Devils in distress: Insights into the composition and genetic population structure of devil rays landed in the Southwestern Indian Ocean

Abstract

Devil ray populations in the Southwestern Indian Ocean (SWIO) are facing alarming declines due to overexploitation driven by high demand for their products in international markets. Despite local and international conservation efforts, insufficient information on their genetic population structure and uncertainties in their taxonomy complicates effective conservation strategies. This study analysed sequences of the cytochrome oxidase subunit I gene (620 base pairs) from 120 devil rays landed in Tanzania to test two hypotheses: (i) whether devil ray populations in the SWIO have lower genetic diversity compared to those in other regions, and (ii) whether Mobula mobular and M. japanica are indeed synonyms of the same species. Our findings identified four devil ray species: Mobula mobular, M. kuhlii, M. tarapacana, and M. thurstoni, all classified as endangered on the IUCN Red List but lacking national protection. The study also confirmed the taxonomic alignment of M. mobular and M. japanica as the same species, validating previous findings. Additionally, M. thurstoni dominated the landed devil rays, accounting for over 70 % of the specimens, and exhibited higher haplotype and nucleotide diversities compared to populations in the Eastern Indian Ocean (EIO). Furthermore, populations of M. thurstoni in Tanzania showed limited historical genetic connectivity (FST = 0.176, p < 0.05) with those in the EIO and Brazil. The study calls for urgent action to revisit existing fishing regulations in Tanzania to ensure the protection of the identified endangered rays. It also advocates for unified conservation measures for the species formerly considered separate, M. mobular and M. japanica. While these findings should be validated with hypervariable nuclear markers, the observed patterns in genetic connectivity highlight the need for regional-specific conservation strategies aimed at addressing local threats such as overfishing and destructive fishing practices to ensure the sustainability of M. thurstoni populations across their range.