Investigating drivers of marine connectivity in the pink sea fan: implications for its conservation and the designation of Marine Protected Areas

Previously, in collaboration with Natural England, we studied two species of soft coral (Eunicella verrucosa; Ever and Alcyonium digitatum; Adig) from sites around Britain and Ireland, and from further afield in northwest Europe. We developed novel genetic markers (microsatellites) for these octocorals and typed them with panels of 14 (Ever) and 11 loci (Adig). We demonstrated that populations of Ever (the pink sea fan) in waters around Britain and northwest Europe showed marked regional genetic structure (Holland & Stevens, 2014, 2017; Figure 1A). In contrast, samples of Adig (dead man's fingers) inhabiting approximately the same geographic area appeared panmictic, and exhibited no significant genetic population structure. However, while both species overlap in the waters around southwest Britain and Ireland, distribution maps indicate that this area is the northern limit for Ever and the southern limit for Adig. Investigation of the possible drivers of population structure in Ever identified two loci as being under selection; no evidence of selection was detected in Adig. Critically, further analysis excluding the loci under selection resulted in the disappearance of structure from the Ever data set (Figure 1B); such a finding highlights the potential importance of selection in driving apparent population structure in this species. Whether the patterns of genetic structure observed are driven by connectivity/barriers to connectivity or by selection has major importance for our understanding of connectivity in these species and for the placement and spacing of marine protected areas, especially for the pink sea fan, an IUCN listed species. Both species are thought to broadcast spawn, with larvae reliant on oceanic currents for dispersal, yet our findings suggest very different patterns of connectivity across the same geographical area. To date, very little is known about the basic biology of these creatures, with little or no knowledge of their ability to deal with thermal stress.

The proposed project will investigate the extent of selection driving genetic differences between populations of the two species, Ever at the northern limit of its range, Adig at its southern limit. We will explore fundamental differences in their basic biology to elucidate biological factors that might be contributing to the markedly different patterns of genetic structure observed, whether due to differences in connectivity or to selection. In the absence of published octocoral genomes, we will use a molecular biology technique known as RAD-seq to identify loci specific to geographical regions and/or under selection (at Exeter). The molecular analysis will be complemented by common garden experiments and heat stress exposures in a controllable marine mesocosm (at Plymouth Marine Laboratory).

Eunicella verrucosa is IUCN red-listed and is a protected species in UK waters. It is listed as a Protected Feature, a key factor in the designation of many of the MCZs designated in southwest England in 2013 and 2016. An understanding of the diversity and connectivity of this species is essential for both conservation of the species and the designation of further candidate MCZs (Natural England, 2016). We anticipate that this research will inform on both these key factors.