Results of field surveys, stomach content analysis, and laboratory measurements of oxygen consumption at different salinities in the exotic shrimp Palaemon macrodactylus and its native counterpart P. longirostris from the Guadalquivir estuary (SW Spain) were used to assess interaction between both species. After its first record in this estuary in 1998, P. macrodactylus has shown a gradual increase in density and its estuarine population has been clearly dominated by juveniles. Feeding habits of both shrimps indicated a strong trophic overlap between them, with mysids as main prey. Their salinity-related and spatial distribution patterns show that maximal inter-specific overlap between the two populations occurs in the inner, less saline part of the estuary. Specific oxygen consumption rates under different salinities and water oxygen concentrations suggest a more efficient metabolism and a higher tolerance to hypoxic conditions in brackish waters by P. macrodactylus than by P. longirostris. Such inter-specific physiological differences could have made it possible for the exotic P. macrodactylus to invade an estuarine stretch that, prior to its arrival, was infra-utilized by native species.
