Prey, feeding rates, and asexual reproduction rates of the introduced oligohaline hydrozoan Moerisia lyonsi

Moerisia lyonsi Boulenger (Hydrozoa) medusae and benthic polyps were found at 0 to 5 parts per thousand salinity in the Choptank River subestuary of C hesapeake Bay, USA. This species was introduced to the bay at least 30 year s before 1996. Medusae and polyps of M. lyonsi are very small and inconspic uous, and may occur widely, but unnoticed, in oligohaline waters of the Che sapeake Bay system and in other estuaries. Medusae consumed copepod nauplii and adults, but not barnacle nauplii, polychaete and ctenophore larvae or tintinnids, in laboratory experiments. Predation rates on copepods by medus ae increased with increasing medusa diameter and prey densities. Feeding ra tes on copepod nauplii were higher than on adults and showed no saturation over the range of prey densities tested (1 to 64 prey l(-1)). By contrast, predation on copepod adults was maximum (1 copepod medusa(-1) h(-1)) at 32 and 64 copepods l(-1). Unexpectedly, M. lyonsi colonized mesocosms at the H orn Point Laboratory during the spring and summer in 4 years (1994 to 1997) , and reached extremely high densities (up to 13.6 medusae l(-1)). Densitie s of copepod adults and nauplii were low when medusa densities were high, a nd estimated predation effects suggested that M. lyonsi predation limited c opepod populations in the mesocosms. Polyps of M. lyonsi asexually produced both polyp buds and medusae. Rates of asexual reproduction increased with increasing prey availability, from an average total during a 38 d experimen t of 9.5 buds polyp(-1) when each polyp was fed 1 copepod d(-1), to an aver age total of 146.7 buds polyp(-1) when fed 8 copepods d(-1). The maximum da ily production measured was 8 polyp buds and 22 medusae polyp(-1). The colo nizing potential of this hydrozoan is great, given the high rates of asexua l reproduction, fairly wide salinity tolerance, and existence of a cyst sta ge.