Trophic ecology of small hydromedusae: a new perspective on their function in coastal ecosystems

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The focus of the research is the trophic role of small (bell diameter 5 mm) hydromedusae. Medusae are important, often highly selective, planktonic predators that can strongly affect standing stocks of metazoan zooplankton as well as fish eggs and larvae. The feeding rates, prey selection patterns and the underlying feeding mechanisms that have led to the success of large, conspicuous medusae have been well studied. However, most medusae are small. These small medusae are abundant, taxonomically diverse, and often have cosmopolitan distributions. Yet their feeding ecology is virtually unknown despite their status as the largest and most diverse group of gelatinous zooplankton in the sea. Preliminary data indicate that many small hydromedusae feed differently and have different trophic roles than their larger relatives. These preliminary studies have shown that some small medusan species (1) feed as omnivores on microplankton, including phytoplankton and (2) are sufficiently abundant and have sufficiently high feeding rates to remove a significant proportion of the phytoplankton standing stock in the coastal ecosystems where they occur seasonally. This project will address the hypothesis that this combination of high abundance and high feeding rates enables small hydromedusae to function as significant grazers of phytoplankton during periods of peak seasonal production. Consequently, the contemporary paradigm that medusae are primarily predators of planktonic metazoans may require significant revision in order to encompass the trophic role of small hydromedusae. The goal of the research is to examine the feeding processes and impact of small hydromedusae by examining their trophic role in Narragansett Bay, Rhode Island. We will use a combined approach of (1) quantifying the trophic impact of small hydromedusae on the plankton community of Narragansett Bay, Rhode Island and (2) quantifying the functional bases of their feeding and prey selection. Intellectual Merit of the Proposed Activity: The research will lead to new understanding of community feeding rates, prey selection patterns and the underlying feeding mechanisms that have led to the success and evolutionary longevity of the most diverse taxonomic group of medusae. If small medusae feed primarily as omnivores on microplankton and are able to significantly affect microplankton standing stocks, then the research will document a new and important trophic link between the metazoan and microbial food webs. The new knowledge will provide protistan ecologists with enhanced understanding of the factors controlling microplankton standing stocks and production rates. The study will also provide evolutionary ecologists with a greater understanding of the biomechanical factors that have influenced the evolution of small hydromedusan body form and function as well as protist behavior and body form. From this perspective, this study directly addresses the issue of understanding factors influencing biological diversity and its ecological consequences in marine systems, a specific theme emphasized by NSF's Biological Oceanography division. Broader impacts of the Proposed Activity: The project will encourage scientific discovery by documenting a new, but presumably common, trophic link in pelagic food webs. The participation of scientists from two primarily undergraduate institutions and one primarily graduate institution enhances opportunities for training and mentorship. Each institution will involve students in every aspect of the research (5 undergraduates during each year of the proposal, 2 from RWU and 2 from PC; 1 undergraduate from the Marine Biology program at URI will do an independent study course (15 credits) with Gifford during the spring academic term of each project year). Upon receipt of funding, Gifford will apply for a budget supplement to support participation in the project by a secondary school science teacher during the summer months. As in the past, we will use our contacts with media involved in education of the general public (radio, television documentaries, news articles and aquarium exhibits) to communicate our new findings about medusan ecology. Our continuing dialog with these outlets to the public allows us avenues to communicate our findings to a wide public audience and to educate the general public.