Bark spider cryptic diversity highlights need for taxonomic expertise

Bark spiders (genus Caerostris) are large spiders distributed across the Old-World tropics. Caerostris are characterized by web gigantism, reaching extremes in Darwin's Bark spider (C. darwini Kuntner & Agnarsson, 2010), which employs the toughest known silk.

The giant web of Darwin's bark spider (Caerostris darwini), Analamazaotra national park, Madagascar (photo: M. Kuntner).

Due to the exceptional silk of C. darwini, recent research provided whole-genome sequencing and silk-gene mapping for C. darwini and a sympatric congener misassigned, due to cryptic diversity, to C. extrusa.

We describe a case of convergent evolution in somatic morphology that further hinders species identification in the field. Conducting a morphological and molecular investigation of exemplars that share the “C. extrusa morphotype”, we conclude they belong to four distinct species. We redescribe the two valid ones, C. extrusa and C. hirsuta (Simon, 1895), elevate the previously synonymized C. bankana Strand, 1915 to species level, and newly describe C. kuntneri sp. nov., the species used in the assembly of its genome. We argue this to be an example of how the global taxonomic impediment can lead to errors in rapidly advancing fields such as genomics.

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