Jellyfish Jellyfish evolution
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Medusae were the earliest animals to evolve muscle-powered swimming in the seas. Although medusae have achieved diverse and prominent ecological roles throughout the world’s oceans, their primitive organization of muscle tissue limits their mechanical alternatives for swimming (see Nemopsis study below). Consequently, despite the taxonomic diversity that has evolved among Medusazoa, it appears that only two primary functional modes have evolved. These functional modes are distinguished by and related to the type of propulsion used by medusae (i.e.; jet versus rowing propulsion). Medusae within each functional group not only share a common mode of propulsion but generally share similar bell morphologies and foraging strategies (See Emergent organization study below).

Medusan diversity and comparison of medusan swimming bell diameter and fineness among major medusan clades. Only extant lineages containing medusae are shown. Parenthetical lineage names reflect historical nomenclature when referring to the medusan portions of life cycles rather than current systematic nomenclature. Histograms represent the average diameter and fineness values of species within various medusan lineages based on Mayer (1910), Kramp (1961), and Pugh (1999). Error bars represent one standard error from the mean