Multiple colonizations of an ‘anomalous island’: origin and phylogeography of Sulawesi’s endemic freshwater shrimp Caridina (Crustacea: Decapoda: Atyidae)

Abstract Sulawesi hosts a remarkable diversity of atyids, with 62 species in five genera, most notably Caridina. Most species are endemic (N = 43), particularly (N = 24) in the well-studied ancient lakes of the island. Nonetheless, the colonization of Sulawesi by atyids and their endemic species diversity in rivers has been poorly studied to date. Based on extensive sampling of the endemic atyids of Sulawesi from all parts of the island and including all described endemic species, we have estimated divergence times based on data from two mitochondrial gene fragments. Our analyses suggest five independent colonization events among the endemic atyids of Sulawesi, with two major Caridina clades of Miocene origin composing the majority of endemic species. These two clades are most closely related to species from Sundaland and the Philippines, respectively. On Sulawesi, they form distinct western and eastern clades that are only marginally overlapping in their distribution. The inferred speciation history on Sulawesi within these clades is only compatible in part with the palaeogeographical history of the island or present-day catchment boundaries. Other intensively studied freshwater taxa from Sulawesi, such as pachychilid snails or ricefishes, owe their origin on the island to single colonization events, and commonalities in diversification patterns on the island are limited. Our results also show that the riverine species diversity of Caridina on Sulawesi is significantly underestimated, with ≥10 undescribed species. The purported distribution of some supposedly widespread described species is not supported by our data. An integrative revision of Sulawesi atyids is overdue.