Occurrence of zebra mussel (Dreissena polymorpha) in the oligohaline Hudson River,New York

Between May and October of 1992,Dreissena polymorpha invaded the oligohaline portion of the Hudson River Estuary to river kilometer (RKM) 65 (as measured upstream from the river mouth), recruiting onto suspended concrete cinder blocks and surviving 2–5‰ salinity.D. polymorpha was significantly most abundant at the most upstream sites examined, RKMs 112 and 99 (0–3‰), decreased significantly at each more downstream site, with none found at the most downstream site, RKM 49 (5–9‰). The size frequencies ofD. polymorpha at the most upstream sites were skewed toward the smallest identifiable size-class (2 mm <shell length≤3 mm). The abundance data suggest thatD. polymorpha passively dispersed downstream from established upstream source populations. By extending its range to the oligohaline zone of the Hudson River,D. polymorpha has encountered a novel suite of species with which to interact, several of which are also introduced species.Dreissena polymorpha now sympatrically occurs with other fouling organisms, including the introduced false dark mussel,Mytilopsis leucophaeata, the introduced hydroidCordylophora caspia, and the barnacleBalanus improvisus. D. polymorpha also established sympatry with a potential predator, the blue crab,Callinectes sapidus, and a potential macrophytic substrate, the introduced water chestnut,Trapa natans.