Not out of the Mediterranean: Atlantic populations of the gorgonian Paramuricea clavata are a separate sister species under further lineage diversification

The accurate delimitation of species boundaries in non-bilaterian marine
taxa is notoriously difficult, with consequences for many studies in
ecology and evolution. Anthozoans are a diverse group of key structural
organisms worldwide, but the lack of reliable morphological characters and
informative genetic markers hampers our ability to understand species
diversification. We investigated population differentiation and species
limits in Atlantic (Iberian Peninsula) and Mediterranean lineages of the
octocoral genus Paramuricea previously identified as P. clavata. We used a
diverse set of molecular markers (microsatellites, RNA-seq derived
single-copy orthologues [SCO] and mt-mutS [mitochondria]) at 49 locations.
Clear segregation of Atlantic and Mediterranean lineages was found with
all markers. Species-tree estimations based on SCO strongly supported
these two clades as distinct, recently diverged sister species with
incomplete lineage sorting, P. cf. grayi and P. clavata, respectively.
Furthermore, a second putative (or ongoing) speciation event was detected
in the Atlantic between two P. cf. grayi colour morphotypes (yellow and
purple) using SCO and supported by microsatellites. While segregating P.
cf. grayi lineages showed considerable geographic structure, dominating
circalittoral communities in southern (yellow) and western (purple)
Portugal, their occurrence in sympatry at some localities suggests a
degree of reproductive isolation. Overall, our results show that previous
molecular and morphological studies have underestimated species diversity
in Paramuricea occurring in the Iberian Peninsula, which has important
implications for conservation planning. Finally, our findings validate the
usefulness of phylotranscriptomics for resolving evolutionary
relationships in octocorals.