The lives and times of jellyfish: Modelling the population dynamics and ecological role of jellyfish in marine pelagic ecosystems

Abstract

Jellyfish are found nearly everywhere in marine environments, and have existed virtually unchanged for more than 500 million years. Jellyfish are often considered marine ‘pests’, as they form large, unpredictable blooms that inconvenience human activities, but despite this, they have historically been largely ignored in marine science. Recently, interest in jellyfish ecology has picked up due to the widespread and increasing human impact on the marine environment that have in many cases benefited jellyfish. However, the study of jellyfish and their role in marine ecosystems is hampered by a lack of data, and the use of theoretical models to investigate jellyfish population dynamics is a promising tool for increasing our knowledge of these fascinating animals. Jellyfish share many common traits that set them apart from other organisms, such as their inflated and simple body plans, fast growth rates and reproduction, efficient swimming and feeding, and high clearance rates. In this thesis, we theoretically investigated characteristic traits of jellyfish, and how they can contribute to the patterns of jellyfish occurrence in the world’s oceans and the ability to form blooms. The thesis is based on three studies that are quite different, but each focused on an important aspect of jellyfish ecology, with emphasis on how traits of jellyfish interact with the environment to regulate jellyfish population dynamics. All jellyfish are tactile predators that rely on direct contact with their prey in order to catch it. This contrasts with the visual feeding of their main competitors, i.e. zooplanktivorous forage fish. In the first study of this thesis, we examined the competition between forage fish and large zooplanktivorous jellyfish in a global context, as influenced by water clarity and fishing. The contrasting feeding traits between the two groups causes jellyfish to have an advantage over fish in unclear water, because they are not dependent on sight to catch their prey. Using a simple food web model with mechanistic descriptions of feeding processes, we showed that this difference may explain much of the global patterns of jellyfish occurrences. In spite of their similarities, different jellyfish can have fundamental differences in key traits, such as feeding mode and life cycle. In the second study, we investigated the interactions between environmental variation and jellyfish life cycles. The aim was to explore how environmental variation can contribute to the fluctuating nature of jellyfish populations, and how the two fundamental life cycles of jellyfish could be expected to respond to different environmental variations. We found that observed patterns of jellyfish blooms can indeed be explained by different types of variation, and predicted different responses of the two main jellyfish life cycles to seasonality and other key environmental factors. The third study was dedicated to the important, but understudied polyps of scyphozoan jellyfish, and their complex asexual reproductive biology. Using an evolutionary model, we investigated the allocation of resources into three basic modes of reproduction in jellyfish polyps; fast local reproduction, dormant and mortality-resistant cyst production and dispersing motile buds. Consistent with observations, our evolutionary model predicted the evolution of more than one strategy in most cases, and we predicted how each strategy should be favored by different environmental effects. Jellyfish are underrepresented in ecosystem models, and there is therefore a drive to better understand and describe jellyfish population dynamics. The lack of good data on jellyfish populations provides an opportunity to form our understanding of the roles of jellyfish in marine systems from the ‘bottom up’, constructing mechanistic descriptions of the key traits and features that sets jellyfish apart, and then comparing the predicted dynamics to the data and observations that are available. Such models can help piece together the key interactions for developing predictive capabilities when it comes to jellyfish populations. The three studies in this thesis represent different approaches and scales of models, and provide examples of how theoretical mathematical models are a flexible tool than can be tailored to specific situations and questions. However, they also highlight the need for more field studies, in order to validate the predictions of models. Developing our understanding of how understudied, but important, groups like jellyfish fit into marine ecosystems, and how they depend on environmental conditions, is a pressing task, as human pressures on the marine environment are only expected to increase in the future