While you explore, the world of BioShock goes on around you. Protectors search for fresh bodies and then call up their 'little sisters', the Gatherers, who can extract the Adam from the carcasses. The Gatherers are basically children, and the Protectors are their giant bodyguards - and once you learn this, you can start to use them to your advantage.
By waiting for the Protector to shepherd his Gatherer to the corpse and then luring the ceiling-crawler towards them, our character doesn't need ammo. So long as you aren't a threat, the Protector will kill the mutated beast and ignore you - leaving the way forward clear.
Above: The story of how the magnificent city of Rapture collapsed is for you to discover
It's interacting with the environment - and everything in it - like this that's making BioShock so much more than just another sci-fi shooter. We'll be back in Rapture for more undersea experiences soon.