Abstract
The drivers behind evolutionary innovations such as contrasting life histories and morphological change are central questions of evolutionary biology. However, the environmental and ecological contexts linked to evolutionary innovations are generally unclear. During the Pleistocene glacial cycles, grounded ice sheets expanded across the Southern Ocean continental shelf. Limited ice‐free areas remained, and fauna were isolated from other refugial populations. Survival in Southern Ocean refugia could present opportunities for ecological adaptation and evolutionary innovation. Here, we reconstructed the phylogeographic patterns of circum‐Antarctic brittle stars Ophionotus victoriae and O. hexactis with contrasting life histories (broadcasting vs brooding) and morphology (5 vs 6 arms). We examined the evolutionary relationship between the two species using cytochrome c oxidase subunit I (COI) data. COI data suggested that O. victoriae is a single species (rather than a species complex) and is closely related to O. hexactis (a separate species). Since their recent divergence in the mid‐Pleistocene, O. victoriae and O. hexactis likely persisted differently throughout glacial maxima, in deep‐sea and Antarctic island refugia, respectively. Genetic connectivity, within and between the Antarctic continental shelf and islands, was also observed and could be linked to the Antarctic Circumpolar Current and local oceanographic regimes. Signatures of a probable seascape corridor linking connectivity between the Scotia Sea and Prydz Bay are also highlighted. We suggest that survival in Antarctic island refugia was associated with increase in arm number and a switch from broadcast spawning to brooding in O. hexactis, and propose that it could be linked to environmental changes (such as salinity) associated with intensified interglacial‐glacial cycles.
Highlights
In marine invertebrates, early life-history strategy influences species dispersal potential, and this, in turn, can shape population-level gene flow and long-term evolutionary histories (Hart & Marko, 2010)
We used c oxidase subunit I (COI) sequence data from samples collected from an expanded distribution to determine (a) whether O. victoriae contains cryptic species, (b) how genetic structure is characterized in O. victoriae and O. hexactis, (c) if there is genetic evidence indicating how O. victoriae and O. hexactis have survived glacial cycles, and (d) whether the divergence between O. victoriae and O. hexactis can be linked to isolation-by-environment and present-day conditions
For the Southern Ocean ophiuroids that are currently living on the continental shelf, O. victoriae historically found refuge in the deep sea while A. agassizii likely persisted on the shelf over glacial periods, highlighting that refugium survival can be different between brittle stars with the same reproductive strategy
Summary
Early life-history strategy influences species dispersal potential, and this, in turn, can shape population-level gene flow and long-term evolutionary histories (Hart & Marko, 2010). In this study, we have incorporated new O. victoriae samples from rarely sampled regions including East Antarctica (Prydz Bay, Davis Sea, Adélie Land) and Antarctic islands (South Georgia, Shag Rocks, Discovery Bank, Herdman Bank, Balleny Islands, Scott Islands, Heard Island) in order to holistically examine evolutionary processes across the Southern Ocean, along a geographical and circumpolar cline, in a species with a circum-Antarctic distribution. We used COI sequence data from samples collected from an expanded distribution to determine (a) whether O. victoriae contains cryptic species, (b) how genetic structure is characterized in O. victoriae and O. hexactis, (c) if there is genetic evidence indicating how O. victoriae and O. hexactis have survived glacial cycles, and (d) whether the divergence between O. victoriae and O. hexactis can be linked to isolation-by-environment and present-day conditions We used these analyses to investigate the ecological and evolutionary context that could explain the life-history and morphological differences between O. victoriae and O. hexactis
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