Introduction
The common octopus (Octopus vulgaris) is an ecologically and economically valuable cephalopod distributed across the eastern Atlantic and Mediterranean Sea. Due to its high commercial demand, fisheries targeting this species are intense, raising concerns about the erosion of genetic diversity and the long-term viability of populations. A clear understanding of genetic structure, variability, and effective population size is essential for designing conservation measures and sustainable management strategies capable of preserving adaptive potential under growing anthropogenic pressures.
Materials and Methods
Samples of 117 individuals were collected from seven Atlantic sites and one Mediterranean location. Restriction-site associated DNA sequencing (RAD-seq) produced genome-wide single nucleotide polymorphisms (SNPs), with 15,548 high-quality markers retained after filtering. Population structure was inferred using discriminant analysis of principal components (DAPC) and ADMIXTURE modelling. Genetic differentiation was quantified with pairwise fixation indices (FST), and isolation-by-distance was tested using a Mantel test. Inbreeding coefficients were estimated, and contemporary effective population sizes (Ne) were calculated based on genome-wide linkage disequilibrium.
Results
Population structure analyses revealed a pronounced latitudinal genetic gradient across the Atlantic distribution of common octopus. ADMIXTURE modelling identified two ancestral clusters (K = 2), corresponding to European and African populations (Figure 1), with the Mediterranean sample exhibiting a partially distinct genetic signature at higher resolution. PCA and DAPC supported this structure, showing a clear north-south separation and highlighting limited differentiation among northern European populations. Pairwise fixation indices (FST) ranged from 0.000 to 0.270, with a mean of 0.116, indicating moderate genetic differentiation overall. The Mantel test confirmed a significant isolation-by-distance pattern (r = 0.857, p < 0.001), suggesting restricted gene flow driven by geographic separation. Inbreeding coefficients were elevated in several populations, particularly Mauritania, where levels of homozygosity were highest. Estimates of contemporary Ne were critically low, ranging from 8 individuals in Mauritania to less than 200 across all other sites (Figure 2).
Discussion
The pronounced latitudinal cline and moderate differentiation suggest that neutral processes, such as genetic drift and limited dispersal, shape the population structure of common octopus. The partial differentiation of the Mediterranean sample likely reflects a historical phylogeographic barrier imposed by the Strait of Gibraltar. Critically low Ne values across all populations raise concerns about the long-term adaptive potential of this species. According to the widely cited 50/500 conservation rule, populations with Ne < 50 are at risk of short-term inbreeding depression, while Ne < 500 limits adaptive capacity over evolutionary timescales. In Mauritania and Morocco, where Ne estimates were below 15, intensive fishing pressure combined with reduced gene flow may exacerbate the risk of genetic erosion and fitness loss. These findings highlight the urgent need for region-specific fishery management plans that integrate genomic indicators, alongside traditional demographic and ecological data. Continuous genetic monitoring will be essential to mitigate overexploitation under changing environmental pressures. Furthermore, the implementation of controlled aquaculture programs for O. vulgaris could help alleviate fishing pressure on natural populations, safeguard wild stocks, and promote a sustainable and economically viable production system.
References
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