Spore crawlers can move, but can only plant themselves on creep. Their mobility is significantly reduced when they are not on creep, however.
Crawlers can uproot themselves from the creep and move about, but can only plant themselves back onto the creep. They move much more quickly on the creep than off it.
Players will want to spread creep onto the map to create more space for their crawlers. Their mobility allows players to simply relocate the crawlers depending on the situation, and as such, they could build them in lesser numbers without weakening the defenses. Like the spine crawler, spore crawlers slowly die if rooted in ground not covered with creep. They can, however, survive indefinitely without creep so long as they are uprooted.
The following section contains information from a previous version of StarCraft II which is no longer valid.
The concept of mobile defenses in StarCraft II began with the phase cannons, but was transferred to spine and spore crawlers when the phase cannon proved to be unbalanced in testing. Prior to this, crawlers were stationary structures. They underwent a visual re-design late in the game's development cycle.
StarCraft II beta patch 7 decreased the cost of spore crawlers from 100 to 75, and Patch 11 increased their root time from 6 to 12. Patch 1.3.3 reverted it to 6 seconds. Balance Update 6 in the Heart of the Swarm beta changed the evolution chamber requirement to spawning pool, and Balance Update 13 increased their damage from 15 to 15 (+15 vs. Biological). The Patch 2.0.8 Balance Update increased their damage again to 15 (+30 vs. Biological). The Patch 2.5.2 Balance Update in the Legacy of the Void beta reverted their damage. The Patch 3.3.2 Balance Update decreased their root time to 4.
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