Phylogeography and demographic inference in Nacella (Patinigera) concinna (Strebel, 1908) in the western Antarctic Peninsula

Abstract

Endemic to Antarctic ecosystems, the limpet Nacella (Patinigera) concinna (Strebel, 1908) is an abundant and dominant marine benthic invertebrate of the intertidal and shallow subtidal zone. In order to examine the phylogeographic pattern and historical demography of the species along the western Antarctic Peninsula, we amplified 663 bp of the mitochondrial DNA cytochrome oxidase subunit I of 161 N. concinna specimens from five localities, as well as two specimens from South Georgia and Sub-Antarctic Marion Island. As two different morphotypes, one characterized by an elevated shell in the intertidal and the other by a flat one in the subtidal, have been recurrently reported for this species, we also compared intertidal and subtidal samples from two localities of King George Island (Admiralty and Fildes Bay) through geometric morphometric and genetic analyses. As a result, elliptic Fourier analyses on shell shape morphology detected highly significant differences between intertidal and subtidal morphotypes. In contrast, mtDNA analyses between these morphotypes did not detect statistical differences between them and support the hypothesis that subtidal and intertidal N. concinna forms correspond to be the same population unit.

Genetic analyses depicted low levels of haplotypic and nucleotide diversity in N. concinna in all localities. Among populations, comparisons did not detect any genetic structure, supporting the existence of a single genetic unit along the western Antarctic Peninsula. A marked L-shaped distribution of pairwise differences and significant negative Tajima's D and Fu's Fs indices suggest the existence of a recent demographic expansion of this species. Time estimations corrected by the "time dependency of molecular rate" hypothesis for this demographic event (7,500-22,000 years ago) fit well with the last glacial-interglacial transition period. Low levels of genetic diversity in N. concinna could reflect the dramatic effect of glacial periods on population sizes, especially in Antarctic species with narrow bathymetric ranges.

Genetic similarities between South Georgia and Antarctic samples, as well as between Nacella delesserti (Philippi, 1849) and N. concinna (Strebel, 1908) fell within the range of intraspecific variation. The genetic proximity between sub-Antarctic N. delesserti and the Antarctic limpet could be explained through north-eastward long-distance dispersion events during the late Pleistocene.