Long range gene flow beyond predictions from oceanographic transport in a tropical marine foundation species

The transport of passively dispersed organisms across tropical margins remains poorly understood. Hypotheses of oceanographic transportation potential lack testing with large scale empirical data. To address this gap, we used the seagrass species, Halodule wrightii, which is unique in spanning the entire tropical Atlantic. We tested the hypothesis that genetic differentiation estimated across its large-scale biogeographic range can be predicted by simulated oceanographic transport. The alternative hypothesis posits that dispersal is independent of ocean currents, such as transport by grazers. We compared empirical genetic estimates and modelled predictions of dispersal along the distribution of H. wrightii. We genotyped eight microsatellite loci on 19 populations distributed across Atlantic Africa, Gulf of Mexico, Caribbean, Brazil and developed a biophysical model with high-resolution ocean currents. Genetic data revealed low gene flow and highest differentiation between (1) the Gulf of Mexico and two other regions: (2) Caribbean-Brazil and (3) Atlantic Africa. These two were more genetically similar despite separation by an ocean. The biophysical model indicated low or no probability of passive dispersal among populations and did not match the empirical genetic data. The results support the alternative hypothesis of a role for active dispersal vectors like grazers.This work was funded by Portuguese National Funds from FCT—Foundation for Science and Technology, through UIDB/04326/2020, UIDP/04326/2020, LA/P/0101/2020, PTDC/BIA-CBI/6515/2020, and fellowships SFRH/BD/138182/2018, SFRH/BSAB/150485/2019, the transitional norm-DL57/2016/CP1361/CT0035, the Individual Call to Scientifc Employment Stimulus 2022.00861.CEECIND, project MARAFRICA: AGA-KHAN/540316524/2019, and by a Pew Marine Fellowship to EAS. Te sampling collections were supported by projects LuandaWaterFront AGKN -FCT-333191101, by the MAVA Foundation projects ResilienSEA, "Consolidation of sea turtle conservation at the Bijagós, Guinea-Bissau" granted to the Instituto da Biodiversidade e das Áreas Protegidas da Guiné-Bissau (IBAP), "PNBA-site emblématique pour les tortues" granted to the National Park of the Banc d’Arguin, project "Survie des Tortues Marines” PRCM/STM POOOA4/OA9 by PRCM (Partenariat Régional pour la Conser vation de la zone côtière et Marine en Afrique de l’Ouest), EU-H20202 854248 (Tropibio), EU-BiodivERsA program, Programa de Pós-Graduação em Ciência para o Desenvolvimento (Fundação Calouste Gulbenkian), FCT scholarship attributed to RLF (SFRH/BD/73947/2010), the Scientifc Council of the National Park of the Banc d’Arguin (funded by BACOMAB) and by the BLUE ROUTE project (PT-INNOVATION-0072—Preparing the new OCEAN economy 2030: the blue route of discovery). Tis study received support from the Portuguese node of EMBRC-ERIC, specifcally EMBRC.PT ALG-01-0145-FEDER-022121 and funding from the European Union’s Horizon 2020 research and innovation programme through project Assemble Plus under grant agreement No. 8329