Mind the gap: DNA barcoding as a tool to delineate evolutionary units in Afrotropical freshwater crabs (Decapoda: Brachyura: Potamoidea: Deckeniidae and Potamonautidae)

Abstract We used field survey data of newly collected freshwater crabs (Potamonautes MacLeay, 1838) from South Africa, combined with sequence data for cytochrome oxidase one (COI) from Afrotropical freshwater crabs to examine the application and the use of barcoding gap. We examined intra and interspecific and intrageneric sequence distances in Afrotropical species and genera. On average uncorrected intraspecific COI sequence distance values of 1.97% and a range of 0.39–4.97% were recovered, while on average uncorrected interspecific COI distance values in the region of 8.76% was detected and a range of 0.22–18.65%. Within the species complexes the mean uncorrected distance was 10.32% and ranged 6.51–15.08%. We observed a mean interspecific sequence-distance value of 6.57% ranging 0.22–12.93% between the 32 southern African Potamonautes sister species pairs. Furthermore, in Potamonautidae Bott, 1970 the mean genetic distance for the COI locus was twice that reported in Deckeniidae Ortmann, 1897. We advocate caution in the use of COI distance values to identify novel lineages since we observed considerable overlap between intra and interspecific distance values. The species complexes we observed were geographically widespread, some were confined to mountainous regions and traversed distinct biogeographic regions, and to date remained poorly studied and require further taxonomic scrutiny. While the COI locus is useful for identifying species range extensions, good reference sequence-data libraries are required, and this may be particularly problematic in poorly surveyed areas or where species complexes are present. We ultimately argue in an Afrotropical context for the inclusion of morphological characters to describe novel diversity alongside the DNA sequencing of this material to build a COI data library.