Enemies

Four main categories of enemies are encountered:

Splicers
Deformed, genetically modified Rapture citizens who are now remnants of Ryan's army, the Aggressors (also known as Splicers) cannot survive without ADAM due to their extensive biological modifications. Early Splicers seem to be referred to according to their weapon preference, such as "Leadhead" for those who prefer guns. They wear little or no armor, and normally roam the levels of Rapture, searching for other inhabitants to kill and steal ADAM from. As their name would suggest, they are aggressive and quick to attack, and will use their enhanced physical strength, group tactics and, sometimes, semi-biological weapons to kill all in their path. Most of the Splicers are wearing the masquerade ball masks that were worn during the 1959 New Year's Day party. (Atlas muses, "I wonder why they wear the masks? Maybe it's because they have a shred of humanity left, and they're ashamed of what they've become.")

There are 5 different types of splicers throughout Rapture.

Thuggish Splicer
The Thuggish Splicer attacks with blunt weapons. These can be found throughout the game and are fairly weak, but make up for it by running fast and attacking in groups whenever they can.

Leadhead Splicer
The Leadhead Splicer attacks with either a pistol or a machine gun. These can also be found throughout the game, but are more dangerous than the "thuggish splicer" since they can attack from a distance and will almost always hurt you a bit even if you can bring them down easily. These will always keep their distance from you whenever they can help it, but at very close range, they will attempt to whack you with their gun. It is worth knowing that they have to reload their guns, just like Jack. They are defenseless while doing so.They also have more health than a thuggish splicer causing them to be harder to take down, especially if there are more than one.

Spider Splicer
The Spider Splicer is first encountered in Neptune's Bounty when you're told by Peach Wilkins to photograph 3 of them in order to gain access into Fontaine Fisheries. These splicers will walk on the ceiling and throw hooks at you from afar. When on the ground, the Spider Splicer will get near enough to physically attack you, then immediately jump onto the ceiling or backflip away and come at you from another angle. They are tough to kill due to their amazing agility.

Houdini Splicer
The Houdini Splicer is first encountered when you enter Arcadia. These splicers have the ability to teleport from one place to another in the middle of combat. They can be tracked through smoke that appears around them whenever they disappear or reappear. The more research you do on this type of splicer, the easier it becomes to predict where it will teleport. The Houdini Splicer will most often attack by shooting balls of fire (and rarely ice) at its enemies, but like the leadhead splicer, will attack physically at close range. An easy way to track them is with an insect swarm. If you set them on it they will follow it even if it is invisible there fast bastards

Nitro Splicer
Nitro Splicer are found rarely throughout the game; mostly in Neptune's Bounty. The first one you will encounter is in the Medical Pavilion, where you must catch his grenade and use it to destroy debris that is blocking your path. They will attack by throwing grenades at you from a distance. They will not attack physically, and at close range, will throw a grenade directly on the ground in order to either kamikaze, or distract you so it can escape to a farther distance. A common way of killing these is by using telekinesis to catch its grenades and throw them back at it.

Little Sisters
The Little Sisters, genetically modified prepubescent girls, were originally conceived as slug-like, genetically modified sea creatures. The Gatherers, who were created by a scientist named Dr. Tennenbaum as a solution to the ADAM shortage, extract the inert ADAM from the dead (which they call "angels") using retractable needles and then consume it, their bodies reprocessing it into a usable form. The BioShock team designed these characters in order to draw out an emotional response from players, who face an ethical choice between saving or murdering little girls in order to obtain much-needed resources. Nevertheless, doing so is severely difficult, due to their symbiotic relationship with the Big Daddies. Once the Big Daddy protecting the Little Sister is defeated, the player is presented with the option to either save or harvest the Little Sister. Saving them, while giving the player less ADAM, cures them of their genetic programming and promises future rewards. Harvesting, on the other hand, gives double the ADAM but kills the Little Sister in the process. Ultimately, the player's decision vis-à-vis the Little Sisters will influence the game's ending.

Big Daddies
Also known as Big Daddies, or Mr. Bubbles and Mr. B as the Little Sisters call them.

Mute, lumbering bio-mechanical monstrosities, created to protect the Little Sisters. They are heavily armored and wield high-powered weaponry, usually including a large drill. Their armor is a large adapted diving suit. Most of the inhabitants stay out of the way of the Gatherers and Protectors. They will not attack the player unless provoked and according to lead designer Paul Hellquist: "Once you mess with them, you find out why no one messes with them." In a recent and extremely gruesome trailer/commentary for BioShock, it was revealed that, despite their size and typically slow movements, they become incredibly fast once engaged, in some cases quicker than the player character. However, the player can defeat them easily enough if the correct plasmids are used, such as shooting bolts of lightning at a Big Daddy standing in a pool of water.

There are two known types of Big Daddies; Bouncers and Rosies. Each of these types also appear as an "Elite" variety later on in the game.

Bouncers
The Bouncer's main weapon is an oversized drill attached to its right arm. They generally attack using powerful blows at close range. They are essentially ineffective in ranged combat, with a fast charge being their only offensive long range ability. These charges can be a pain in the bottom, especially when the bouncer follows the charge by knocking you onto the floor and sitting on your face.

Rosies
Rosies are much less agile than their counterpart, the Bouncer. They shoot with a 'rivet gun' and throw proximity mines if they're far enough away from the player. They also occasionally use a charge attack and hit the player with their gun.

Security Bots
Throughout the city of Rapture, they are called via alarm by various security cameras. Security bots appear to hover and fly using the same method as helicopters and are armed with machine guns. When disturbed, the security cameras' siren will sound and an unlimited supply of security bots will pour out for a limited time. There are three ways of taking down security bots: the player can attempt to destroy them, shut off the security system by paying the money demanded to access a Bot Shutdown switch, or manually hack into their systems. Hacking bots and other mechanical devices prompts a mini-game resembling Pipe Dream. The security system can also be suborned by the player, either by "hacking" it or using the "Security Bullseye" plasmid on an enemy.

Occasionally the player will come across Splicers using hacked security bots of their own. If the controlling Splicer is killed, the bot will shut down, allowing Jack to claim it. Splicer/bot pairings are identified by pulsating red signal waves emanating from the bot and the Splicer's head.

The maximum number of hacked Security bots protecting the player at any one moment is two (2).

Security Camera
The security camera is a mounted bot with a red searchlight that becomes white once it detects a hostile entity and attempts to identify it. If the camera detects the player, they have a about 2-4 seconds, depending on their equipped tonics, before the alarm goes off, summoning a wave of security bots. The camera is susceptible to most weapons, but is especially vulnerable by armor-piercing ammunition, which deals considerably more damage, and electricity, which causes the camera to short temporarily, allowing safe passage while the camera is still disabled. Once the player is within a certain proximity, they may choose to Hack the camera, causing its light to become green, and the camera to ignore the player while seeking the player's enemies. If destroyed, cameras may provide film for the research camera.

Big Daddies and Little Sisters are immune to camera detection in either mode. However, using the Security Bullseye plasmid power on a Big Daddy will cause cameras to initiate an alarm against the Big Daddy, and the repeated summoning of security bots against the Big Daddy for the duration of the alarm (one minute) can inflict a lot of damage.

Turrets
Turrets are fixed gun positions that attack the player on sight. Turrets can have either a machine gun, a RPG launcher or a flamethrower. The first turret appears in Welcome to Rapture at the blocked off exit to Neptune's Bounty. Turrets can be hacked, although they need to be frozen, disabled via the Electrobolt, or distracted from targeting the player (either plant a Target Dummy or Security Bullseye an enemy in view of the turret) to do so and are among the few things in the game that can be shattered while frozen and can still be looted.