Serris is an aquatic serpent kept aboard the BSL research station for study. The specimen's main distinguishing feature is its ability to move at incredibly high speeds both in and out of the water. After the X infect it, Samus is forced to kill it, and in so doing she recovers the speed booster.
This particular fight is not overly difficult, as Serris tends to move in a very specific pattern. Its only method of attack is ramming, but this is compensated by its ability to move at very high speeds, though it generally only does this in short bursts.. The only portion of Serris that is vulnerable is its head, and it is vulnerable to both the charge beam and missiles. When it is boosting though, it is invulnerable to damage, so at that time the only thing that can be done is staying out of its way, and staying out of the water.