2.2.1 Sampling

The sampling was developed in two periods with a 2-year interval (2015–2017), for each reef. Each reef complex was divided into three main areas: north, central, and south. We established 35 sites for Alacranes reef and 36 stations for Chinchorro Bank, positioned with GPS (Garmín inReach Explorer+). The stations were distributed in the following ways: 16 sites in the southern zone of both reefs; 10 and 17 for Alacranes and Chinchorro, respectively, in the central zone and 9 and 3 for Alacranes and Chinchorro, respectively, in the north zone. The differences between the south and central areas with the northern area were mainly the difficulty and risks of navigating the low sand and shallow depths.

Two 20-m-long randomized photographic transects [16] for each site, transect consisted of 20 photographs that covered each one an area of (56 � 34 cm) 1904 cm2 , were used [17]. The photographs were taken with a Nikonos V camera, and the total number of photographs analyzed was 2802 (533.5 m<sup>2</sup> ); 38 photographs were disposed by out of focus. To obtain the coverage of the species, each photograph was superimposed on a grid with 10 cm2 divisions for the coverage calculation. In parallel with the photographic transects, a selective collection of species was carried out for their precise identification in the laboratory and to serve as a basis for photointerpretations. The bibliography used depended on the phylum [18–23].
