Two-shelled organisms, such as bivalves and seed shrimps, protect themselves from external stresses by firmly closing both shells. We found that the diving beetle Hyphydrus japonicus Sharp, 1873 (Dytiscidae: Hydroporinae) larvae frequently prey on seed shrimps. Other predatory aquatic insects have difficulty in capturing seed shrimps owing to the general form of their mandibles. Larvae of H. japonicus bear a projection on the head (the nasale), which is used in conjunction with vertical movements of the mandibles to fixate the valves of their prey and gain access to the soft body within them. This capture method does not destroy shells and is extremely efficient. The present study provides the first report on the use of a unique morphological trait for attacking prey animals without destroying their shells.