Environmental drivers of spatial distribution of two sympatric blue crabs (Callinectes danae and C. sapidus), key fishery resources in a subtropical Brazilian estuary

Estuaries are particularly dynamic systems that support diverse assemblages of marine and estuarine species. In the Laguna Estuarine System (LES), southern Brazilian coast, the blue crabs Callinectes danae and C. sapidus occur in sympatry. Their coexistence likely depends on mechanisms that reduce niche overlap or mitigate competition interactions. We hypothesise that both species spatially segregate along environmental gradients, especially salinity, and according to ontogenetic stage. To test this, we assessed how abundance and biomass of Callinectes species vary across a subtropical estuary in response to physico-chemical factors. Blue crabs and environmental variables were sampled across three estuarine sectors (inner, channel, shallow) over 18 months. Demographic groups of Callinectes differed in relative abundance among sectors, with adult males and females of C. sapidus predominating in the inner sector, and juvenile males and females of both species concentrated in the shallow areas. Fine-scale spatial partitioning was mainly driven by chemical gradients—particularly salinity and dissolved oxygen. These patterns indicated that ontogenetic groups of both species occupy distinct spatial niches, reducing potential competitions and promoting coexistence. The results support our hypothesis that spatial segregation along abiotic gradients facilitates niche differentiation. This dynamic embeds classical coexistence mechanisms within the spatial complexity of estuarine mosaics. By advancing on ontogeny-mediated competition framework, our study offers a pathway to better anticipate species responses to environmental changes.