Interannual fluctuations in connectivity among crab populations (Liocarcinus depurator) along the Atlantic-Mediterranean transition

Abstract

An interesting evolutionary question that still remains open is the connectivity between marine populations. Marine currents can favour the dispersal of larvae or adults, but they can also produce eddies and gyres generating oceanographic fronts, thus limiting gene flow. To address this subject, we selected the Atlantic-Mediterranean transition, where several fronts are located: Gibraltar Strait (GS), Almeria-Oran Front (AOF) and Ibiza Channel (IC). Seven populations of the marine crab Liocarcinus depurator (Cadiz, West and East Alboran, Alacant, Valencia, Ebro Delta and North Catalonia) located along this transition were analysed in six consecutive years (2014-2019) using a fragment of the COI (Cytochrome Oxidase subunit I) gene. All sequences (966) belonged to two well defined haplogroups: ATL (most abundant in Atlantic waters) and MED (predominant in Mediterranean waters). Following a geographic variation, the frequency of ATL decreased significantly from Cadiz to North Catalonia. However, this variation presented steps due to the effect of oceanographic restrictions/fronts. Significant effects were recorded for GS (2015, 2017, 2018 and 2019), AOF (all years except 2018) and IC (2016). The intensity and precise location of these fronts changed over time. Multivariate analyses distinguished three main population groups: Cadiz, Alboran Sea and the remaining Mediterranean populations. These findings could be relevant to properly define Marine Protected Areas and for conservation and fisheries policies.

Figure 1

Sampling populations of Liocarcinus depurator in the Atlantic-Mediterranean transition. Most populations were sampled in the period 2014–2019 (see text for details). Sampling areas appear in red colour. Location numbers and acronyms are: 1. Cadiz (CADI), 2. West Alboran Sea (WALB), 3. East Alboran Sea (EALB), 4. Alacant (ALAC), 5. Valencia (VALE), 6. Ebro Delta (DELT) and 7. North Catalonia (NCAT). Dashed lines identify the main currents and gyres in the studied area. Oceanographic fronts appear as solid red lines: Gibraltar Strait (GS), Almeria-Oran Front (AOF) and Ibiza Channel (IC). This figure was prepared with the help of Surfer (Golden software Inc.) (https://www.goldensoftware.com/products/surfer).

Figure 2

Linear regression model between the frequency of ATL haplogroup and geographic distance considering all years of the research. The model showed a significant decrease in ATL frequency when the geographic distance increases.

Figure 3

Percentages of ATL (blue) and MED (red) haplogroups in the populations of the Atlantic-Mediterranean transition, differences significant between populations and detection of the oceanographic front’s effects. For each year, the differences in haplogroup frequencies between neighbouring populations are presented (continuous yellow lines and dashed green lines stand for significant and a trend of differentiation, respectively). Each histogram is accompanied by a map showing the effect of the corresponding oceanographic front, that could be intense (continuous red lines, dashed red lines stand for significant and trend effect of the oceanographic front, respectively). (a) 2016, (b) 2017, (c) 2018 and (d) 2019. The population acronyms are the same as in Fig. 1. This figure was prepared with the help of Surfer (Golden software Inc.) (https://www.goldensoftware.com/products/surfer).

Figure 4

PCoA analysis using the genetic distances between all population pairs. The first and second axes explain 75.81 and 2.28% of the variability, respectively. Three main groups of population can be distinguished: Gulf of Cadiz, Alboran Sea and the remaining populations. Each population is indicated by an initial and a different colour and followed by the year (CADI: C, dark blue; WALB: W, light blue; EALB: E, light green; ALAC: A red; VALE: V, light brown; DELT: D, dark brown; NCAT: N, black).