At 60-80 mm, this is one of the larger, sturdier damselflies of Japan, and one of the most beautiful. The male has a bright aquamarine metallic abdomen and thorax, with chocolate-bronze wings. The female also sports a metallic body, with smoky wings. 

Males perch over suitable egg-laying sites, and advertise them to passing females through an elaborate display. Males fly only with the forewings, beating faster than normal, and with the hindwings held still below. The wings form a conspicuous X shape. The male then throws himself onto the water above roots visible below. If the female is interested, she'll perch and allow him to mate. Females lay their eggs underwater. They grip onto the rootlets of trees and bushes, and drag themselves under, spending between 20 and 120 minutes submerged. They achieve this by absorbing oxygen from a thin layer of air trapped on the body and the wings. Experiments have shown that females can survive for 120 minutes without coming to the surface. Fortunately, this is more than enough for them to lay their entire clutch of eggs, which take around 15 days to develop and hatch into aquatic larvae. Both sexes benefit from the submerged arrangement. The male, because no other male can disturb his female's egg-laying, can be assured of fathering all the eggs she lays. The female, on the other hand, doesn't have to put up with the unwanted attention of other eager young males.

Coenagrion mercuriale is classified as Least Concern on the IUCN Red List.