Exploring the interplay between evolution and climate change in shaping Potamonautes freshwater crab distributions of South Africa

Thesis (MSc)--Stellenbosch University, 2025.

Grobler, P. C. J. 2025. Exploring the interplay between evolution and climate change in shaping Potamonautes freshwater crab distributions of South Africa. Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/433e29c8-98f0-44ed-9411-5de6979535b3

ENGLISH ABSTRACT: Freshwater ecosystems, among the most biodiverse and ecologically significant habitats on Earth, are increasingly threatened by anthropogenic pressures and climate change. This study investigates the evolutionary dynamics, genetic diversity, and climate-driven distribution patterns of South African freshwater crabs of the genus Potamonautes. Focusing on Potamonautes flavusjo, a wetland specialist, and Potamonautes sidneyi, a generalist species, this research provides insights into the interplay between habitat preference, genetic diversity, and the potential impacts of climate change on species distributions. Phylogeographic analyses using mitochondrial cytochrome oxidase subunit one (COI) sequence data reveal contrasting genetic structures for the two species. Potamonautes flavusjo exhibits limited genetic differentiation across its range, indicative of high dispersal capabilities, likely facilitated by its semi-terrestrial lifestyle and burrowing behaviour in wetland habitats. Conversely, P. sidneyi displays marked genetic differentiation, with two distinct haploclusters and evidence of cryptic lineages. Bayesian inference and species delimitation methods further support the presence of evolutionary divergence within P. sidneyi, with cladogenesis occurring during the middle to late Pleistocene. These results suggest that habitat specificity and dispersal capacity are key factors influencing genetic structure in freshwater crabs. Morphological analyses were conducted to examine the relationship between carapace dimensions and dispersal capabilities in lentic (standing water) and lotic (flowing water) habitats. Although P. flavusjo is associated with heightened carapace dimensions, indicative of semi-terrestrial adaptations, and P. sidneyi with flatter carapaces typical of riverine species, no significant correlation was found between carapace morphology and genetic differentiation across the broader Potamonautes dataset. This finding challenges assumptions about the role of morphology in shaping dispersal and genetic connectivity in freshwater crabs. To assess the impacts of climate change, species distribution models (SDMs) were employed to project the current and future distributions of South African Potamonautes species under different climate scenarios. The models predict substantial shifts in distribution ranges, with some species experiencing range contractions and others showing potential expansions. The models indicate that six species, including P. barbarai and P. isimangaliso, are likely to experience a net loss in suitable habitat, with P. isimangaliso particularly vulnerable due to substantial reductions in protected habitat coverage. In contrast, seven species, including P. flavusjo and P. sidneyi, are predicted to experience net gains in suitable habitat, reflecting their adaptability to changing conditions. Four additional species, such as P. perlatus and P. barnardi, exhibit variable responses depending on the general circulation model (GCM) applied. An analysis of protected areas revealed notable gaps in their effectiveness under future climatic conditions. Species such as P. depressus and P. warreni are expected to benefit from increases in protected habitat, while others, notably P. barbarai and P. isimangaliso, face substantial declines, underscoring the need to reassess conservation priorities. Precipitation-related variables, particularly precipitation seasonality and precipitation of the coldest quarter, emerged as the most influential predictors of species distributions, highlighting the critical role of water availability in shaping freshwater crab habitats. The findings of this study underscore the importance of integrating genetic, ecological, and climatic data to inform conservation planning for freshwater ecosystems. This research contributes to the broader understanding of how freshwater species respond to environmental change, with implications for the management of biodiversity in the face of global climate change. It emphasizes the role of habitat specificity, genetic diversity, and protected areas in shaping species' resilience to climatic and anthropogenic pressures. By addressing critical gaps in the conservation of freshwater crabs, this study provides a framework for preserving the ecological integrity of South Africa's freshwater ecosystems and the services they provide.

AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.

Masters