Role of mesophotic ecosystems to preserve reef fish biodiversity under a climate change scenario in the eastern tropical Pacific

Mesophotic ecosystems have been proposed as life rafts against anthropogenic impacts such as pollution, tourism, fisheries, sea warming, ocean acidification, and extreme meteorological events (storms, hurricanes, and waves). However, evidence supporting this role is conflicting. Between 2021 and 2023, we conducted 384 visual transects using remotely operated vehicles (ROVs) across shallow (> 10% incident light) and mesophotic (< 10% incident light) strata, assuming that similar fish assemblages across depths indicate potential refuge capacity. Three sampling locations along a latitude gradient (16°–24° N) were selected based on the presence of coral or rocky reefs, steep depth gradients, and varying human pressures and conservation statuses. We identified 141 fish species across all sites and depths, 85 species were shared between strata, 37 were exclusive to the shallow stratum, and 19 to the mesophotic stratum. We calculated taxonomic (α diversity, Shannon, Simpson, and β-diversity) and functional (richness, divergence, evenness, and mean pairwise distance) diversity indices, finding no significant differences (P > 0.05) between depth strata. This similarity likely reflects the shallow mesophotic zones in the eastern tropical Pacific, a result of turbid waters limiting light penetration, and does not necessarily indicate that the mesophotic zone functions as a refuge.