Background/Question/Methods Many populations of gelatinous zooplankton (‘jellies’) appear to be increasing worldwide. These increases are often associated with large bloom events that can lead to severe ecological disruptions, especially when jellies invade new ecosystems. Three species of Ponto-Caspian hydrozoan jellies have become established in the brackish water habitats of the San Francisco Estuary (Maeotias marginata, Blackfordia virginica, Moerisia sp.). Their impact on the aquatic community may be great as they are novel predators in this system and have high density seasonal blooms. Despite their likely ecological importance, relatively little is known about these non-natives. To increase understanding, we conducted a large scale, multi-year study investigating the ecology of Moerisia sp. in Suisun Marsh in the upper San Francisco Estuary. We collected monthly samples of medusae and polyps and associated environmental parameters from sites throughout Suisun Marsh during the Moerisia sp. blooms in 2007 and 2008. We used these samples to conduct genetic analyses of diversity and reproduction, determine habitat requirements, and analyze population dynamics and seasonality.
Results/Conclusions We developed a suite of microsatellite genetic markers to investigate the genetic diversity of polyps and medusae. We found relatively high levels of overall genetic diversity, but also a substantial amount of asexual propagation.  From our field studies, we found polyp abundance is correlated with mid-levels of salinity, water transparency, temperature, and depth, and also high dissolved oxygen. Level of asexual reproduction of medusae buds is influenced by temperature and salinity conditions, whereas the production of polyp buds is impacted by dissolved oxygen and water transparency levels. Amount of sexual reproduction, as defined by the presence of eggs on medusae, is correlated with the size of medusae. Our findings shed light on the basic ecology of this understudied hydromedusa and the impact this non-native may be having on an already highly stressed ecosystem. This understanding can aid in developing predictive tools for bloom formation and ecological impact, as well as inform restoration and management actions so conditions that promote this non-native species can be avoided.