JELLYFISH BLOOMS A character-based analysis of the evolution of jellyfish blooms: adaptation and exaptation Michael N Dawson Æ William M. Hamner Ó Springer Science+Business Media B.V. 2008 Abstract Mass occurrence—aggregation, bloom- ing, or swarming—is a remarkable feature of a subset of usually diverse scyphozoan clades, sug- gesting it is evolutionarily beneficial. If so, it should be associated with one or more phenotypic charac- teristics that are advantageous and which facilitate occurrence en masse. Here, we examine the evolution of morphological, ecological, and life history char- acteristics of medusozoans, focusing on the taxa that occur en masse. By tracing the evolution of aggre- gating, blooming, and swarming phenotypes, organismal traits, and environmental settings on an up-to-date synoptic phylogeny of classes and orders of Medusozoa, we are able to hypothesize circum- stances that enable taxa to occur en masse. These include character states and character complexes related to podocyst formation, strobilation, oral arms, large size, and shallow-water habitat. These evolu- tionarily advantageous traits may be adaptations that evolved in response to selection for individual traits such as survival during periods of few resources, feeding on pulsed resources, and fecundity. These adaptations were apparently subsequently coopted by selection for reproductive success which favored mass occurrence. By considering the distribution of traits describing other phylogenetic lineages—when appropriately detailed ecological and systematic descriptions become available—it may be possible to predict which species are evolutionarily predis- posed to form problematic blooms if environmental conditions permit. Keywords Ecology Á Evolution Á Environment Á Morphology Á Phylogeny Á Phenotype Á Scyphozoa Introduction Large accumulations of jellyfish—aggregations, blooms, and swarms—are natural phenomena that apparently occur unpredictably spatially and tempo- rally. Yet, taxonomically and phylogenetically, there are systematic patterns to their occurrence suggesting a certain suite of characteristics may predispose some medusozoans to occur en masse (Hamner & Dawson, 2008). If true, this would suggest that, assuming Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances Electronic supplementary material The online version of this article (doi:10.1007/s10750-008-9591-x) contains supplementary material, which is available to authorized users. M. N Dawson (&) School of Natural Sciences, University of California, Merced, CA 95344, USA e-mail: mdawson@ucmerced.edu W. M. Hamner Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA 123 Hydrobiologia DOI 10.1007/s10750-008-9591-x