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Three species of hydrozoans, Maeotias marginata, Moerisia sp., and Cordylophora caspia, are established in the San Francisco Estuary, CA (SFE). These species are of concern as they may be competing with important fish in the system and disrupting the planktivorous food web. They are novel predators in the SFE and, thus, have an especially high likelihood of impacting the system. Very little is known about their basic biology and life history. In my dissertation research, I aimed to address some basic questions in invasion biology in order to understand these species and the impact they may be having. In Chapter 1, I developed a suite of molecular markers to use as a tool for understanding these populations. I characterized 10 new microsatellite markers each for Maeotias marginata and Moerisia sp. In Chapter 2, I used these molecular markers to investigate population structure, genetic diversity, and determine reproductive strategies of Maeotias marginata and Moerisia sp. I found both species had high levels of overall genetic diversity (Average HE̳ = 0.61 and 0.57 for M. marginata and Moerisia sp. respectively) but also detected evidence of asexual reproduction. In addition, I conducted genetic sequence analyses using samples of Moerisia sp. from the SFE and M. lyonsi from Chesapeake Bay. I found 100% sequence similarity showing that Moerisia sp. in the SFE is M. lyonsi. In Chapter 3, I conducted laboratory experiments to determine temperature and salinity tolerances and feeding ecology in C. caspia. Warmer temperatures resulted in higher growth rates, with the maximum growth at 25 C, but salinity did not impact growth. I found C. caspia eats a wide range of mostly planktonic prey items, proportional to prey availability, and digestion times for one prey item ranged from 48-69 minutes. Collectively, this body of work aids in the understanding of what leads these species to be successful invaders and how they may be impacting the system.