From coral reefs to gorgonian forests: Shared cues and hierarchical settlement in a temperate octocoral (Paramuricea sp.)

Larval settlement is a critical bottleneck in coral life cycles, yet mechanistic understanding is largely derived from tropical scleractinians, however lacking for non-tropical octocorals. Tropical crustose coralline algae (CCA) and their associated microbiome are known to be a major trigger for settlement processes. We investigated settlement drivers in a temperate octocoral ( Paramuricea sp.) to assess whether similar processes operate in temperate and cold-water coral forests: (1) In a choice experiment larvae were exposed to different types of substrates. Settlement occurred exclusively on substrates naturally colonized by crustose coralline algae (CCA), independent of rock lithology, indicating that biogenic surface properties, rather than mineral composition, govern settlement. Substrate covered with Lithophyllaceae -associated CCA induced settlement across depth ranges, whereas other CCA taxa and bare fragments did not. (2) Experimental exposure to the bacterial metabolite cycloprodigiosin (CYPRO) triggered settlement behaviour in up to 44% of the exposed larvae, suggesting chemical cues as primary inducers. (3) Spatial analyses of settlement patterns revealed that attachment rarely occurred directly on CCA surfaces. Instead, larvae settled near CCA and within millimetre-scale crevices and depressions. Small-scale heterogeneity suggests that microtopographic features act as secondary filters after chemical induction. These findings indicate that key elements of settlement regulation might be shared between tropical reefs and temperate coral forests highlighting the joint importance of CCA communities and structural complexity for recruitment and conservation.