Niche appropriation by a widespread nereidid polychaete within estuarine-like modern coastal stromatolite pools

Determining how organisms respond to environmental gradients or variability is a central theme in ecology, which can be used to predict niche occupation. In this study we assessed the natural population variability of a locally widespread nereidid Ceratonereis (Composetia) keiskama to determine whether its broad distribution can be used to infer a generalist autecological niche. This was examined in the recently discovered estuarine -like stromatolite-forming pools in South Africa where this polychaete also occurs. Results suggest that neither temperature nor salinity were important predictors of C. keiskama density, attesting to its broad ecophysiological tolerance. Instead, sediment properties were correlated with presence in estuaries and the stromatolite pools, which suggests that foraging conditions or biotic interactions are more important autecological drivers. Notably, C. keiskama density in the stromatolite pools was on average an order of magnitude higher than in most estuaries and this species is the dominant infaunal and epibenthic macroinvertebrate within the stromatolite sediment, suggesting an opportunistic occupation of this niche. These findings highlight the importance of linking species ranges to niche occupation of diverse habitat types, and it frames future work which should address the population level connectivity and gene transfer between such different environments.