The single-celled amoeba Dictyostelium discoideum, commonly known as slime mould is unique in that part of its lifespan is multicellular, with many amoeba aggregating together and forming a motile ‘slug’ which moves directionally towards stimuli such as light. Once a suitable location is found, they will develop and differentiate into a fruiting body to distribute spores.
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So how do they do this? When food supply is low they emit pulses of cyclic Adenosine Monophosphate (cAMP), this is detected by other amoeba that respond by emitting their own pulses of cAMP and by moving towards the signal (chemotaxis)
Pictured is the various stages of the multicellular lifecycle.