Dispersal and biodiversity of Antarctic marine species
One of the challenges for scientists is determining how far, and where, organisms disperse. For large animals such as seals or birds, we can use satellite tracking to monitor an individual’s movement. However, for animals such as marine invertebrates – many of which are small or have microscopic larval phases – we simply can’t track them this way. Instead we can use their DNA as a unique type of tag, to understand dispersal.
Through the Australian Antarctic Division and the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS), my team of postgraduate students and I are using modern genetic methods to study the connectedness of marine invertebrate populations in Antarctica. We are beginning to show that many Antarctic species don’t disperse as far as we originally thought and that the spatial scale over which populations exist is actually quite small.
For many years scientists have assumed that populations of marine invertebrates around Antarctica would be highly connected due to the circum-Antarctic current that flows in a clockwise direction around the continent. This is akin to Nemo travelling from the Great Barrier Reef to Sydney on the East Australian Current. In fact my team is showing that even for species that have a circum-Antarctic distribution dispersal is quite limited. When we compare DNA sequences in animals such as pycnogonids (sea spiders), sea urchins, amphipods and deep sea corals from areas including the Weddell Sea, Ross Sea, Antarctic Peninsula, Dumont d’Urville Sea and the Davis Sea, we find that the populations from different regions are only distantly related. From this we can infer that there is very limited dispersal, at least across large spatial scales, around Antarctica.
If the larvae of marine invertebrates aren’t travelling on the circum-polar current and dispersing all around Antarctica, how far do they go? Another aspect of our research is understanding dispersal on much smaller scales – for example, between bays and headlands around Casey or Davis stations. We have developed microsatellite DNA markers (highly variable regions of DNA that enable us to look at differences between individuals rather than just populations) for three species – the amphipod Orchomanella franklini and the sea urchins Sterechinus neumayeri and Abatus ingens.During the summers of 2008–09 and 2009–10 we collected samples of these three species from Casey and Davis stations for genetic analysis. Importantly these three species have quite different larval types; O. franklini and A. ingens are both ‘brooders’. This means their larvae develop internally and they are not released until they are fully developed, so we expect they would settle quickly and not disperse far from their parents. In fact our genetic data so far is showing that the amphipods don’t disperse much more than a few tens of metres. In contrast, S. neumeyeri is a ‘broadcast spawner’, which releases gametes into the water column where fertilisation and subsequent development takes place. For this species we expect that larvae may be carried some distance before they metamorphose and settle, and so dispersal could be relatively high. By comparing the genetic structure of species which brood and broadcast spawn, we will be able to develop a better understanding of dispersal processes across a wide range of life-history types, which will help inform management and marine protected area design in Antarctica.
Genetic data is also incredibly important for understanding biodiversity. As a result of recent discovery voyages associated with the Census of Antarctic Marine Life project, we have been able to access samples of marine invertebrates from around Antarctica. Using genetic data from these samples we’ve shown that within many animal groups there are many unknown and unidentified species. PhD student Helena Baird has found at least seven new species of Eusirus amphipods within what was initially considered only two species. PhD student Narissa Bax has also found evidence of cryptic species (physically alike but genetically different) in the stylasterid coral Errina fissurata in the Ross Sea and we are certain there are many more species yet to be discovered.
Genetic diversity is important for the survival of populations – the more genetic variation that exists in Antarctic communities, the greater the likelihood that they will be able to adapt to environmental change. Our genetic research is beginning to help us understand that Antarctic marine communities are actually very diverse, and this genetic variability will be important in terms of how resilient they are to climate change.
Australian Antarctic Division and Institute for Marine and Antarctic Studies
C. Arango, A. Soler Membrives and K. Miller (2010). Genetic differentiation in the Circum-Antarctic sea spider Nymphon australe (Pycnogonida; Nymphonidae). Deep Sea Research II, doi:10.1016/j.dsr2.2010.05.019