INVERTEBRATE INTRODUCTIONS IN MARINE HABITATS - 2 SPECIES OF HYDROMEDUSAE (CNIDARIA) NATIVE TO THE BLACK-SEA, MAEOTIAS-INEXSPECTATA AND BLACKFORDIA-VIRGINICA, INVADE SAN-FRANCISCO BAY

The hydrozoans Maeotias inexspectata Ostroumoff, 1896 and Blackfordia virginica Mayer, 1910, believed to be native to the Black Sea (i.e. Sa rmatic) and resident in a variety of estuarine habitats worldwide, wer e found as introduced species in the Petaluma River and Napa River, Ca lifornia, in 1992 and 1993. These rivers are mostly-estuarine tributar ies that flow into north San Francisco Bay. Both species appeared to b e well-established in this brackishwater habitat. Salinities at the co llection sites were about 11 parts per thousand during the summer, ris ing to nearly 20 parts per thousand in the early autumn and falling to near 0 parts per thousand in the winter. Large numbers of all sizes o f both species of medusae were observed and collected, indicating that the hydroid stages of the life cycles of the two are also well-establ ished in these rivers. In the Petaluma River, populations of both spec ies were at maximum in late July, with numbers of individuals declinin g through August and into September; the Napa River was sampled only i n October, and at that time only B. virginica was found. Examination o f full guts of M. inexspectata and B. virginica medusae revealed that both species had fed nearly exclusively on small crustaceans, principa lly barnacle nauplii, copepods and their eggs and nauplii, and crab zo ea larvae (M. inexspectata only). All the M. inexspectata medusae were males, indicating that the population has probably developed from the introduction of perhaps only a single male polyp or polyp bud. In spi te of its inability to reproduce sexually, this population appears to be maintained by the prodigious ability of the polyp to bud and reprod uce asexually, and is fully capable of invading additional low-salinit y habitats from its present Petaluma River site. Male and female B. vi rginica medusae were collected in both the Petaluma River and the Napa River, indicating that B. virginica may have been introduced by eithe r the polyp or medusa stage (or both), but that multiple individuals ( of both sexes) must have arrived from another port in one or more inva sions. As indicated for M. inexspectata, the B. virginica population w ill also probably seed new populations in San Francisco Bay and elsewh ere. Based on its cnidome as well as the morphology of both medusa and polyp, M. inexspectata has been reclassified by moving it from the fa mily Olindiidae, Limnomedusae, to the family Moerisiidae, Anthomedusae .