Seascape Genetics and Distinct Intraspecific Diversification of the Decapod Nephrops norvegicus in the Adriatic Sea

Abstract

Norway lobster Nephrops norvegicus, a prized decapod crustacean species, is found at different depths across the East Atlantic Ocean and Mediterranean Sea. Despite management efforts, the stocks are globally characterised as overexploited. In the present study, the impact of biogeographical boundaries on the phylogeographical and demographic population status was investigated within the Adriatic Sea, addressing important genetic indices for decapod functional conservation management. Central Mediterranean, Adriatic Sea A total of 482 individuals of Nephrops divided into the 12 samples were collected across biogeographical range of the Adriatic Sea. Using the mtDNA D-loop and microsatellite markers, methods of phylogeography and seascape genetics were applied to infer offshore versus coastal population divergence, demography and structure. Significant findings include genetic differentiation between offshore and coastal samples, with higher diversity indices in open waters. The limited gene flow observed between these two areas emphasises the self-sustained nature of coastal populations. Recent demographic changes in coastal populations reflect geographical constraints, fishing pressures and fluctuations in self-recruitment success. Additionally, the study reveals historical biogeographic events shaping the Adriatic populations, with evidence suggesting lineage divergence during the upper Pleistocene and postglacial recolonisation from southern Adriatic refugia. The role of biogeographical conditions in shaping genetic structure and limited gene flow between inshore and offshore areas underscore the need for improved management strategies, emphasising the importance of marine protected areas in conserving coastal populations and maintaining overall genetic diversity of the Norway lobster in the Adriatic Sea. Genomic monitoring within current management practices is recommended.

Keywords: Central Mediterranean; Norway lobster; decapods; gene flow; management; population structure.

FIGURE 1

Adriatic Sea bathymetry with sampling locations of decapod Nephrops norvegicus where the first two letters of location abbreviations denote the sampling year (2018, 2019) and the second two letters denote the abbreviations for sampling sites (AL, Valona; AN, Ancona; BR, Brač channel; HV, Hvar channel; JA, Jabuka Pit; JK, South Adriatic Pit; MD, Mošćenička Draga; PA, Palagruža; VE, Velebit channel). More information about samples abbreviations and sampling years are provided in Table 1. The boundaries of the three Adriatic sub‐basins and FAO Geographical Subareas (GSAs) 17 and 18 are highlighted. Isobaths indicate depths of 250, 500, 1000 and 1200 m. The colours represent different depths. The figure was prepared in MATLAB 2014a (www.mathworks.com) and GIMP 2.8.16 (www.gimp.org) software.

FIGURE 2

Median‐joining network and geographic distribution of Nephrops norvegicus haplotypes in the Adriatic Sea. (A) Network of 95 D‐loop sequences. Coloured circles represent haplotypes found in sampled populations whose sizes are proportional to the number of individuals. Number of mutation steps is shown as hatch marks. (B) Geographic distributions of the mitochondrial haplotypes. The pie charts on the map display the haplotype frequencies found at each location. Samples codes are given in Table 1.

FIGURE 3

Genetic structure of N. norvegicus. (A) MCC tree from BEAST analysis based on the mtDNA dataset of 16 haplotypes. Values above the branches of the tree indicate (i) posterior probability values greater than 0.5 and (ii) estimates of divergence times (million years, Ma). H. americanus and H. gammarus were used as outgroups. The scale bar shows the time in millions of years from the present day. Lineages I and II are indicated by red and green lines, respectively. (B) A neighbour‐net tree constructed from the mtDNA dataset with 31 haplotypes using SplitsTree4, where the red and green circles represent haplotypes from lineage I and lineage II, respectively.

FIGURE 4

Bayesian clustering of Nephrops norvegicus inferred from STRUCTURE analysis of 10 loci dataset across 12 sample sites, where three clusters (K = 3) were assumed. For comparison, the analysis for K = 2 is also presented. See Table 1 for additional information on sample names.