Despite the alarming rates of deforestation and forest fragmentation, Madagascar still harbors extraordinary biodiversity. However, in many arthropod groups, such as spiders, this biodiversity remains mostly unexplored and undescribed. The first subsocial Madagascan species of the theridiid spider genus Anelosimus were described in 2005 when six new species were found to coexist in the Périnet forest fragment within Andasibe-Mantadia NP. However, this discovery was based only on a few specimens and the extent of this Madagascan radiation has remained unknown. We here report on a thorough survey of >350 colonies from Périnet, and three pilot surveys into additional Madagascar forests (Ambohitantely, Ranamofana, and Montagne d’Ambre). The morphological, molecular and natural history data from these surveys facilitated a revised taxonomy and phylogenetic hypothesis of Madagascan Anelosimus. This subsocial clade currently comprises six previously known (A. andasibe Agnarsson & Kuntner, 2005, A. may Agnarsson, 2005, A. nazariani Agnarsson & Kuntner, 2005, A. sallee Agnarsson & Kuntner, 2005, A. salut Agnarsson & Kuntner, 2005, A. vondrona Agnarsson & Kuntner, 2005) and 10 new species: A. ata sp. n., A. buffoni sp. n., A. darwini sp. n., A. hookeri sp. n., A. huxleyi sp. n., A. lamarcki sp. n., A. moramora sp. n., A. tita sp. n., A. torfi sp. n., A. wallacei sp. n.. With the exception of A. may and A. vondrona, all other species appear to be single forest endemics. While additional sampling is necessary, these data imply a much higher local richness and endemism in Madagascan forests than in any other comparable area globally. The phylogenetic results establish a sister clade relationship between the subsocial Anelosimus in Madagascar and the American ‘eximius group’, and between the solitary A. decaryi on Madagascar and a solitary American clade. These findings imply duplicate colonizations from America, an otherwise rare biogeographical pattern, calling for more detailed investigation of Anelosimus biogeography.
