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About a mesophotic ‘giant’ Paramuricea clavata forest: ecological and genomic characterisation of a remarkable natural heritage

Stéphane Sartoretto*, Jean-Baptiste Ledoux, Elise Gueret, Dorian Guillemain, Christophe Ravel, Laurine Moirand, Didier Aurelle

*Corresponding author:

ABSTRACT: Paramuricea clavata is an ecosystem engineer, structuring hard bottom communities in the Mediterranean Sea, from 10 to 200 m depth. For more than two decades, shallow populations of P. clavata have been impacted by marine heatwaves, resulting in massive mortality events. Recently, a new dense population characterised by the tallest colonies ever recorded (up to 1.80 m) in the Mediterranean Sea was discovered between 50 and 60 m depth. Here, we analyzed the size and genetic structure of this “deep giant population” and conducted a reciprocal transplant experiment with a shallow population inhabiting the same area to test for local adaptation. The experiment showed no significant difference in the survival rate despite the high temperatures registered in the shallow area. Nevertheless, we observed a significant differentiation by depth in this area, with low estimates of gene flow. The particular local oceanographic conditions could lead to a relative isolation of the “deep giant colony” population. In conclusion, the reduced connectivity of this newly discovered giant populations with shallower ones, question its potential role as a climate refuge. Additionally, this population constitutes a unique natural heritage site that should be effectively protected from physical impacts and other direct consequences of anthropogenic activities.