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Teeth (c. 1 per second in HL, 10 in HL2)
Leeches appear as small, fish-like creatures that seem to be based on the terrestrial medicinal leeches. They are usually white in color.
During the Resonance Cascade, Leeches were teleported to Earth, infesting the oceans. They seem to have endangered or caused the extinction of most earthly sea creatures. Indeed they are the only creatures commonly seen in the sea, while it seems that a few crabs remain. Some Earth creatures remain in inland bodies of water, where insects and frogs can be heard. Ichthyosaurs also seem to occupy freshwater bodies.
 Half-Life and its expansions
Leeches make their first appearance in Half-Life in some of the flooded sections of the Black Mesa Research Facility after the Resonance Cascade. In addition to being teleported to Earth, they are kept in the Black Mesa Biodome Complex for study. They appear to have six eyes.
 Half-Life 2
In Half-Life 2, they attack Gordon Freeman in a swarm (an attack similar to that of the terrestrial piranha) if he ventures too far into the ocean (it is made to prevent the player to venture to the "end" of the map and force them to follow the proper path (much like the chopper in the video game Far Cry, for instance). Here the eye count is brought to only two.
 Half-Life 2: Lost Coast
In Lost Coast, Leeches behave like in Half-Life 2. Furthermore, the Fisherman reveals that Leeches are used as food. Since ocean fish are now very rare or extinct, the human occupants of coastal areas seem to catch and eat Leeches instead. He also states that their population is kept down by seagulls and that they ate most of the crabs.
 Behind the scenes
- In Half-Life, Leeches are minor enemies that do not cause much damage and can be easily killed. In Half-Life 2, they cannot be killed and will drain the players health in a matter of seconds.
- The Ichthyosaur was originally intended to fulfill the role of keeping the player from venturing too far into the ocean. It can be encountered underwater in "
d2_coast_02", among others, in the playable Half-Life 2 leak, although it is only heard, not seen, before the player dies.
 List of appearances
- Half-Life (First appearance)
- Half-Life: Opposing Force
- Half-Life 2
- Half-Life 2: Lost Coast (Non-canonical appearance)