Data collection
We sampled 116 reflective beaches in the Western Mediterranean marine province, across Spain (22 beaches, including Mallorca), France (25 beaches, including Corsica), Italy (63 beaches, including Sardinia, Sicily, and Lampedusa), and Tunisia (6 beaches). All sampled beaches were microtidal, wave dominated, and exhibit a reflective morphodynamics stage. In each beach, we collected three samples at three levels corresponding to the wave-breaking zone, the shoaling, and the subtidal level. The wave-breaking zone in reflective beaches corresponds to the swash + surging breaking zone (thereafter referred to as swash level, for simplicity) (McLachlan and Defeo, 2017) (Supplementary methods, section 1.1). Total meiofauna were extracted from each sample using the magnesium chloride decantation methods and a minimum mesh size of 0.0063 mm. From each sample, 100–150 proseriates were then sorted into different genera under the stereomicroscope and identified to species using a compound microscope. Animals belonging to undescribed species were assigned to the same taxonomic unit using consistent codes. For each individual, we examined 14 morphological traits for each species, focusing on body size and shape, ability to withstand turbulence by adhering to or digging into the sediments, trophic specialization, and the reproductive structures (Figure 2A) (Supplementary Methods, section 1.2).
We ecologically characterized each sample measuring its granulometry, and each beach by measuring its length as well as collecting a set of bioclimatic and oceanographic variables from open-access databases (Supplementary Methods, section 1.3).