From SC2Pod
The Sunken Colony is the Zerg means of ground defense. Using deadly Subterranean Tentacles to suprise and puncture enemy ground based units, the Sunken Colony can quickly turn the tide of a battle. The best use of the Sunken Colony lies in building them in clusters and using them in conjunction with Spore Colonies since they have no detection abilities of their own and would be unable to defend against cloaked or burrowed units.
Weapons
StarCraft 2
The Sunken Colony has been replaced by the somewhat similar
Spine Crawler. This new defensive building has the ability to uproot and move arond the battlefield, making it very mobile compared to most static defense buildings. The Spine Crawler's attack also seems to be an above-ground version of the Sunken Colony's tentacles.