Let's get high: Cladogenesis in freshwater crabs (Decapoda: Potamonautidae: <i>Potamonautes</i>) supports the mountain gradient speciation hypothesis in the Cape Fold and Drakensberg Mountains, South Africa

Abstract

During the present study, the evolutionary relationship within a clade of mountain clade of freshwater crabs (<i>Potamonautes</i>) was examined using mtDNA sequence data for species from the Cape Fold Mountain (CFM) and Great Escarpment (Drakensberg Mountain range). We undertook phylogenetic analyses, divergence time estimation, and an ancestral area reconstruction to explore the period of cladogenesis and understand the biogeographic history in this high-altitude clade. Furthermore, we applied four species delimitation methods using ASAP, bPTP, bGMYC, and STACEY on the latter clade. Bayesian phylogenetic analyses retrieved a monophyletic freshwater crab clade comprised of two major sister clades, one comprised of the Cape Fold (clade A) and two comprised of Drakensberg Mountains (clade B) species. Divergence time estimation indicated that the two clades underwent Mio/Pliocene cladogenesis. Within the CFM clade (A), <i>P. amathole</i> (Amathola Mountains) was sister to <i>P. parvispina</i> (Cederberg and Kouebokkeveld Mountains) and the latter species were sister to <i>P. parvicorpus</i> (Cape Peninsula, Jonkershoek, and Helderberg Mountains) sister to <i>P. tuerkayi</i> (Overberg Mountains) and <i>P. brincki</i> (Hottentots Holland Mountains). Within the Drakensberg Mountain clade (B), we observed <i>in situ</i> diversification. Specimens from the southcentral Drakensberg Mountains (Dargle Forest, Injasuti, Karkloof, and Impendle) represent a new undescribed lineage <i>Potamonautes</i> sp. nov. 1. The second clade from the northern Drakensberg, representing <i>P. clarus</i>, was sister to a central Drakensberg Mountain clade that comprised <i>P. depressus</i> that was in turn sister to <i>P. baziya</i> from the Eastern Cape Province. The application of species delimitation methods generally overestimated the number of species. The biogeographic analyses indicated that the Eastern Cape Province is the most likely ancestral range area. Ecological niche modelling of representative species in clades A (Cape Fold Mountains) and B (Drakensberg Mountains) demonstrated that temperature and rainfall were the major abiotic drivers that differentiated the two clades. Our data favours the mountain gradient speciation hypothesis.