Sea Slug

The Sea Slug or ADAM Slug is an underwater creature known to naturally produce raw ADAM. It is apparently indigenous to the sea bed around Rapture, and is utilized in the city to create ADAM, and in turn, Plasmids. The slugs' ability to produce ADAM is the result of mutation caused by a large luminescent biomass in ocean rifts deep below Rapture.

In order to produce the ADAM that is known to the citizens of Rapture, the slugs must first go through the embedding process. The embedding process is the implanting of the slug into a small child, specifically, a young girl, aged 5-8 (possibly). Through this procedure, the young girl becomes a Little Sister, and the slug creates ADAM with the child as its host. The ADAM is then regurgitated by the Little Sister and can be used to purchase Plasmids at a Gatherer's Garden.

Over the course of BioShock, Jack encounters none to multiple sea slugs, depending on whether or not he harvests or rescues the Little Sisters he obtains. These sea slugs are shown to be a brownish-black color, with reddish orange luminescent spots on its exterior.

BioShock 2
During BioShock 2 the slugs can be seen when walking outside in the Atlantic Ocean around Rapture. They are lying on the sea floor, gripping walls, etc. Subject Delta can then pick them up to supply himself with 10 ADAM per slug. There are 5 in total: 3 between Siren Alley and Dionysus Park and 2 in Fontaine Futuristics. The slugs are also visible during the process of harvesting a Little Sister but it is faintly seen.

Minerva's Den
In the downloadable content Minerva's Den, the player encounters at least 5 of these slugs throughout the level, (not all of them are in the water.)