Complex genetic patterns in the mangrove wood-borer Sphaeroma terebrans Bate, 1866 (Isopoda, Crustacea, Sphaeromatidae) generated by shoreline topography and rafting dispersal

Abstract

A phylogeographic analysis of the widely distributed marine wood-boring isopod Sphaeroma terebrans Bate 1866 was carried out to test for potential genetic differentiation and geographical structure. The species spends its entire life cycle in mangrove roots and is characterized by low active dispersal ability and no early dispersal stages. As the species has a mainly circumtropical distribution, long-range dispersal mechanisms, such as rafting on floating wood, could have played important roles in the distribution of the species throughout the world. However, as high genetic diversity and very low gene flow levels have been found among very close localities, physical barriers related to changes in coastal topography could have contributed to the contemporary genetic structure. Mitochondrial (COI and 16S) and nuclear (Histone3) sequences were analysed from 13 populations of S. terebrans. High levels of genetic population differentiation (Φst = 0.84) were observed among the 13 populations analysed and they were not explainable by an isolation by distance model (Mantel test: p > 0.13). Four major lineages were recognized by phylogenetic and spatial genetic analyses. One lineage (Clade D), highly divergent from the others, was only found in one of the two populations from Mahè (Seychelles Is.) and it shows morphological affinities with South Asian populations. The other 12 populations are represented by three haplogroups. Clades B and C are not highly differentiated from each other, but both of them appear very distant from Clade A, probably representing a different species with significant departure from neutrality, as suggested by Mismatch distribution, Tajima's and Fu's Fs tests. Contrary to expectation, the cosmopolitian S. terebrans seems to comprise more than one species.Research Highlights► We investigate the genetic structure of an isopod marine species, living in mangrove ecosystems. ► High genetic diversity and very low gene flow levels have been found among very close localities. ► Phylogenetic molecular analyses reveal the existence of some cryptic species.