4.5 to 8 mm with the females being generally larger than males. Broadly-oval and widest behind the middle, entirely shiny black with a bluish or bronzy lustre and red legs with dark claws. This last feature is present in only one other UK species, Gyrinus aeratus.

Gyrinus aeratus is similar, but here the elytral margin between the outer row of punctures and the edge is narrow throughout whereas in G. marinus it becomes wider from the middle then tapers before the apex.

Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

Still water bodies. Usually found in larger pools but can also occur in small ponds.

Most records fall between March and October when the species is most active.

They feed on small insects.

Widespread and locally fairly frequent in Britain.

Fairly frequent in Leicestershire and Rutland.