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

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.