An Ocean of data: eDNA metabarcoding provides useful data on aquatic species and metacommunity

Advancements in sequencing technologies have boosted the expansion of environmen tal DNA ( applications as a powerful tool for aquatic biodiversity monitoring and marine resources management. In this study, we employed universal primers Tele02 and MiFish [ which target the 12s rRNA gene of teleosts, along with MarVer1 for marin e mammals [ to investigate the presence of bottlenose dolphins Tursiops truncatus and sympatric fish species along the southwest coast of Ireland. During the survey on the Celtic Mist Research Vessel, as part of the Shannon Dolphin Project, a dual sam pling approach was used: active sampling via filtration of 5 L seawater through 0.45 µm filters, and passive sampling using roll gauzes [ Extracted eDNA underwent PCR amplification and high throughput Illumina sequencing on a Next2000 sequencer (600 cyc le cartridge). Reads were processed with Vsearch and taxonomically assigned with the fish database MitoFish and the general NCBI database. T. truncatus was recorded at 12 of the 17 surveyed sites, basking sharks Cetorhinus maximus) at two, and the common dolphins Delphinus delphis) at one. Among the primers tested, Tele02 showed higher amplification efficiency and detection sensitivity, corroborating T. truncatus visual sightings (detected in 10/12 filters and all rolls). Notably, Tele02 also outperformed MiFish in detecting C. maximus at low relative abundance ( in one passive sample. Unexpectedly, at a site where four D. delphis were visually confirmed, eDNA analysis detected only T. truncatus (RRA filter 10.29%, roll 3.84%), suggesting either inco mplete primer specificity or potential species misassignment. Interestingly, the most recurrent fish taxa detected during dolphin sightings included Scomber scombrus, Sardinia pilchardus, Hyperoplus lanceolatus/Ammodytes marinus, Clupea harengus, Chelon la brosus, Dicentrarchus labrax Overall, this study highlights the versatility of the Tele02 primer in the simultaneous detection of marine mammals, cartilaginous fish, and teleosts, offering multi species data to support ecosystem monitoring and conservati on efforts.