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A 400% increase in Dolomedes diversity on Madagascar

Madagascar is well known for its diverse and unique biota. The combination of a unique geological history of Madagascar and its long isolation for around 84 million years has affected the evolution of organisms inhabiting the island resulting in extremely high endemism rates. Despite a long and lively history of biodiversity research of Madagascar its biota continues to be vastly underestimated.

The globally distributed spider genus Dolomedes Latreille, 1804—known as raft-or fishing spiders—contains around 100 species. Being large and iconic, with semi-aquatic lifestyles, and often predators of freshwater vertebrates in addition to invertebrates, Dolomedes species are model organisms in diverse fields such as behavioral ecology and conservation biology. Our knowledge of Dolomedes in the tropics is limited. One region that stands out in this respect is Madagascar: literature only reports two species.

Our single expedition to humid forests of eastern and northern Madagascar, however, yielded a series of Dolomedes exemplars representing both sexes of five morphospecies. we devised and tested an integrative taxonomic model for Dolomedes based on the unified species concept. The model first determines morphospecies within a morphometrics framework, then tests their validity via species delimitation using COI. It then incorporates habitat preferences, geological barriers, and dispersal related traits to form hypotheses about gene flow limitations. Our results reveal four new Dolomedes species that we describe from both sexes as Dolomedes gregoric sp. nov., D. bedjanic sp. nov., D. hydatostella sp. nov., and D. rotundus sp. nov. 

By increasing the known raft spider diversity from one valid species to five, our results merely scratch the surface of the true Dolomedes species diversity on Madagascar. Our integrative taxonomic model provides the framework for future revisions of raft spiders anywhere.


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