

From: Saigusa, Tetsu, et al. "Amoebae anticipate periodic events." Physical Review Letters 100.1 (2008): 018101.

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    $\begingroup$ Very interesting find! +1 and bravo $\endgroup$
    – CDB
    May 11 '15 at 20:10

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