Factors limiting marine connectivity at a species' range edge - the case of the pink sea fan, Eunicella verrucosa

The pink sea fan, 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.

The proposed project will investigate the extent of oceanographic pathways and barriers to gene flow, e.g. due to residual currents such as tidal mixing fronts and coastal/estuarine circulation patterns, and the role of selection in driving genetic differences between populations of pink sea fan at the northern limit of its range in southwest Britain. E. verrucosa is a broadcast spawner, with larvae reliant on oceanic currents for dispersal, and yet, to date, very little is known about the basic biology of this species, with little or no knowledge of its retention time in the water column or its ability to deal with thermal stress. Accordingly, we will undertake modelling and aquarium studies to begin to explore key biological factors that might be contributing to the lack of recruitment into these range edge populations. Obvious avenues for investigation include: heat stress, factors affecting spawning time, spawning duration, survival in the water column and pelagic larval duration. As these data become available these can subsequently be parameterised into our oceanographic models. High quality physical (currents, temperatures) and biological (e.g. population distributions, larval behaviour) data will be used to ground-truth the models, and a sensitivity framework will be designed to set the dispersal/connectivity results within uncertainties based on natural variability and model parameterisations.

We hold at Exeter an extensive selection of sea fan tissue samples collected from across the species' range in the northeast Atlantic and the western Mediterranean and, critical to this proposal, we have now obtained permission to work with NRW staff at the Skomer Marine Reserve in Pembrokeshire, Wales, to take tissue samples from new recruits which have recently settled within the Skomer Marine Reserve. A key objective will be to establish whether the low level of recruitment of new sea fans at Skomer is due to connectivity, selection (possibly temperature, esp., minimum winter temperature), or whether (or to what extent) the Allee effect is acting, whereby the population density of mature adult fans is now so low (few fans, widely spaced) that the chances of gametes finding each other are reduced below an as yet unknown critical threshold. Findings for Skomer will be compared with those obtained at Lundy, a site at a similar latitude, but with potentially more robust in-water connectivity to major populations of pink sea fan in north and west Cornwall and the Isles of Scilly.

We will use a molecular technique known as RAD-seq to identify single nucleotide polymorphism (SNPs) loci associated with particular geographical regions and/or with particular selection pressures. Our on-going project (Exeter) has recently finalised the screening of several thousand SNPs from across the pink sea fan genome - a panel of the most discriminating SNPs will be selected based on their ability to discriminate populations of interest (based on FST).

In summary, the project offers a unique, trans-disciplinary research training for a postgraduate student with opportunities to gain experience in molecular genetics, oceanographic modelling and aquarium manipulations of sensitive marine organisims, generating results that will feed directly into UK marine policy through our CASE partner, Natural England, and our associate partner, Natural Resources Wales. Such a mix should deliver a highly skilled and employable young researcher.