31 Past, Present and Future Connectivity of Mediterranean Cold-Water Corals: Patterns, Drivers and Fate in a Technically and Environmentally Changing World

Abstract

Given the vastness of the oceans and the small size of larvae and other marine propagules, tracking marine organisms’ dispersal particles is unrealistic. When propagules successfully recruit, a genetic profile can be tracked that integrates the movements of multiple generations. Molecular analyses thus provide an indirect means of estimating connectivity among subdivided populations, at any given spatial and/or temporal scale of interest. In general, it remains challenging to disentangle the relative influence of the various processes that cause the observed patterns of genetic differentiation among subpopulations, notably in far-off deep-sea environments. In the past decade only a handful of studies have reported on the genetic patterns in Mediterranean cold-water corals. This represents a substantial limitation for any researcher attempting to understand the dynamics of Mediterranean cold-water coral populations. It affects as well conservation decisions involving these vulnerable species and habitats. Until recently, hypotheses for gene flow of some deep-sea corals in the Atlantic have included moderate to high connectivity at broad and regional scales, through sporadic larval transport mediated by ocean currents, and simultaneous strong discontinuities between ocean basins caused by vicariance or local adaptation. However, progress has been made that is allowing researchers to retrace past major modifications in the patterns of cold-water coral migrations at evolutionary time-scales, in and out of the Mediterranean Sea. Growing evidence of the influence of clonality and its effect on estimates of genetic diversity now stimulate researchers to engage in optimised sampling strategies. Improved estimates are possible to attain, provided a rigorous sampling strategy. Studies of the cosmopolitan corals Lophelia pertusa and Madrepora oculata have made evident that Atlantic populations are clearly differentiated from Mediterranean ones, suggesting that contemporary gene flow between the two genetic backgrounds is very limited, if at all present. Results support several non-mutually exclusive hypotheses: that subpopulations in the North East Atlantic were recolonised from Mediterranean refugia following the last glacial maximum; that Mediterranean L. pertusa reefs appear to have been relevant glacial refugia during the Pleistocene glaciations and the main source for North East Atlantic recolonisation; and that M. oculata in the NE Atlantic may have had multiple sources of post-Last Glacial Maximum colonisation, which remain elusive. The solitary Desmophyllum dianthus has been shown to share the same genetic identity between very distant populations (Mediterranean and NE Atlantic). Yet, it remains unclear if this is solely due to high contemporary gene flow or if the genetic resemblance is a remnant left by an Atlantic post-glacial recolonisation from a unique Mediterranean source with high population size. Future genome-wide studies with next generation sequencing techniques will undoubtedly help clarifying the interpretations of molecular data in terms of past and present cold-water corals migration pathways. This is strongly needed, as the development of effective conservation strategies relies on well-informed, knowledge-based, policies. These must include thorough species inventories, relatedness, connectivity metrics and clear identification of genetic units, all of which depending on the use of robust techniques. Indeed, information on the genetic connectivity of Mediterranean cold-water coral populations has proven to be key to the establishment of a protected site under the European Union’s Natura 2000 Network of protected areas. We conclude with perspectives on how Next Generation Sequencing will strengthen inferences on connectivity of the majestic cold-water coral habitats in the coming years.