The ecology of the mesopelagic hydromedusae in Monterey Bay, California

Cnidarians are frequently the dominant members of the gelatinous fauna, and hydromedusae are often the most common sub-group. Although hydromedusae have been well studied in some epipelagic areas, there is little known about the ecology of mesopelagic species. This is primarily due to the fact that nets, which were the principle methods of collecting and studying the organisms, were typically unable to sample these fragile, gelatinous animals without destroying them.; It was not until the advent of blue water diving techniques, specialized nets, submersibles, and remotely operated vehicles (ROVs) that the role of gelatinous organisms in the mesopelagic ecosystem could be clarified. However, much remained to be learned. Questions of distribution, trophic interaction, and behavior were unanswered for most of the mesopelagic cnidarian taxa. This dissertation research used ROVs to collect both high-quality video footage and animals in pristine condition to study the distributions, trophic interactions, and behaviors of the mesopelagic hydromedusae in Monterey Bay.; This dissertation provides a broad range of ecological information on hydromedusae. The vertical distributions, seasonal and yearly variations, and the abundance of the 35 known mesopelagic hydromedusae in Monterey Bay are provided. Profiles of the physical characteristics of the water column in which these medusae were found are also shown. Novel forms of predation by medusae are described with information on gut contents and foraging behavior. El Niño events were found to have pronounced effects on populations of mesopelagic hydromedusae, with a seldom-seen species observed in high numbers in the mesopelagic during El Niño events, while historically common species became rare. A description of the first known cnidarian-pelagic tunicate mutualistic interaction is presented with a description of a unique tentacle-based asexual reproduction strategy.