The population of the non-indigenous hydrozoan Blackfordia virginica medusa was studied in the Mira estuary, Portugal.

B. virginica medusae occurred seasonally in the system, from May to December.

Their diet included copepods, barnacle nauplii, decapod crustacean larvae and anchovy eggs.

The predation impact may be important and it was within the range documented for other gelatinous predators.