Leviathan

Leviathan is an octopus-like alien lifeform about 1 meter wide.

Overview
Leviathans is not very fast, pretty weak but very dangerous alien. Their attack is an acid spit that damages crewmember over time. Each of them dangerous by itself, but Leviathans appears mostly in packs, which makes them considerable threat, especially in service tunnels. Moreover, they may infect your Clones crew with (unknown) desease, even if they are in Alien Research cell.

Alien nests
Leviathan alien nests will spread very fast in service tunnels.