No abstract

Shell shape variation of the freshwater mussel Castalia ambigua in relation to environmental and genetic drivers remains unresolved in South America. Along an increasing upstream to downstream hydrological gradient in the Caeté River, eastern Amazon, Brazil, we evaluated two co-occurring morphotypes of C. ambigua: the rst is elongated and compressed; the second is rounded and in ated. We performed multivariate shape analysis on the lateral view of the right valve and the umbonal view, to identify potential ecophenotypic trends. We also investigated whether the degree of shell in ation was associated with sexual dimorphism. Castalia ambigua shell shape differed along the hydrological gradient and between morphotypes. However, in high hydraulic energy habitats the two morphotypes tend to converge.In ated shells of C. ambigua corresponded to female specimens, implying that shell shape variation in low hydraulic energy habitats was associated with sexual dimorphism. We found strong evidence of ecophenotypic plasticity and variation in sexual dimorphism in the freshwater mussel C. ambigua, associated with a hydrological gradient. Ecophenotypic plasticity appears to override sex-linked shell shape determination in high energy habitats. Differences in shell shape and the identi cation of ecomorphotypes are important for management efforts, such as protecting the speci c habitats associated with both morphotypes.

No abstract

Novel insights into habitat suitability for two Unionida freshwater mussels, Castalia ambigua Lamarck, 1819 (Hyriidae) and Anodontites elongatus (Swainson, 1823) (Mycetopodidae), are presented on the basis of hydraulic variables linked with the riverbed in six 500‐m reaches in an eastern Amazonian river basin. Within the reaches, there was strong habitat heterogeneity in hydrodynamics and substrate composition. In addition, we investigated stressors based on landscape modification that are associated with declines in mussel density. We measured hydraulic variables for each 500‐m reach, and landscape stressors at two spatial scales (subcatchment and riparian buffer forest). We used the Random Forest algorithm, a tree‐based model, to predict the hydraulic variables linked with habitat suitability for mussels, and to predict which landscape stressors were most associated with mussel density declines. Both mussel species were linked with low substrate heterogeneity and greater riverbed stability (low Froude and Reynolds numbers), especially at high flow (low stream power). Different sediment grain size preferences were observed between mussel species: Castalia ambigua was associated with medium sand and Anodontites elongatus with medium and fine sand. Declines in mussel density were associated with modifications linked to urbanization at small scales (riparian buffer forest), especially with percent of and distance from rural settlements, distance to the nearest street, and road density. In summary, the high variance explained in both hydraulic and landscape models indicated high predictive power, suggesting that our findings may be extrapolated and used as a baseline to test hypotheses of habitat suitability in other Amazonian rivers for Castalia ambigua and Anodontites elongatus and also for other freshwater mussel species. Our results highlight the urgent need for aquatic habitat conservation to maintain sheltered habitats during high flow as well as mitigate the effects of landscape modifications at the riparian buffer scale, both of which are important for maintaining dense mussel populations and habitat quality.