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Biogenic reefs provide a wide spectrum of ecosystem functions and services, such as biodiversity hotspot, coastal protection, and fishing practices. Honeycomb worm (Sabellaria alveolata) reefs, in the Bay of Mont-Saint-Michel (France), constitute the largest intertidal bioconstruction in Europe but undergo anthropogenic pressures (aquaculture-stemmed food/space competition and siltation, fishingdriven trampling). Very high-resolution (VHR) airborne optical data enable cost-efficient biophysical measurements of reef colonies, strongly expected for conservation approaches. A synergy of remotely sensed airborne optical imagery, calibration/validation photoquadrat ground-truth (202/101, respectively), and artificial neural network (ANN) modelling is first used to map S. alveolata relative abundance, over the largest bioconstruction in Europe. The best prediction of S. alveolata abundance was reached with the infrared-red-green (IRRG) spectral combination and ANN model structured with six neurons (R 2 = 0.72, RMSE = 0.08, and r = 0.85). The six hyperbolic tangent formulas were applied to the three input spectral bands (IRRG) in order to build six hidden neuronal images, resulting in VHR digital S. alveolata abundance model (6547 × 6566 pixels with 0.5 m pixel size). The innovative model revealed undescribed spatial patterns, namely a reef polarization (perpendicular to the shoreline) of S. alveolata abundance: high abundance on forereef and low abundance on backreef. i e w O n l y 2 1. Introduction 26 Coastal reef builders are able to primarily shape the ecology of local environment 27 through the sediment reworking. By trapping and binding carbonate sands, some 28 cyanobacteria and diatoms produce the stromatolites (Andres and Reid 2006), crustose 29 coralline algae form coatings (Gherardi and Bosence 2001), molluscan vermetidae build 30 bioconstructions (Donnarumma et al. 2017), and cnidarian corals create large barriers 31 (Mumby et al. 2003). Less renown despite their very high productivity, annelids (i.e. 32 worms) can create substantial reefs along tropical and temperate coasts (terebellidae: 33 Degraer et al. 2008; serpulidae: Moore et al. 2009; and sabellariidae: Naylor and Viles 34 2000). 35 Honeycomb worm reefs erected by the gregarious tube-building polychaete 36 Sabellaria alveolata (Linnaeus, 1767) in the megatidal Bay of Mont-Saint-Michel 37 (BMSM, France) consist of the largest intertidal bioconstruction in Europe (Noernberg 38 et al. 2010; Desroy et al. 2011). Contrary to more common encrusting veneers or 39 hummocks on rocky shores, the Sainte-Anne population in the BMSM develops on soft 40 sediment. It is currently structured as three extensive reef entities within the tidal flats. 41 Such biogenic reefs largely contributes to ecosystem functioning and provide a wide 42 panel of ecosystem services: (1) support, with the significant amount of ecological 43 niches (Dubois et al. 2002; Jones et al. 2018); (2) regulation, through the sediment 44 stabilization and trapping (Dubois et al. 2005); and (3) cu...