Early Establishment of the Invasive Atlantic Blue Crab Callinectes sapidus Rathbun, 1896 (Decapoda: Portunidae) in Mauritanian Coastal Waters: Population Structure, Morphometrics and Reproductive Condition at the Southern Edge of Its Northwest Africa

The Atlantic blue crab, Callinectes sapidus Rathbun, 1896, is classified as one of the world’s most invasive and difficult species to control once introduced. This study reports the first confirmed occurrence of this species in Mauritanian waters and assesses the population structure, morphometric features and reproductive condition of the sampled population as evidence of early establishment. Samples were collected monthly in Baie de l’Étoile (Nouadhibou, Mauritania) in 12 sampling campaigns between December 2024 and January 2026 (no surveys in January 2025 or December 2025). In total, 148 specimens of C. sapidus were captured from the shallow waters of the bay’s intertidal and subtidal zones, of which 98 (66.2%) were sexually mature and 50 (33.8%) were immature. Carapace width among captured specimens ranged from 48.5 to 193.4 mm and weight ranged from 10.0 to 255.8 g. The sex ratio did not differ significantly from parity (75 males and 73 females; male: female = 1.03:1; χ2 = 0.027, df = 1, p = 0.870), whereas mature individuals were significantly more frequent than immature ones (χ2 = 15.57, df = 1, p < 0.001). Five of the captured females were ovigerous (June–September). This study provides the first confirmed record of the species along the Mauritanian coast and, based on the presence of multiple size classes in all sampled months and of ovigerous females in summer, indicates an early established (incipient) population, warranting further biological assessment, genetic characterization and regional monitoring to anticipate potential impacts on the local ecosystem.