Gigantis

Gigantis (GIGANTIS-ジャイガンティス-, Jaigantisu) is a light novel series by Yoichi Komori. It received a manga adaptation by Kenichi Tachibana.

Plot
On that day, humanity was turned into “prey”―.

Gen is a high school student living on the island of Tsushima, a door to mainland Japan. Along with his sickly sister, Misaki, and his heartwarming mother, Gen was living a peaceful life, all until an "asymmetrical monster" appears on the island. All hell breaks loose on the island of Tsushima. These monsters devour everything from plants to animals, to humans, and mutate together. Will humanity stand a chance against these mutant creatures? Is there any hope?