The Aquatoids are the stellar cousins of the Sectoids from the First Alien War, and served as the backbone of the alien army in the Second Alien War. Besides certain biological features, particularly being amphibious, the Aquatoids are essentially carbon copies of the Sectoids.

It is unknown where the Aquatoids originated, but it is clear they share a close relationship with the Sectoids, since the two races are anatomically similar in many ways. The Aquatoids were one of the alien species that came to Earth several million years ago, when the Colony Ship T'leth crash landed in the final days of the Cretaceous. Since all Aquatoids are exact clones, it is possible that all Aquatoids but one died in the crash. Aquatoids differ from Sectoids in several ways. Their hands and feet are webbed like a frogs, although the Sectoids were said to have webbed hands, so it is possible that the Aquatoids are the ancestors of the former. They have green skin, bulbous black eyes with red pupils, a ridge of yellow knobs running along their forehead and short fangs. Aquatoids share the strengths and (many) weaknesses of their counterparts, with high-rank units being capable of M.C., while all are physically weak and easy to dispatch.

Aquatoids, like Sectoids, are frequently encountered early in the game, then virtually disappear until Terror Missions begin using Mixed Crews. They are also found at the first phase of a Colony Strike, defending the entrance to an Alien Colony alongside the Tasoths. They are supported by Calcinites on Terror Missions.

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