Dynamics of production and mortality of Aurelia SP. in thau lagoon, northwerstern Mediterranean

Dissertação de mestrado, Biologia Marinha, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2015

Jellyfish are conspicuous components of the marine ecosystems and can have severe impacts on ecosystems functioning and human activities. With the apparent increasing of jellyfish blooms, scientific interest on jellyfish ecology is mounting worldwide. However, investigations are still required with regards the processes of top-down and bottom up control of jellyfish populations. Here we focused on the jellyfish Aurelia sp., which is responsible for numerous outbreaks around the world. Thau lagoon, Northwestern Mediterranean, harbours a resident population of Aurelia sp., as the entire life cycle is suggested to occur in the lagoon. Therefore, this coastal lagoon offers an ideal framework to investigate the complex benthic-pelagic life cycle of Aurelia sp.. We used a multiple approach, comprising field and laboratory investigations, to assess processes of bottom-up and top-down control of Aurelia sp. population. The distribution, habitat use and population dynamics of Aurelia sp. benthic population were assessed in situ by free diving and underwater picture analysis, between April and June 2014. We revealed a well established population of polyps over the entire lagoon and underlined the crucial role of man-made structures to their development. A 4-year field survey (2010-2014) of the pelagic stages unveils the dynamics of the population and its driving forces. Finally, to explore top-down control, fish gut contents were analysed using PCR-based techniques, to identify potential natural fish predators of Aurelia sp.. This approach was further complemented by laboratory experiments on fish predation and selectivity on different life stages of Aurelia sp.. We provide evidences that jellyfish might be a source of food, potentially important for opportunistic fishes. Moreover, we hypothesise that diversity of fish predators is largely underestimated, as even herbivorous fishes might consume gelatinous organisms. Overall, these results further contribute to the mounting acknowledgment that jellyfish are not ‘dead end’ in food webs.