Invasion note Seasonal abundance and occurrence of the Asian isopod Synidotea laevidorsalis in Delaware Bay, USA David Bushek* & Sean Boyd Haskin Shellfish Research Laboratory, Institute of Marine and Coastal Sciences, New Jersey Agriculture Experiment Station Rutgers, The State University of New Jersey, 6959 Miller Avenue, Port Norris, 08349, NJ, USA; *Author for correspondence (e-mail: bushek@hsrl.rutgers.edu; fax: +1-856-785-1544) Received 8 August 2005; accepted in revised form 7 September 2005 Key words: Asian, benthic, Delaware Bay, Hurricane Ivan, invasion, isopod, New Jersey, non-native, Synidotea laevidorsalis Abstract In 1999 the marine isopod Synidotea laevidorsalis (Miers 1881), indigenous to the northwest Pacific, was first documented in Delaware Bay, USA. We monitored weekly recruitment of this isopod and several other motile species in the Maurice River, a tributary of Delaware Bay. A spatial survey was also conducted. Abundance of S. laevidorsalis varied seasonally but overwhelmingly dominated other co-occurring species by an order of magnitude or more throughout most of the year. Isopod abundance increased through the summer of 2004 and peaked in September, coincident with the passing of Hurricane Ivan. Field obser- vations documented large populations, frequently associated with pilings and buoy lines, throughout Delaware Bay in salinities of 4 through 22 ppt. The dramatic abundance of this isopod indicates that there is considerable potential for altering community structure. This isopod has yet to be observed along the Atlantic Coast of New Jersey or in Chesapeake Bay, but it has been reported near Charleston, SC. Abbreviations: cm – centimeter; ° – degrees; DRBC – Delaware River Basin Commission; HSRL – Haskin Shellfish Research Laboratory, Rutgers, The State University of New Jersey; mm – millimeter; N – north; NJ – New Jersey; NJDEP – New Jersey Department of Environmental Protection; ppt – salinity of water as measured in parts per thousand; SC – South Carolina; USGS – United States Geological Survey; W – west Introduction The marine isopod Synidotea laevidorsalis (Miers 1881) has appeared in the literature to illustrate the global spread of non-indigenous species (Chapman and Carlton 1991, 1994). A native to the boreal waters of the northwest Pacific, S. laevidorsalis has reached Europe, Australia and North America, presumably in ballast water (Chapman and Carlton 1994; Moore 2004). The Delaware Bay, located on the Atlantic coast of North America (Figure 1), contains some of the busiest ports in the United States with approxi- mately 3000 cargo vessels arriving to the tri-state docking facilities in Delaware, New Jersey, and Pennsylvania each year (DRBC 2005a). In 1999 S. laevidorsalis was discovered at the Rutgers University, Haskin Shellfish Research Laboratory (HSRL), located at the mouth of the Maurice River on the Delaware Bay (R. Barber, personal communication). It is one of twenty-five non- indigenous aquatic species reported from Biological Invasions (2006) 00:1–6 Ó Springer 2006 DOI 10.1007/s10530-005-2890-5