Molecular and morphological data suggest weak phylogeographic structure in the fairy shrimp Streptocephalus torvicornis (Branchiopoda, Anostraca)

Inland aquatic organisms almost ubiquitously display a pattern of marked provincialism characterized by substantial population differentiation and genealogical discontinuities. This is the result of strong priority effects and local adaptation following dispersal and colonization of new habitats. We present a case that defies this biogeographic paradigm. We have investigated the phylogeography of the fairy shrimp Streptocephalus torvicornis across its circum-Mediterranean and Eurasian distribution. Based on three independent datasets, namely sequence variation at 12S and 16S rRNA, cyst morphology and male second antenna characters, we discern a pattern of extensive genetic and morphological homogeneity pointing to unhindered gene flow and widespread connectivity among populations. These intriguing findings may provisionally be explained by (i) a high dispersal frequency overwhelming the ability of a population to maintain resource monopolization, (ii) an outbreeding vigour opportunity offered to secondary immigrants, (iii) an ecological equivalence of genotypes generating long-term immigration–extinction equilibria and buffering genetic diversity over spatial scales, (iv) enhanced bird-mediated dispersal in open habitats as opposed to ponds surrounded by forests or shrub, or (v) a shallow population history with little time for substantial genetic differentiation.