BIOGEOGRAPHICAL DISTRIBUTION AND ECOLOGY OF THE PLANKTONIC COPEPOD OITHONA DAVISAE: RAPID INVASION IN LAKES FARO AND GANZIRRI (CENTRAL MEDITERRANEAN SEA)

The Asiatic copepod species Oithona davisae was initially described from the type locality of the Sacramento San Joaquin Estuary, California, USA (Ferrari and Orsi, 1984). Following recent reports of O. davisae in the Black Sea, we researched the biogeographical distribution and ecology of this species, based on original data and on a critical review of the literature. O. davisae was not recorded for the Mediterranean Sea before 2003, although it is now becoming established in Italian transitional environments. The literature indicates that O. davisae is a typical coastal and estuarine copepod species that was originally endemic in the temperate coastal waters of East Asia, and its spread to other regions was due to human-related vectors. O. davisae is less than 1 mm in length, and its abundance has often been underestimated because many sampling programmes have used net mesh sizes of 200 μm or larger, which cannot quantify small cyclopoid species. O. davisae has most often been recorded in transitional environments that are characterised by high trophic levels and stable waters, which allow its reproduction throughout the year. In Lakes Faro and Ganzirri, its highest abundance occurs in the late spring to early summer, coincident with the highest abundance of small-sized flagellates. The occurrence of dense populations of O. davisae in these transitional environments that are used for aquaculture activities confirms its invasive behaviour, with competitive exclusion of the indigenous species Oithona nana. After its introduction, O. davisae has become the most abundant species of the copepod assemblage in a short time. To date, there do not appear to be negative consequences at the community and ecosystem levels. Ship-ballast waters and bivalve transplants appear to be the main means of dispersion of O. davisae.