A review and synthesis on the systematics and evolution of jellyfish blooms: advantageous aggregations and adaptive assemblages

Pelagic gelatinous invertebrates in many diverse phyla aggregate, bloom, or swarm. Although typically portrayed as annoying to humans, such accumulations probably are evolutionary adaptations to the environments of pelagic gelatinous zooplankton. We explore this proposition by systematic analysis completed in three steps. First, using the current morphological taxonomic framework for Scyphozoa, we summarize relevant information on species that aggregate, bloom, and swarm and on those species that do not. Second, we establish a molecular phylogenetic framework for assessing evolutionary relationships among classes and many orders of Medusozoa and among most families of Scyphozoa (particularly Discomedusae). Third, we interpret the phylogenetic distribution of taxa and of characteristics of jellyfish that aggregate, bloom, or swarm, in terms of species diversity-a proxy for evolutionary success. We found that: (1) Medusae that occur en masse are not randomly distributed within the Phylum Cnidaria but instead they are found primarily within the Scyphozoa which have a metagenic life history. (2) Midwater and deep-sea medusae rarely bloom or swarm. (3) Epibenthic medusae do not swarm. (4) Large carnivores that feed on large prey do not bloom strongly. (5) Large medusae that feed exclusively on small prey both bloom and swarm. (6) Pelagia, the only holoplanktonic, epipelagic scyphomedusan, both blooms and swarms, demonstrating that a metagenic life cycle is not required for blooming or swarming at sea. (7) Environmental change (overfishing, species introductions, and eutrophication) may induce or inhibit blooms. (8) Taxa that bloom or swarm are often more diverse than taxa that do not. (9) Speciation in scyphozoans can occur rapidly. (10) Morphological stasis in holozooplankton masks genetic variability. (11) Selection for convergent evolution in the sea is strong because mass occurrence has evolved multiple times in independent evolutionary lineages under similar circumstances. Thus, attributes possessed by many taxa that occur en masse appear to be evolutionarily advantageous, i.e., adaptations.