Range sizes of groundwater amphipods (Crustacea) are not smaller than range sizes of surface amphipods: a case study from Iran

The connectivity of groundwater aquifers is lower compared to surface waters. Consequently, groundwater species are expected to have smaller distributional ranges than their surface relatives. Molecular taxonomy, however, unveiled that many species comprise complexes of morphologically cryptic species, with geographically restricted distributional ranges in subterranean as well as in surface waters. Hence, the range sizes of surface and groundwater species might be more similar in size than hitherto thought. We tested this hypothesis by comparing the range size of surface amphipods of the genus Gammarus and subterranean amphipods of the genus Niphargus in Iran. We re-analyzed the taxonomic structure of both genera using two unilocus species delimitation methods applied to a fragment of the COI mitochondrial marker, to identify molecular operational taxonomic units (MOTUs), and assessed the maximum linear extent (MLE) of the ranges of MOTUs from both genera. Genus Gammarus comprised 44-58 MOTUs while genus Niphargus comprised 20-22 MOTUs. The MLEs of the two genera were not significantly different, regardless the delimitation method applied. The results remained unchanged also after exclusion of single site MOTUs. We tentatively conclude that in this case there is no evidence to consider that groundwater species are geographically more restricted than surface species.