Abstract
The Southern Ocean is anomalously rich in benthos. This biodiversity is native, mostly endemic and perceived to be uniquely threatened from climate- and anthropogenically- mediated invasions. Major international scientific effort throughout the last decade has revealed more connectivity than expected between fauna north and south of the worlds strongest marine barrier – the Polar Front (the strongest jet of the Antarctic Circumpolar Current). To date though, no research has demonstrated any radiations of marine taxa out from the Southern Ocean, except at abyssal depths (where conditions differ much less). Our phylogeographic investigation of one of the most ubiquitous and abundant clades at high southern latitudes, the ophiuroids (brittlestars), shows that one of them, Ophiura lymani, has gone against the flow. Remarkably our genetic data suggest that O. lymani has successfully invaded the South American shelf from Antarctica at least three times, in recent (Pleistocene) radiation. Many previous studies have demonstrated links within clades across the PF this is the first in which northwards directional movement of a shelf-restricted species is the only convincing explanation. Rapid, recent, regional warming is likely to facilitate multiple range shift invasions into the Southern Ocean, whereas movement of cold adapted fauna (considered highly stenothermal) out of the Antarctic to warmer shelves has, until now, seemed highly unlikely.
Highlights
Marine animals inhabiting continental shelves can disperse widely over ecological time scales due to extensive shelf connectivity, current regimes, and reasonably consistent ocean chemistry (Paulay and Meyer, 2006)
O. carinifera, the sister taxon to O. lymani, is clearly identifiable by strongly elevated and granulated disc plates and radial shields, and narrow dorsal arm plates with a serrated keel
There is considerable debate as to whether or not biological species should be delimited based solely on mitochondrial DNA sequence data (DeSalle et al, 2005; Taylor and Harris, 2012; Zhang et al, 2013). In this case, based on clear genetic clades that correspond to discrete geographic areas, distinct morphology, and prior evidence that c oxidase subunit 1 gene (CO1) reliably distinguishes ophiuroid species (Ward et al, 2008), we can assume that O. lymani could be conservatively divided into a Bouvetoya species, a South Sandwich Island species and a diverging Patagonian shelf/South Georgia species
Summary
Marine animals inhabiting continental shelves can disperse widely over ecological time scales due to extensive shelf connectivity, current regimes, and reasonably consistent ocean chemistry (Paulay and Meyer, 2006). They are surrounded by deep seas (>2000 m depth), the strongest oceanic current on the planet, the Antarctic Circumpolar Current (ACC) (Whitworth, 1980; Orsi et al, 1995), and encircled by the Polar Front, a narrow jet within the ACC, across which there are steep gradients in temperature and salinity. These isolating conditions, established in the Oligocene (Barker and Thomas, 2004), have left a signature of high endemicity of the Southern Ocean shelf fauna (Dell, 1972; Clarke and Crame, 1989; Griffiths et al, 2009)
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