Birds (Clark's grebe, Aechmophorus clarkii; Forster's Tern, Sterna forsteri), fish (Peacock bass, Cichla temensis, Cichla monoculus; Pescada, Plagioscion squamosissimus) and earthworms were analyzed for selenium, total mercury, and methyl mercury concentration and mercury speciation. A Clark's grebe was collected from Lake Berryessa (California, United States) in September 2012. A Forster's Tern was collected from the San Francisco Bay Estuary (California, United States) in June 2018. Bird necropsies were performed at the U.S. Geological Survey (Dixon, CA) to obtain the following tissues: head feather, breast feather, brain, skeletal muscle, kidney, and liver. Fishes were collected in September 2018 from the Tapajos River (Brazil), which is close to artisanal gold mining areas where liquid elemental mercury is used. Fish necropsies were performed to obtain muscle and liver. Earthworms were collected in 2017-2018 from two soils (Site S1 Champ-sur-Drac, Site S2 Champ-sur-Drac; France) near the chlor-alkali plant of Champ-sur-Drac. Five endogeic and one anecic earthworm were sampled at Site S1 and five endogeic and two anecic at Site S2. Chemical analyses for total mercury and methylmercury were made at the U.S. Geological Survey Mercury Research Laboratory (Middleton, Wisconsin). Chemical analyses for total selenium were made at the U.S. Geological Survey (Menlo Park, California) and University of La Rochelle (La Rochelle, France). Mercury speciation was measured by high energy-resolution X-ray absorption fine structure (HR-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy at beamline ID26 at the European Synchrotron Radiation Facility. The screening of selenium and mercury associated with protein extracts was measured by double affinity (AF) high performance liquid chromatography-inductively coupled plasma mass spectrometry (AF-HPLC-ICPM) at the LGC Group (Teddington, United Kingdom). ORCA, a geometry optimization platform, was used to predict the structures of mercury bound to selenium in two configurations: Hg(selenoeine)4 and Hg10(methaneselenolate)20 (Hg10(SeMe)20). There are 7 files (*.csv) provided in this product: 1) site descriptions 2) analysis descriptions, 3) HR-XANES results and chemical analyses, 4) HR-XANES spectra, 5) EXAFS data, 6) EXAFS spectrum, and 7) modeled cartesian coordinates of the mercury-selenium clusters.