The invasive oyster, Magallana gigas, and mussel, Xenostrobus securis, threaten ecological function in the rías of Galicia, Spain, but little is known about pathways of invasion. This study empirically measures connectivity between subpopulations using the geochemical composition of their shells as a natural marker of origin. Since logistical problems prevented complete analysis of M. gigas samples, this study focuses mostly on X. securis connectivity between the two southern rías where it has naturalized. The shell composition of larvae incubated in Ría de Vigo and Ría de Pontevedra was compared to recruits collected in both rías during the summer spawning season. Thirteen elements were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS), and natal origin of recruits was predicted using random forest models and hierarchical clustering. Connectivity matrices were created based on natal origin assignment and used to evaluate dispersal pathways. Greater dispersal was observed from Ría de Vigo to Ría de Pontevedra, with the Pontesampaio (Ría de Vigo) subpopulation contributing most to this northward spread. The outermost subpopulation in Ría de Pontevedra, Piraguas, had the greatest mixing of recruits from different predicted natal origins, aligning with estuarine circulation. This study provides data-informed recommendations for monitoring X. securis and critical insight into invasion pathways.
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