The results of our study confirm, over a more extended time scale, the strong trends of local retention of blue crab larvae, primarily in the western and central basins observed by Marchessaux et al. (2023). However, they also provide further insights into these observations in the eastern basins and shed light on their regularity over time, from 2010 to 2020. For instance, although the current study and those by Marchesseaux et al. (2023) do not share the same experimental setup (different dates and release periods), the results from both studies appear to show comparable trends, particularly with more precise hydrodynamic models and local zooms. Therefore, an interesting perspective to add to our study would be to consider longer spawning periods and use models with finer resolutions.
Our study shows that within 40 days of pelagic larval duration, larvae travel a minimum of 250 km, even in areas with strong retention due to ocean currents, and can travel distances approaching 1 000 km, but typically fall between the range of 200 km to 500 km. These long distances facilitate connectivity between various coastlines and areas conducive to the development of blue crab populations. This helps explain the formation, over the eleven years studied, of connectivity clusters or groups with regular larval exchange between source and sink areas. For example, clusters formed between the larval spawning zones along the Corsican, Sardinian, and Tunisian coastlines and the same potential recruitment zones in these areas are connected durably over time. Consequently, these connections create stable clusters where larvae are consistently exchanged between specific spawning and recruitment areas. This is evident in regions such as Tunisia, Sicily, Sardinia, Corsica, and the coastal areas of the Ligurian Sea and Tyrrhenian Sea.
The present study highlights the need of using two models with different resolutions, covering different extents and multiple basins, to investigate larval dispersal routes and connectivity, aligning with the recommendations of Swearer et al. (2019). While the large scale MedMFC model can be used for assessing larval travel distances, it lacks precision in coastal phenomena and requires coupling with a finer model like MARS3DMed, as demonstrated in the focus on the Northwestern Mediterranean.
The results of this study highlight, over a recent eleven-year period, trends in the dispersal routes of blue crab larvae, as well as the connectivity between the habitats where this species has been observed, which serve as favourable spawning and settlement habitats. Consequently, certain dispersal routes exhibit remarkable persistence over time, particularly in areas of local retention, leading to strong self-recruitment. These observations align with genetic studies on marine species with planktonic larvae, which have revealed limited oceanographic connectivity in Australia's coasts, even without major dispersal barriers, and found high levels of self-recruitment in a marine gastropod due to low-velocity nearshore currents retaining planktonic larvae near their birth sites (Teske et al., 2016). Thus, over a period of eleven years or more, coastal areas and the presence of islands, as is the case in various sub-basins of the Mediterranean Sea, could play a significant role in the larval dispersal dynamics of this invasive crab, and the results appear to align with on-ground findings. For instance, in the Northwestern Mediterranean, the Ebro Delta and the Gulf of Lion (Fuentes et al., 2019; Labrune et al., 2019) have witnessed and continue to experience a surge in blue crab abundances.
More recent studies such as the one conducted by Schubart et al. (2023), aimed at comparing the phylogeography of blue crab both in its native range and in the Mediterranean Sea. On one hand, this study highlighted that genetic composition of blue crab populations from the native American range and the entire Mediterranean should be included and used for the overall comparison and appropriate management measures should be taken in these two different areas. On the other hand, it appears that only a few founding individuals are responsible for the invasion of Spanish and Italian waters, supporting a dispersal theory. Thus, recent genetic studies in the Mediterranean reveal that invasive populations of blue crabs are characterized by remarkably low genetic diversity (except for Turkey, where reintroductions have occurred), and there is surprisingly little connectivity between established populations, observations that align with the strong tendencies toward local retention observed in the present modeling study. Studies on blue crab in its native range have shown notably high genetic diversity, suggesting that short-term gene flows are regional, whereas long-term gene flows are long-distance (McMillen-Jackson & Bert, 2004). Thus, the highlighted connectivity clusters from our larval dispersal model, such as the grouping between Corsica, Sardinia, the coastlines of the Ligurian and Tyrrhenian Seas, as well as between Tunisia and Sicily, will be important to analyse from a population genetics perspective in the near future to guide species management measures.
The methods and outputs from our study can inform the development of adaptive management tools, particularly in the context of rapidly advancing biological invasions, as exemplified in this study over a decade. Thus, the predictions from these models could help to better direct management measures (Mangano et al., 2020). For example, the production of productivity indicators for spawning zones and receptivity indicators for nursery zones, and their stability over time, could guide managers towards a more spatially focused approach to fisheries management. It is evident that larval dispersion routes and connectivity are key processes to study for the implementation of more effective management measures and protected areas (Hidalgo et al., 2017).
Regarding the case of Callinectes sapidus, this study confirms the essential role of islands in the dispersal and colonization dynamics of this invasive species, over a more extended timeframe and considering two spatial extents. This aligns with ecological island theories (MacArthur, 1963), which appear to function both as intermediate recruitment zones from year to year in colonization dynamics and as exchanges of genetic pools of larvae due to the proximity of suitable crab spawning habitats. This could explain the presence of populations that seem to persist over time and whose biomass increases in certain areas (Culurgioni et al., 2020; Mancinelli et al., 2021), as is also the case in areas with high local retention due to ocean currents, such as the Gulf of Lion, the Gulf of Gabès, etc. It appears that areas with both high local retention (or clusters of geographically close zones) are zones of high larval productivity, a conclusion supported by recent literature (Marchessaux et al., 2023). Furthermore, due to connections with adjacent, sometimes distant areas (such as islands), these larval productivity pools can supply sub-basins that, over an eleven-year period, seem to enable effective species circulation, potentially leading to rapid colonization and widespread distribution.
The trends observed in this study would benefit from further interpretation through comprehensive research on the biology and population dynamics of the blue crab, including growth, survival, reproduction, thermal tolerance conditions, and mortality (Reynes et al., 2021). The temperature of the seawater significantly impacts the life cycles, reproductive periods, and overall metabolism of planktonic organisms, especially those inhabiting surface waters. Consequently, even a slight rise in water temperatures can have significant repercussions on individuals and populations (Edmunds et al., 2005), notably affecting larval development and dispersal (O'Connor et al., 2007). Heightened global warming has been described in the Mediterranean region in recent decades, particularly impacting surface waters (Pastor et al., 2020), within various Mediterranean basins and sub-basins (Margirier et al., 2020). Simultaneously, research has shown that this warming of waters can facilitate biological invasions on a global scale (Stachowicz et al., 2002), and more locally in the Mediterranean region (Albano et al., 2021). Consequently, the expansion of tropical and subtropical species into the cooler waters of the Aegean, the Adriatic, and the western Mediterranean Seas indicates that the warming of Mediterranean waters due to climate change is also promoting the geographic expansion of non-indigenous species (Zenetos & Galanidi, 2020).
Climate change-related shifts, such as variations in the frequency and intensity of storms, can also disrupt the oceanographic characteristics of fronts that typically act as barriers to larval dispersal (Yamada et al., 2017). These powerful storms alter coastal circulation by changing dominant wind patterns, creating temporary opportunities for invasive larvae and sometimes influencing species distribution (Yamada et al., 2017). For instance, during storms, a change in wind direction can redirect post-larvae of blue crabs towards bays, thereby promoting their recruitment (Etherington & Eggleston, 2003). Additionally, strong cyclones usually expand nursery habitats for juveniles, leading to increased recruitment of this species (Eggleston et al., 2010). Consequently, a potential intensification of cyclones (Kang & Elsner, 2015) could enhance the abundance of species reliant on these extreme events for specific critical recruitment years. Hence, investigating blue crab larval dispersal and population connectivity in the context of climate change and integrating these studies with temporal assessments of surface temperatures and other climate-related environmental parameters appears to be a promising avenue for enhancing these models.