Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal,[3][6] although in practice individuals can still die. In nature, most Turritopsis dohrnii are likely to succumb to predation or disease in the medusa stage without reverting to the polyp form.[7] --WP
I was actually just looking up locusts the other day. They are fascinating, they are various species of grasshopper that have a swarming phase. During the swarming phase they go through crazy physical and behavioral changes, they change color, eat a lot more, and breed like crazy. The swarming phase starts with overcrowding causing increased tactile stimulation of the legs, which massively spikes their serotonin levels. Absolutely insane creatures.