In a relationship: Sister species in mixed colonies, with a description of new Chikunia species (Theridiidae)

Group-living behaviour is rare in spiders but has evolved repeatedly, yielding several species, some showing cooperation among close kin, and others living in colonies where each female builds its own web and is territorial. The most frequent origins of group living are seen in the cobweb spiders (Theridiidae) that commonly build threedimensional webs and show extensive maternal care, both putatively pre-adaptive traits to spider sociality. A very unusual behaviour was recently discovered in the theridiid genus Chikunia, where two distinct but related species occur in mixed-species colonies with potentially indiscriminate brood care. These mixed colonies consist of Chikunia nigra and a newly discovered species. Here, we describe the new species, Chikunia bilde sp. nov., and summarize the unique biology of this species pair. We also place the origin of mixed-species group living in a phylogenetic context, firmly confirming the placement of Chikunia within the clade (lost colulus clade) previously characterized as containing a concentration of independent origins of group living. The two Chikunia studied here are sister species, representing a rare case of close genetic and behavioural interspecific relationship. We conclude that the loss of aggression that accompanies group living and social behaviour in cobweb spiders might help to explain the origin of mixed-species colonies.