Diversity and historical demographics of Southern Ocean brittle stars
The rich fauna of the Southern Ocean is managed by a complexity of treaties and conventions coupled with geo-political boundaries. The fauna is generally considered an “Antarctic” assemblage with some influence from South American shelf fauna around the sub-Antarctic islands (Hedgpeth 1969; Griffiths et al. 2009). Sequence data from the mitochondrial DNA barcoding gene, targeting species of Antarctic benthos, has identified more genetic and geographic structure than the “Antarctic Assemblage” model accounts for (Linse et al. 2007; Leese and Held 2008; Havermans et al. 2011; Dietz et al. 2015; Dömel et al. 2017). The congruence of patterns across some faunal elements indicate a shared evolutionary history and, perhaps, demographic processes, suggesting that the assumptions of wide distributions, large populations sizes and broad connectivity of metapopulations – key attributes that reflect a resilient assemblage - are likely to be violated. It appears that there are deep divergences in lineages with many cryptic species, some with reduced ranges and little or no connectivity (e.g. Sands et al. 2015; Jossart et al. 2019). This has considerable management implications, particularly for management bodies such as the Government of South Georgia and the South Sandwich Islands that may have a two or three fold increase in diversity compared with that which is recognised, and many of the new species elements are likely to be endemics with low effective population sizes making them vulnerable to environmental changes (see Barnes et al. 2011).
One group of organisms that is of particular interest is the brittle stars – or class Ophiuroidea. They are ubiquitous across the Southern Ocean, they are diverse with 219 described species (Martín-Ledo and López-González 2013) and often the most abundant in terms of numbers and biomass. Several studies have identified cryptic diversity in ophiuroids. In many cases these are “first pass” assessments where the mitochondrial barcoding gene has identified deep, cryptic divergences that tend to be geographically explicit. For example, in the sub-Antarctic species Ophiuroglypha lymani forms a morphological complex with even its sister species, the Antarctic endemic Ophiuroglypha carinifera is subsumed Sands et al. (2015). In other cases high resolution genomic markers are required for more rigorous genetic assessment of speciation processes, such as applied in a recent study to resolve ecological speciation in the snake star Astrotoma agassizii (Galaska et al. 2017). In contrast to these genetic assessment, no distinguishing morphological characters have yet been identified that could be used to reconcile the majority of cryptic genetic groups within Astrotoma and Ophiuroglypha with corresponding morphologies. Morphology is, however, still the most convenient tool to use in the field for identifications and often the only measure supporting biodiversity assessments supplied to environmental managers. Good taxonomic expertise specific to Southern Ocean ophiuroids is therefore a fundamental skill that needs to be developed alongside 21st century genetic toolkits to facilitate the surveying of Southern Ocean benthos, and by implication, its conservation (Saucède et al. 2020).
Hunter and Halanych (2008) described three deeply divergent clades in the snake star Astrotoma agassizii, one specific to individuals collected around the Antarctic continental shelf and two from the Patagonian shelf. Further work identified that this brooding species appeared to have a pelagic or lecithotrophic larvae around the Antarctic, giving an indication that there may be a species divide as indicated by the divergent mitochondrial clades (Heimeier et al. 2010). High resolution genomic markers confirmed strong separation between the Antarctic population and the two Patagonian Shelf populations, despite the clear ability to hybridize, indicating ecological speciation had taken place or was in the process of taking place. Further work identified as many as seven distinct clades, two different reproductive strategies, and that the sister species, Astrotoma drachi, belongs to one of the 7 clades of A. agassizii (Jossart et al. 2019) in much the same way as O. carinifera is nested within the O. lymani complex. There is strong evidence that both Astrotoma and Ophiuroglypha are cryptic species complexes with no distinguishing morphological characters, apart from the clade in which the sister species sits. In these two examples, diversity has increased while population size has decreased. If this turns out to be a pattern common to Southern Ocean species, it has profound implications for conservation management.
This leads to some key research questions:
- Are the patterns of cryptic diversity identified a generality of the Southern Ocean brittle stars?
- To what extent do high resolution genomic markers alter perceptions of cryptic brittle star species derived from DNA barcode data?
- Can cryptic brittle star species be reconciled with morphology?
- Shared diversity patterns may be due to shared evolutionary histories. Using coalescent techniques, can common historical demographics be inferred?
- What generalities can be gained from combined molecular and morphological analysis of brittle stars that can be applied to facilitate conservation and management of Southern Ocean benthos / ecosystems?
Dr William Goodall Copestake (SAMS)
Prof Bhavani Narayanaswamy (SAMS)
Prof Peter Convey (British Antarctic Survey)
Dr Sabine Stöhr (Swedish Museum of Natural History)
British Antarctic Survey
University of the Highlands and Islands
Council member of the Systematics Association
Fellow of the Linnean Society
Member of the British Ecological Society
Member of the Challenger Society
2020 ICBERGS 3 and Marine Planning survey of Burdwood Bank RRS James Clark Ross (3 weeks)
2018 ICEBERGS 2 and Marine Planning survey of Burdwood Bank Principal Science Officer RRS James Clark Ross (4 weeks)
2018 ODA / Bluebelt Assessing ecosystem resilience and ecosystem services in South Atlantic overseas territories RRS James Clark Ross (Tristan da Cunha and St Helena 6 weeks)
2017 ICEBERGS 1 Biological response to retreating glaciers along the Antarctic Peninsula RRS James Clark Ross (4 weeks)
2017 Ascension Island Sea Mounts with Pristine Seas (National Geographic) on the RRS James Clark Ross. (3 weeks)
2017 Antarctic Circumpolar Expedition leg 2 (Hobart – Punta Arenas, Akademic Tryoshnikov), group leader. (5 weeks)
2015 Benthic collections cruise on RV Polarstern ANTXXIX/3 (PS96) (12 weeks)
2015 Biodiversity survey around Ascension Island on the RRS James Clark Ross (JR15003). Shift leader, benthic collections (1 week) 2014 Benthic sampling cruise on RV Polarstern ANTXXIX/9 (PS82) sampling Filchner Trough, Weddell Sea. Group leader of Benthic collections (11 Weeks).
2013 Benthic sampling cruise on RV Nathaniel B Palmer (NBP B281) through Bellingshausen, Amundsen and Ross Seas (40 days).
2012 Benthic sampling cruise on RRS James Clark Ross (JR275) to South Sandwich Islands and Southern Weddell Sea (Filchner or Theil Trough) using Agassiz trawl and epibenthic sledge with deep water camera system. (7 weeks)
2011 Leading benthopelagic near shore sampling project. Samples taken from modified RIB trawler using Rauchert dredge and plankton net around Cumberland Bay, South Georgia. (4 weeks)
2011 Benthic survey of South Georgia continental shelf on RRS James Clark Ross (JR262) as part of a Darwin Initiative grant using Agassiz trawl, epibenthic sledge and shallow water camera system. (4 weeks)
2011 Benthic team co-ordinator on RV Polarstern ANTXXVII/3 (PS77). Sampling shallow water (200-300m) benthos with Agassiz trawl throughout Scotia Arc, Antarctic Sound, Larson A, B and C, southeastern Weddel Sea (BENDEX project) and Bouvet Island (10 weeks)
2009 Ecosystem coupling cruise on board RRS James Clark Ross (JR230 - BASWAP) using epibenthic sledge and Agassiz trawl in Margurite Bay, West Antarctic Peninsula (4 weeks)
2008 Benthic sampling cruise on board RRS James Clark Ross (JR179 – BIOPEARL II) using Agassiz Trawl and epibethic sledge to sample benthic organisms from Bellingshausen and Amundsen Seas, Antarctica. (7 weeks)
2002 – 2004 Sampling for saproxylic (log dependent) invertebrates throughout southern highlands NSW. (3 x 2 weeks, 1 x 1 week) 2001 Setting up and maintained squid tracking experiment using VEMCO sonic hydrophones, Picnic Bay, Magnetic Island, Queensland (9 weeks).
Guest Lecturer of Molecular Evolution for the Polar Microbiology summer course at University of Norway at Svlabard (UNIS) 2010 – 2015)
16 years - Molecular Ecologist, British Antarctic Survey
2000 - B Antarcitc Studies Hons (1st), Institute of Antarctic and Southern Ocean Studies, University of Tasmania
1999 - B. Science, University of Tasmania
1991 - Ass. Dip. Appl. Sci. Aq, University of Tasmania