Hydrozoa (phylum Cnidaria) is one of the most diverse and widespread classes of gelatinous zooplankton. They are understudied because they are often inconspicuous and overlooked in many planktonic studies. Due to their complex life cycle, they undergo blooms and seasonal fluctuations. However, the factors that cause their fluctuations and blooms are unknown. Hydromedusae are top predators and are in direct competition with fish for resources. They can thus significantly impact the marine ecosystem during their seasonal blooms. Therefore, it is important to understand their seasonality, both in diversity and abundance, to better understand marine food webs and manage fishing grounds such as Galveston Bay and the Gulf of Mexico. To enhance our taxonomic knowledge of Hydrozoa in Galveston Bay and understand their seasonality, plankton samples were collected locally four times a week over thirteen months. These samples were examined for both abundance and species diversity to understand how the Hydrozoa population fluctuates in response to seasonal abiotic factors such as temperature, salinity, dissolved oxygen, and chlorophyll a. Twenty-five different species were found in Galveston Bay with strong seasonality in overall abundance and species richness. Dominant species included Blackfordia virginica, Liriope tetraphylla, Clytia gracilis, Malagazzia carolinae, Nemopsis bachei and the genus Obelia. Temperature alone had strong correlation with overall medusa abundance and the majority of the dominant species. This study provides a first assessment of the composition of hydromedusa in Galveston Bay and their seasonal response to environmental factors.
