Recent introduction of an Asian estuarine copepod,Pseudodiaptomus marinus (Copepoda: Calanoida), into southern California embayments

Past studies made between 1938 and the mid-1970's of coastal embayments between northern Baja California and Oregon (USA) established the presence of only one species of the calanoid copepod genus Pseudodiaptomus, P. euryhalinus. This species is endemic to warm temperate latitudes of western North America. We now report the presence of breeding populations of the Asian calanoid copepod P. marinus in two Southern California embayments, Mission Bay and Agua Hedionda Lagoon. The Mission Bay population was monitored monthly from December 1986 to June 1987 and showed evidence of continuous breeding and growth. The regional, endemic species, P. euryhalinus, was not observed in either embayment. Mechanisms that might account for the apparent recent introductions of P. marinus into Southern California include dispersion by ocean circulations and transport by transoceanic ships. In the present case, the explanation is that the introduction is most likely a direct consequence of aquaculture projects. Remaining to be confirmed by systematic sampling is the apparent disappearance of P. euryhalinus from embayments dominated by P. marinus.