Interpreting genetic distances for species recognition: The case of Macrobrachium amazonicum Heller, 1862 and the recently described M. Pantanalense Dos Santos, Hayd & Anger, 2013 (Decapoda, Palaemonidae) from Brazilian fresh waters

The river prawn Macrobrachium amazonicum Heller, 1862 occurs in limnic and brackish habitats in northeastern South America, with a broad distribution range and a wide phenotypic variability. Recently, representatives of populations from the Pantanal were reassigned to the newly described species M. pantanalense Dos Santos, Hayd & Anger, 2013, according to distinct morphology, lifehistory, and geographic distribution. Genetic distinctness is in a range that may be recognized as either inter- or intraspecific, which raises the question to what extent genetic distance data alone should be used to determine whether two closely related populations belong to the same or different species. Here, we discuss the case of M. amazonicum and compare it with other cases of conflicting conclusions from studies on decapod crustaceans, mainly using morphological or genetic criteria. Based on these comparisons, we caution against a too simiplistic view of genetic distance as an exclusive criterion for the definition of "species" vs. "conspecific populations".