A Well-Kept Treasure at Depth: Precious Red Coral Rediscovered in Atlantic Deep Coral Gardens (SW Portugal) after 300 Years

Background
The highly valuable red coral Corallium rubrum is listed in several Mediterranean Conventions
for species protection and management since the 1980s. Yet, the lack of data about its
Atlantic distribution has hindered its protection there. This culminated in the recent discovery
of poaching activities harvesting tens of kg of coral per day from deep rocky reefs off
SW Portugal. Red coral was irregularly exploited in Portugal between the 1200s and 1700s,
until the fishery collapsed. Its occurrence has not been reported for the last 300 years.
Results
Here we provide the first description of an Atlantic red coral assemblage, recently rediscovered
dwelling at 60–100 m depth in southern Portugal.We report a very slow growth rate
(0.23 mm year-1), comparable to Mediterranean specimens. In comparison with most of the
Mediterranean reports, the population reaches much larger sizes, estimated to be over one
century old, and has a more complex coral branch architecture that promotes a rich assemblage
of associated species, with boreal and Mediterranean affinities. Atlantic red coral is
genetically distinct, yet mitochondrial analyses suggest that red corals from the Atlantic
may have introgressed the Mediterranean ones after migration via the Algeria current. Our
underwater surveys, using advanced mixed-gas diving, retrieved lost fishing gear in all
coral sites. Besides illegal harvesting, the use and loss of fishing gears, particularly nets, by
local fisheries are likely sources of direct impacts on these benthic assemblages.
Conclusions
We extended the knowledge on the distribution of C. rubrum in the Atlantic, discovered its
genetic distinctiveness, and reveal a rich deep-dwelling fauna associated to these coral assemblages. These findings support a barrier role of the Atlantic-Mediterranean transition
zone, but reveal also hints of connectivity along its southern margin. The results highlight
the genetic and demographic uniqueness of red coral populations from SW Iberia. However,
we also report threats to these vulnerable populations by direct and indirect fishing
activities and argue that its protection from any mechanically destructive activities is urgent
as a precautionary approach. This study advances our understanding of phylogeographic
barriers and range edge genetic diversity, and serves as a baseline against which to monitor
future human and environmental disturbances to Atlantic C. rubrum.