Multigene phylogeny of the scyphozoan jellyfish family Pelagiidae reveals that the common U.S. Atlantic sea nettle comprises two distinct species ( Chrysaora quinquecirrha and C. chesapeakei )

Background Species of the scyphozoan family Pelagiidae (e.g., Pelagia noctiluca , Chrysaora quinquecirrha ) are well-known for impacting fisheries, aquaculture, and tourism, especially for the painful sting they can inflict on swimmers. However, historical taxonomic uncertainty at the genus (e.g., new genus Mawia ) and species levels hinders progress in studying their biology and evolutionary adaptations that make them nuisance species, as well as ability to understand and/or mitigate their ecological and economic impacts. Methods We collected nuclear ( 28S rDNA) and mitochondrial (cytochrome c oxidase I and 16S rDNA) sequence data from individuals of all four pelagiid genera, including 11 of 13 currently recognized species of Chrysaora . To examine species boundaries in the U.S. Atlantic sea nettle Chrysaora quinquecirrha , specimens were included from its entire range along the U.S. Atlantic and Gulf of Mexico coasts, with representatives also examined morphologically (macromorphology and cnidome). Results Phylogenetic analyses show that the genus Chrysaora is paraphyletic with respect to other pelagiid genera. In combined analyses, Mawia , sampled from the coast of Senegal, is most closely related to Sanderia malayensis , and Pelagia forms a close relationship to a clade of Pacific Chrysaora species ( Chrysaora achlyos, Chrysaora colorata , Chrysaora fuscescens , and Chrysaora melanaster ). Chrysaora quinquecirrha is polyphyletic, with one clade from the U.S. coastal Atlantic and another in U.S. Atlantic estuaries and Gulf of Mexico. These genetic differences are reflected in morphology, e.g., tentacle and lappet number, oral arm length, and nematocyst dimensions. Caribbean sea nettles (Jamaica and Panama) are genetically similar to the U.S. Atlantic estuaries and Gulf of Mexico clade of Chrysaora quinquecirrha . Discussion Our phylogenetic hypothesis for Pelagiidae contradicts current generic definitions, revealing major disagreements between DNA-based and morphology-based phylogenies. A paraphyletic Chrysaora raises systematic questions at the genus level for Pelagiidae; accepting the validity of the recently erected genus Mawia , as well as past genera, will require the creation of additional pelagiid genera. Historical review of the species-delineating genetic and morphological differences indicates that Chrysaora quinquecirrha Desor 1848 applies to the U.S. Coastal Atlantic Chrysaora species (U.S. Atlantic sea nettle), while the name C. chesapeakei Papenfuss 1936 applies to the U.S. Atlantic estuarine and Gulf of Mexico Chrysaora species (Atlantic bay nettle). We provide a detailed redescription, with designation of a neotype for Chrysaora chesapeakei , and clarify the description of Chrysaora quinquecirrha . Since Caribbean Chrysaora are genetically similar to Chrysaora chesapeakei , we provisionally term them Chrysaora c.f. chesapeakei . The presence of Mawia benovici off the coast of Western Africa provides a potential source region for jellyfish introduced into the Adriatic Sea in 2013.