What Are Jellyfishes and Thaliaceans and Why Do They Bloom

The jellyfishes and thaliaceans comprise primarily planktonic species of cnidarians, ctenophores (jellyfishes) and chordates (thaliaceans or pelagic tunicates). Grouped together because of their gelatinous bodies, these diverse species nonetheless differ in their evolutionary histories and may have distinct morphologies, life histories, ecologies and other traits. Subsets of these species occur at some times and places in highly elevated concentrations, i.e. they accumulate, aggregate, bloom or swarm. Why jellyfishes and thaliaceans occur in such masses is, however, somewhat unclear; the reasons obscured in part by a tendency to treat many gelatinous zooplankton, including jellyfishes and thaliaceans, as a single functional group. Here we summarize the evolutionary relationships among gelatinous zooplankton and review the characteristics of blooms, before focusing on comparing and contrasting medusae, ctenophores and thaliaceans. We highlight some substantial knowledge gaps, emphasize biological factors that likely contribute to blooms and outline a population genetic framework for investigating the ecological causes of boom and bust population dynamics in the plankton.