ALBORAN: A reassessment of the diversity patterns in the transition zone between the Mediterranean and the Atlantic

The Mediterranean Sea is one of the most important hot-spots of marine biodiversity. In the recent Census of Marine Life (www.coml.org), it ranked fourth among 25 marine areas distributed worldwide, with an estimate of 16,848 species described, only behind the richest seas of Australia, Japan and China [1]. But despite being one of the most studied areas in the world, it is also among the ones where most discrepancies exist in geographical and taxonomic coverage. Whilst the northwestern margin is exhaustively studied, the North African Mediterranean coast is among the least explored areas worldwide [2]. This is particularly evident for smaller taxa such as Platyhelminthes, non decapod Crustacea and other minor invertebrate Phyla, where the number of species yet unknown is estimated to vary between 30% up to more than 50% [2]. But can this incomplete knowledge compromise our understanding of the patterns of diversity, and the underlying processes that drive them, in the Mediterranean? Apparently it can, and surprisingly this shows up in one of the most well studied regions of the basin, the Mediterranean-Atlantic transition zone (MATZ). It is generally accepted that the Strait of Gibraltar is no barrier to marine organisms. Instead, the whole Alboran basin (Fig. 1) is a transitional area between the Atlantic and the Mediterranean. Its homogeneous biota, with clear Atlantic affinities, granted it the status of an independent biogeographic unit within the Mediterranean [3]. At intra-specific level, a major genetic break has been identified eastwards, at the Almeria-Oran front (AOF), as shown by many phylogeographic studies of species that span across the area [4]. However, recent evidence shows that, within the MATZ, patterns of diversity and their causes may be more complex than once thought. Not only differences in species diversity between the two margins of the Alboran basin were found [5], but molecular data also suggest that for some organisms those margins have been historically disconnected [6]. Additionally, diversity in North African coasts might be underestimated, as shown by the numerous cryptic species recently discovered [6-8]. The biogeographic affinities of the Alboran island, which lies at the centre of the Alboran Sea, have also been questioned [9,10]. Furthermore, the presence of eastern genetic lineages in the Alboran island (similar to those from Algeria or eastern Spain), but not in the neighbouring margins of the Alboran Sea, challenges the effectiveness of the AOF as a barrier [6,11]. The emergence of these surprising results clearly stems from the effort put on the the study of small invertebrate taxa and, particularly, in the exploration of the North African coasts, two major flaws already pinpointed in recent revisions dealing with the Mediterranean biodiversity [1,2]. Many small invertebrates are brooders with direct developing embryos. Their intrinsic limitations to dispersal make them more prone to retain genetic signs of past events [12]. Even so, data are still fragmentary, and more exhaustive and systematic surveys are needed to fully understand the implications of the aforementioned results. Several questions remain unanswered. How different are the two margins of the Alboran Sea? What are the biogeographic affinities of the Alboran island? How effective is the AOF as a barrier? Are these results related to species’ modes of dispersal or not? The aim of the ALBORAN project is to answer those questions, thus contributing to the understanding of the factors that drive the distribution of diversity in the marine environment. The MATZ is exceptionally suited for this endeavour. It is a well defined area, limited westwards by the Strait of Gibraltar and eastwards by the AOF, with an island in the middle. It is clearly a transition zone between the less diversified biota of the Atlantic and the richer Mediterranean. More importantly, contrary to other marine regions, there is a good knowledge of the prevailing oceanographic patterns in the area [13] and models exist that simulate these currents under different climatic scenarios [14]. To tackle the above mentioned questions, we propose a multi-species phylogeographic approach coupled with particle-dispersion modelling. Phylogeography has proved useful to reveal speciation processes related to adaptive radiations and geological events [8] and may be used as a first step to describe patterns of population history in understudied biotas, particularly in areas with inadequate sampling and taxonomy [15]. Particle-dispersion models can be used to assess levels of contemporary connectivity for passive dispersers and their relation to species’ modes of dispersal. We think that the combination of historical and contemporary data is the key to a more comprehensive understanding of the processes driving species distribution in the MATZ, and will certainly serve as a baseline for studies in other unexplored marine regions.