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

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.

The following hypotheses were tested:
(1) Small hydromedusae feed in ways that enable them to utilize protistan prey, including microzooplankton as well as autotrophic protists typically regarded as phytoplankton. Consequently, these hydromedusae are omnivores utilizing microplankton prey rather than strict predators on metazoans.
(2) The combination of high seasonal abundances of these small medusae in coastal waters and their omnivorous diet allows them to function as significant grazers of phytoplankton during periods of peak seasonal production.

Specific objectives of the research were:
(1) Quantification of the ecological impact of small hydromedusae on their prey community. This objective will be accomplished by measuring the temporal distribution and abundance of hydromedusae and their potential microplankton and metazoan prey, quantifying feeding rates and prey selection, and confirming that the prey ingested are utilized for growth by the medusae.

(2) Quantification of the mechanical basis of prey selection by the target organisms. This objective will be accomplished by evaluating the sequential components of feeding for each of the target species, predicting prey selection patterns from measured encounter, retention and capture efficiencies of different prey types, and comparing predictions derived from this analysis with empirical results from in situ feeding and prey selection studies.

Field Sampling: Field sampling was done twice weekly in 2005 and once weekly in 2006 during the hydromedusan growth season. Hydrographic data was collected using a HydroLab Datasonde 4 profiling package outfitted with sensors for depth, conductivity,and temperature. The potential prey field of hydromedusae consists of mesozooplankton (>200 μm) and microplankton (autotrophs and heterotrophs <200 μm). Mesozooplankton samples include both the hydromedusae and their potential mesozooplankton prey. Mesozooplankton were collected by vertical hauls of a metered 64 μm-mesh 0.5m diameter ring net. Samples were preserved in 4% buffered formaldehyde. Samples for analysis of autotrophic and heterotrophic microplankton were collected using a Niskin bottle deployed at targeted depths, typically 3 depths perstation (surface, mid-depth and bottom of the well-mixed water column). Samples were preserved with 10% (vol/vol) acid Lugols solution, settled, and then processed by inverted microscopy (Gifford and Caron 2001).