Integrative molecular and taxonomic techniques reveal larval fish biodiversity patterns in the Azores UNESCO biosphere reserves

Oceanic islands are important habitats for marine fish larvae, providing suitable conditions for their development and growth, contributing to fisheries production. This study characterises the early summer larval communities around the Azores UNESCO Biosphere Reserves (Mid North Atlantic, Portugal), focusing on four islands: São Jorge and Graciosa (central group), Flores and Corvo (western group). Species identification was performed using a combined approach of molecular DNA barcoding (Cytochrome oxidase subunit, COI and 12S ribosomal RNA markers) and morphological techniques. Results showed high diversity, with 49 taxa (27 families, 35 species) identified, of which 23 species and 13 families were recorded for the first time as larval stages in the region. Common families included Blenniidae, Pomacentridae, Labridae, Scombridae, Myctophidae, and Sparidae, while Parablennius incognitus, Chromis cadenati and Sarda sarda , were the most abundant species. Larval communities included species from several habitats (mainly demersal and reef-associated), reflecting the high fish diversity of the Azores. The western islands showed higher species abundance and diversity, compared to those in the central group. Community structure varied between island groups and was mainly linked to water temperature, dissolved oxygen, and microzooplankton biomass. When used alongside morphological techniques, molecular tools increased the efficiency of species identification, especially for taxa that are difficult to identify by morphology. The results allowed the morphological identification of two abundant species, Parablennius incognitus and Symphodus mediterraneus, for which detailed descriptions are lacking. This study provides a valuable baseline for understanding larval fish biodiversity in the Azores, essential for biodiversity conservation and fisheries management. • Early summer ichthyoplankton communities were sampled in the Azores. • We used a combination of morphological and molecular identification techniques. • Higher abundance and diversity of fish larvae occurred in the western group islands. • 23 species, 3 genera and 13 families recorded as larvae in the region for the first time. • Water temperature was the main environmental factor shaping these communities.