Dispersal and speciation: The cross Atlantic relationship of two parasitic cnidarians

Abstract How dispersal strategies impact the distribution of species and subsequent speciation events is a fundamental question in evolutionary biology. Sedentary benthic marine organisms, such as corals or sea anemonesusually rely on motile larval stages for dispersal and therefore have a relatively restricted distribution along coasts. Edwardsiella lineataand Edwardsiellacarnea are virtually indistinguishable edwardsiid sea anemonesnative to the east American and the Northern European coast, respectively. E. lineata is a facultative parasiteto the ctenophore Mnemiopsisleidyi , while the life cycle of E. carnea is unknown. Recently M. leidyi was found in the Skagerrak carrying Edwardsiellasp. parasites, which raised the intriguing possibility that the invasive comb jelliesacted as cargo for the facultativeE. lineata parasites to establish a new population in Northern Europe. Here, we assessed the genetic differences between these two cryptic Edwardsiellaspecies and isolated parasites from the invasive comb jellyM. leidyi in Sweden by comparing rRNA, whole transcriptomes, SNPs, ITS2 sequences and the gene complements of key developmental regulators, the Wnt gene family. We show that E. carnea and the parasite transcriptomesare more than 99% identical, hence demonstrating that E. carnea has a previously unknown parasitic life stage. ITS2 sequence analysis of E. carnea and E. lineata suggest that they may not be reproductively isolated. The transcriptomesof E. lineata and E. carnea are ∼97% identical. We also estimate that the species diverged between 18.7 and 21.6 million years ago.