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.

Variation in mesopelagic fish community composition and structure between Mediterranean and Atlantic waters around the Iberian Peninsula

“…Therefore, in the event of transport of the night surface fish from the Atlantic to the Mediterranean, they will encounter very different conditions when migrating to mesopelagic layers during the daytime, particularly higher salinity. Osmoregulation to deal with changes in salinity implies high metabolic expenses (Taylor, 1988;Peŕès and Mantelatto, 2020) and has been advocated as the reason for differences in populations of crustaceans from both sides of Gibraltar Strait (Ojeda et al, 2022). It is difficult to hypothesize the fate of larvae that could have been displaced from the Atlantic to the Alboran Sea.…”

Indicators to assess interannual variability in marine connectivity processes: a semi-theoretical approach

“…The temporal variability of the connectivity was influenced by the spatial distribution of zones along the NW Mediterranean margin. On the one hand, their geographical distribution in the area played a role in the variability of connectivity due to their exposure to the dominant water circulation, as often observed in other regions [34, 35]. The formation of multiple connections over time, defined by the Occurrence indicator P X>1 and its inverse (i.e., 1- P X>1 , when a source zone tends to connect a single time to another zone) highlighted how some zones were more exposed to the temporal variability of currents than other zones.…”

Genetic clines across the Atlantic-Mediterranean transition in the harbour crab Liocarcinus depurator

“…In Liocarcinus depurator populations, nuclear markers showed a clinal pattern from the Gibraltar strait to the Ibiza channel, and agree with the mtDNA gradient, also observed in previous studies 12 . However the gradient seems to uctuate due to the interannual variation in the strength of oceanographic fronts preventing the income of Atlantic waters towards the Mediterranean coast of the Iberian Peninsula 13,15 . Most gene ow in marine invertebrates is effectively carried out through larval dispersal 17 , and sea fronts are known to enhance retention of L. depurator larvae 18,19 .…”

“…Presently available evidence does not discern whether this mitochondrial DNA (mtDNA) cline is due to reduced larval dispersal through oceanographic barriers (i.e. neutral) or in uenced by natural selection, although interannual variation across these oceanographic fronts has been reported suggesting potential selection 13,15 . If the mtDNA genetic gradient is due to limited dispersal, genetic patterns at putatively neutral nuclear markers such as microsatellites should also be shaped by restricted gene ow.…”