This photo shows a great depth of field because the snail is in focus and the bridge isnt, its blurred. the photo isnt dark, it's quite light. I think a large number of aperture has been used for example F4 or F5. The mood this photo puts you in, is a scary mood because the snail and the photo is taken on a train track and it creeps you out to think a train cold come flying round the corner.
Use this easy-to-understand photography cheat sheet to help you next time you get confused about aperture, shutter speed, ISO, and depth of field. http://mcpactions.com/2015/01/05/free-photography-cheat-sheet/