A new Amazonian species of Colostethus with sky blue digits

We describe a new species of Colostethus that is unusual in that males have blue digits on the hands and blue toe discs; females have blue discs on th e fingers and toes but lack blue digits. The new species occurs in lowland tropical rainforest south of Manaus, Amazonas, Brazil, in the municipality of Castanho, at km 12 on the road to Autazes. Six species of Colostethus oc cur north of the Amazon River, and four, including the new species and thre e undescribed species, occur south of the river. Four of these six species have small body size; i.e,. adult male SVLs are less than or equal to 18.0 mm. Two of the species, both undescribed, from south of the Amazon have adu lt male SVLs are greater than or equal to 19.5 mm. Call of all six species are unique; calls are produced either continuously, as in the case of the n ew species, C. marchesianus, and one of the large undescribed species south of the Amazon river, or in bouts of varying lengths. The function of the b lue digits in males of the new species is unknown but may be associated wit h visual communication during reproductive behavior. Blue toes or discs wer e present only in adults during the breeding season.