Functional trait convergence in benthic macrofauna between estuaries and estuarine-like stromatolite pools

Estuaries are shaped by fluctuating temperature and salinity levels, which select for highly tolerant species. Coastal stromatolite-forming pools occur where fresh groundwater meets the ocean, thereby creating habitats analogous to estuaries. It has been hypothesised that the benthic macroinvertebrate communities that inhabit modern supratidal stromatolite pools form a subset of the most stress-tolerant species found in estuaries. Such an ecological overlap would suggest a functional convergence that could be driven by environmental pressures. To investigate this hypothesis, we conducted a meta-analysis comparing functional traits of benthic macroinvertebrate communities across the southeastern coast of South Africa, the region with the highest density of supratidal stromatolite pools globally. Furthermore, a detailed analysis of morphometric traits of a numerically dominant macrofauna that occurs in both habitats, the amphipod Melita zeylanica , was conducted. Our findings showed that the functional traits of macroinvertebrates in stromatolite pools closely resemble those in estuaries, despite the lower species richness in these pools. Morphometric analyses of Melita zeylanica revealed no significant differences between individuals from these two habitats, suggesting that similar selective pressures may be shaping morphological traits. Fluctuations in environmental conditions such as temperature and salinity may be acting as environmental filters, favouring species with functional traits that are well-adapted to both environments. The prevalence of crawling species in stromatolite pools could provide insight into how these ancient structures which dominated most shallow aquatic coasts in the Proterozoic are able to persist in modern times, as they are less destructive than burrowing species. The lower species richness in stromatolite pools may be a consequence of their isolated nature, smaller size, and extreme environmental variability. Our findings highlight the role of environmental filtering in shaping benthic communities and suggest that functional trait convergence occurs in response to shared abiotic constraints. Future studies should investigate genetic connectivity and dispersal patterns between these habitats to better understand the mechanisms driving community similarity. • Functional traits of stromatolite macrofauna overlap with estuarine assemblages. • Environmental filtering drives trait similarity under shared salinity and temperature stress. • Melita zeylanica shows morphological consistency across coastal habitats.