Unravelling Hidden Trophic Interactions Among Sea Urchin Juveniles and Macroinvertebrates by <scp>DNA</scp> Amplification

Rocky reefs may shift between two distinct stable states: productive algal forests, characterised by high abundance and biodiversity of macrofauna, and impoverished barrens, dominated by overgrazing sea urchins. Barren states may persist despite the recovery of adult sea urchin predators, suggesting additional stabilising mechanisms. Sea urchin settlers equally colonise forests and barrens in large numbers, but in forests only a few of them reach adult size, suggesting that post-settlement predation might play a crucial role in determining sea urchin population density. Visual assessment of predation events in the field is unfeasible due to the microscopic scale of both predators and prey and the complexity of the arena. In this study, we designed and tested specific primers for the detection of mtDNA of settlers of the Mediterranean sympatric sea urchin species Paracentrotus lividus and Arbacia lixula in the stomach content of macroinvertebrates. By testing 360 invertebrates collected in algal forests during an urchin settling event at five mtDNA loci, we identified 60 (17%) samples positive for P. lividus DNA. Presence of urchin DNA was confirmed by sequencing and NGS metabarcoding analyses. Our results suggest that predation by macroinvertebrates may represent an important process in controlling sea urchin population density and maintaining the forest state in temperate rocky reefs.