Contributions of the BioMar-ACG Project to the exploration and cataloging of marine macroalgae in the North Pacific of Costa Rica

Introduction: Costa Rica hosts remarkable marine biodiversity, vital for conserving its marine and coastal ecosystems. However, marine macroalgae remains understudied, especially in the Área de Conservación Guanacaste (ACG). To address this, the BioMar-ACG Project began in 2015 as a collaboration between the government, academic institutions, and NGOs, aiming to document marine biodiversity using morphological and molecular tools, with local community involvement. The project has since expanded macroalgal collections, identified new species, corrected misidentifications, and revealed high regional endemism. Objective: To update the marine macroalgae inventory of the ACG using 2009-2024 collections from the BioMar-ACG Project and CIMAR-UCR previous projects, through morphological and molecular identification. Methods: Surveys were conducted at 35 coastal and island sites, from intertidal zones to 20 m depth, using snorkeling and SCUBA techniques. Sampling occurred during both upwelling and non-upwelling seasons to capture environmental variability. Macroalgae were manually collected, documented in situ, and preserved for morphological and molecular analyses. DNA was extracted using commercial kits, targeting six molecular markers (COX, ITS, psbA, rbcL, tufA, and UPA) for genetic identification. Final taxonomic identifications integrated molecular and morphological data. Results: A total of 562 specimens were analyzed, representing 101 species distributed among Rhodophyta (59 spp.), Heterokontophyta (22 spp.), and Chlorophyta (20 spp.). Thirty-one species were newly recorded for Costa Rica, 22 for the Pacific of Central America, and nine genera were documented for the first time in the region. Molecular analyses revealed nine potential new species to science. Conclusions: The BioMar-ACG Project substantially increased knowledge of the marine flora of Costa Rica’s northern Pacific, doubling the number of species previously reported for the ACG. The discovery of new genera and species, together with high genetic divergence within groups such as Ceramiales and Dictyotales, highlights the underestimated and cryptic nature of macroalgal diversity in the Eastern Tropical Pacific. These findings emphasize the need for continuous exploration, molecular re-evaluation of historical records, and inclusion of diverse habitats and depth gradients in future studies. The integration of morphological and molecular data, combined with community-based research, has proven an effective model for strengthening biodiversity documentation and marine conservation in Costa Rica.