Marine rove beetles in a low-salinity lake: Cafius pectoralis (Coleoptera: Staphylinidae) prey on beach hopper in brackish water

A marine rove beetle, Cafius pectoralis (Boheman, 1858), inhabits the coast and is found in seaweed washed ashore on sandy beaches. This species also inhabits Lake Shinji, a brackish lake with low-salinity, but there is no seaweed or kelp flies in this lake. Cafius pectoralis prefers to prey on beach hoppers, and many hoppers live in Lake Shinji. One rove beetle and one beach hopper were placed together in a container for 24 h. As a control, beach hoppers were placed in similar containers alone for 24 h. While the beach hoppers that were alone all survived, those with rove beetles were all dead after 24 h. Consequently, it is speculated that the beetles feed on beach hoppers and that they have become habituated to the brackish lake.