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Out of Asia: Evolutionary History of the Invasive Supralittoral Isopod Ligia exotica
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The supralittoral isopod Ligia exotica Roux, 1828 is an invasive species with a tropical and temperate cosmopolitan distribution, frequently found in harbors and ports. In the New World, this isopod has a broad distribution along the Atlantic coast, being particularly common in the US Gulf of Mexico, where it inhabits rocky artificial substrates. Although it has been suggested that L. exotica has an Old World origin, from where it was introduced to other regions via wooden ships and solid ballast, the native range of this isopod remains uncertain. Recent molecular work in East Asia uncovered the presence of two highly divergent lineages of L. exotica, and suggests that this region is the source of the nonindigenous US populations of Georgia and O’ahu, Hawaii. The goal of the present study was to better understand the evolution and invasion history of this isopod, based on phylogenetic analyses of a fragment of the mitochondrial 16S ribosomal rDNA gene. The dataset examined included publicly available sequences associated with published and unpublished work, as well as newly generated sequences from the Gulf of Mexico, South America, Hawaii, Africa, and Asia. Different Maximum Likelihood (ML) and Bayesian Inference (BI) programs were implemented to reconstruct the phylogeny of L. exotica. Ligia exotica was comprised of several highly genetically divergent lineages, probably corresponding to a cryptic species complex. Most of the genetic diversity was detected in the region spanning Southeast to East Asia, which appears to constitute the native range of L. exotica. Temperature appears to influence the distribution and levels of genetic diversity of L. exotica clades. Greater opportunities for diversification of L. exotica appear to have occurred in the warmer waters. Phylogenetic patterns suggest that multiple independent invasions of L. exotica have occurred around the world. Haplotypes observed in nonnative populations belong only to two sister clades, suggesting that the potential to become invasive may be phylogenetically constrained. In Asia, these clades akin to the nonnative populations are distributed in warmer regions; thus, environmental similarity between donor and recipient regions might have increased the chance of a successful invasion.
Wang, Chang (2016). Out of Asia: Evolutionary History of the Invasive Supralittoral Isopod Ligia exotica. Master's thesis, Texas A & M University. Available electronically from