Abstract

BackgroundLarge numbers of endemic species inhabit subantarctic continental coasts and islands that are characterised by highly variable environmental conditions. Southern hemisphere populations of taxa that are morphologically similar to northern counterparts have traditionally been considered to be extensions of such Northern hemisphere taxa, and may not exhibit differentiation amongst geographically isolated populations in the Southern Ocean. Smooth-shelled blue mussels of the genus Mytilus that exhibit an anti-tropical distribution are a model group to study phylogeography, speciation and hybridisation in the sea, and contribute to the theory and practice of marine biosecurity.MethodsWe used a single nucleotide polymorphism (SNPs) panel that has the ability to accurately identify reference Northern and Southern hemisphere Mytilus taxa to test for evolutionary differentiation amongst native Southern Ocean island populations.ResultsNative mussels from the Falkland Islands and the Kerguelen Islands exhibited greatest affinity to native M. platensis d’Orbigny 1846 from the Atlantic coast of South America. The major Southern Ocean current flow from west to east is likely to explain the spreading of M. platensis to remote offshore islands, as adults via the process of rafting or perhaps directly as larvae. SNPs variation revealed that mussels from Tasmania were native and clearly differentiated from all other blue mussel groups in the Southern and Northern hemispheres. The native mussels M. planulatus from Tasmania and from mainland New Zealand (NZ), and tentatively M. aoteanus from the two NZ Southern Ocean offshore island groups (the Auckland Islands and Campbell Island), formed a distinct M. galloprovincialis–like Southern hemisphere group with closest affinity to Northern hemisphere M. galloprovincialis from the Mediterranean Sea. In all cases, the SNPs revealed evidence of hybridisation between two or more distinct taxa. The invasive Northern hemisphere M. galloprovincialis was identified only in Tasmania, amongst native mussels of a distinct Australian M. planulatus lineage.ConclusionOverall, our results reveal that Southern hemisphere island mussels have mixed genome ancestry and are native, not introduced by human activities. The preservation of distinct evolutionary lineages of Southern hemisphere species needs to be an ongoing focus of conservation efforts, given that population sizes on some of the remote offshore oceanic islands will be small and may be more easily adversely affected by invasion and subsequent hybridisation and introgression than larger populations elsewhere.

Highlights

  • Large numbers of endemic species inhabit subantarctic continental coasts and islands that are characterised by highly variable environmental conditions

  • Genetic diversity Eight samples from four regions encompassing the South Atlantic Ocean, the South Indian Ocean and the South Pacific Ocean were analysed for Single nucleotide polymorphism (SNP) variation, along with reference populations from both the Northern and Southern hemispheres

  • Phylogeography Building on the recent descriptions of native blue mussels from Southern hemisphere locations, such as New Zealand [4], Chile [13] and Argentina [63], we show that mussels from the Southern Ocean islands are native and very different from the three recognised Northern hemisphere reference taxa of M. edulis, M. galloprovincialis and M. trossulus

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Summary

Introduction

Large numbers of endemic species inhabit subantarctic continental coasts and islands that are characterised by highly variable environmental conditions. Southern hemisphere populations of taxa that are morphologically similar to northern counterparts have traditionally been considered to be extensions of such Northern hemisphere taxa, and may not exhibit differentiation amongst geographically isolated populations in the Southern Ocean. Because of increased human activities, including maritime traffic, geographic ranges of endemic marine species may be extended, their populations mixed via hybridisation or endangered by invasions of alien species [5]. Ocean rafting is increasingly recognised as an important natural method of range expansion of some marine taxa [6, 7] and there is evidence that increased storm activity resulting from global warming may help to break down the geographic isolation of regions such as Antarctica [8]. Understanding the natural patterns of species distributions in the Southern Ocean have long been a challenge, given the scale of the endeavour, but this challenge is increasingly becoming more difficult due to recent mixing of species and the blurring of species’ natural distributions

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