Among the most well-known, prioritized, and historically implemented types of connectivity, Ecological Corridors (ECs) represent clearly defined geographical spaces (biological or physical strips), regulated throughout time in order to preserve or restore effective ecological connectivity allowing movement of species and related ecological processes such as energy and gene fluxes and nutrient cycles (Good, 1998; Benson et al., 2007; Van der Windt and Swart, 2008; Palmeri et al., 2017; Hilty et al., 2020; Velázquez et al., 2022).

According to the EU Biodiversity Strategy for 2030, Member States are called to create ECs between protected sites (European Commission, 2021). In addition, IUCN Guidelines for Conserving Connectivity through Ecological Networks and Corridors is meant to guide global connectivity conservation efforts to design, govern, and manage for effective ecological connectivity (Hilty et al., 2020; Zhao et al., 2022). The EU Biodiversity Strategy asks for ECs to be included in the network of protected areas as a method of creating a cohesive transboundary network of protected areas (Hilty et al., 2020). Ecologically, ECs play an important role in the reconnecting fragmented ecosystems, regulating climate (Pataki et al., 2011; Gratani and Varone, 2013) and water (Cettner et al., 2014; Nickel et al., 2014), and providing food (Barthel and Isendahl, 2013), with the common aim of fostering, protecting and conserving biodiversity, (Hilty et al., 2006; Kattwinkel et al., 2011; Klaus, 2013; Garmendia et al., 2016; Fung et al., 2017). From a sociocultural standpoint, ECs connect valuable locations, offer working and leisure areas, and cultivate a sense of place associated with cultural heritage, in addition to being key components in the growth of tourism (McHarg, 1969; Beger et al., 2015; Beyer et al., 2018; Hilty et al., 2020).

In this context, remains still lacking the volume in MSP scientific research on ECs, and in particular, into the design of networks between MPAs and OECMs, supporting connectivity of marine systems (Balbar and Metaxas, 2019). MPAs and OECMs as elements for the conservation of the biodiversity may not be enough for addressing threats, such as, for instance, global climate change (Bates et al., 2019). For this reason, further environmental attributes, such us connectivity through ECs, might be considered to achieve biodiversity protection goals.

The aim of this paper is to review the scientific literature to investigate the current approaches and tools applied for investigating marine ECs between MPAs and OECMs, in an MSP perspective, and to identify gaps and opportunities to improve ecological connectivity between current areas to promote marine ecosystem conservation.

Research to investigate ecological connectivity in marine environments has focused mainly on models of larval dispersal by oceanic currents (Sanvicente-Añorve et al., 2014; Soria et al., 2014; Thomas Y. et al., 2014; Roberts et al., 2021), as a key driver of population connectivity (Treml et al., 2008; Andrello et al., 2015; Magris et al., 2016). Understanding dispersal trends between MPAs and OECMs is difficult since these phenomena are influenced by currents, season, time, and depth, all of which vary by area and species (Kinlan and Gaines, 2003; Basterretxea et al., 2012; Sayol et al., 2013). To deepen connectivity in seawaters by using larval dispersal, it is also important to use oceanographic data when addressing dispersion direction, since they constitute key variables for recruitment estimates (Schunter et al., 2011). Chemical tags (Hüssy et al., 2020), parentage analysis (Bode et al., 2019), or individual-based biophysical models are often used to evaluate larval, fragments or organisms’ dispersion distance in coastal marine ecosystems (Lett et al., 2020; Mari et al., 2020; Cecino and Treml, 2021) providing a spatial representativeness needed for conservation planning more easier than other techniques (Beger et al., 2022).

In the past ten years, also adults’ movement has received a novel attention. It is well understood that knowledge on adult-mediated population connection is crucial for understanding the complicated dynamics of connectivity (Frisk et al., 2014; Pittman et al., 2014; Bryan-Brown et al., 2017; Holyoak et al., 2020; Keeley et al., 2021; Jetz et al., 2022). Connectivity analysis combines information about, for example, adult habitat preferences, resistance to mobility within and across habitat types, influence of oceanic currents, density dependency, and interactions between species where species-specific data are available (Rocha et al., 2002; Gillanders et al., 2003; Treml et al., 2008; Baggio et al., 2011; Caldwell and Gergel, 2013; Pittman et al., 2014; Magris et al., 2016; Allan et al., 2021). The most interesting species for assessing connectedness are those with modest adult mobility distances since the movement of widely dispersed species may limit the efficacy of conservation interventions (e.g.: MPAs) (Moffitt et al., 2009; Green et al., 2014; Friesen et al., 2021). Even if species with low dispersion distances might be confined, for example, within single MPAs (Kaplan et al., 2009; Carr et al., 2017; Friesen et al., 2021).

Furthermore, it was found that narrow coastal channels effectively function as ECs for mobile marine species migration, and earlier research suggests that marine predators may seek these habitats for feeding (e.g.: Tursiops truncatus and Phoca vitulina) (Brown and Mate, 1983; Thompson et al., 1991; Suryan and Harvey, 1998; Hastie et al., 2004; Wilson et al., 2007; Hastie et al., 2016; Krost et al., 2018). This because narrow coastal channels are characterized by hydrographic features (e.g.: current direction and velocity, tide) have been demonstrated to alter nutrient availability and movement, plankton retention, and fish aggregation, as well as potentially give greater feeding chances for predators (Benjamins et al., 2015).

Technological advancements in microelectronics for telemetry, spatial analytical approaches, and marine remote sensing favor the possibility of filling substantial information gaps in marine animal migrations (Pittman et al., 2014). In all cases, tagging highly mobile marine organisms with acoustic transmitters is the most successful and widely used approach for studying their movements in time and space (Pittman and McAlpine, 2003; Heupel et al., 2006).

In contrast, it is unclear how connectivity patterns, discovered using benthic habitat as a proxy, match with the effective population connectivity of a single species. (Friesen et al., 2019). By combining landscape characteristics with knowledge about a species’ capacity for dispersal (Calabrese and Fagan, 2004) or likelihood of dispersal between patches (Watson et al., 2010), this potential connectivity provides a more thorough understanding of species-specific connectivity patterns than a habitat proxy approach. However, connectivity is an important component in marine habitats, particularly for benthic species (Carr et al., 2003). Indeed, several physical drivers such as ocean currents are involved in connectivity (Brock et al., 2012). Surface currents can help to detect main connections and ECs which should be considered in MSP (Muñoz et al., 2015) and MPA network design (Schill et al., 2015).

Another factor to consider is the tridimensional character of pelagic ecosystems, as well as the fact that most MSP techniques do not take this third vertical dimension into account (Muñoz et al., 2017). In fact, vertical connectivity can be attributed to physical (upwelling, downwelling, particle settling) or biological mechanisms (migration) (Robinson et al., 2010). For instance, the upwelling from the thermocline to the photic zone may promote the growth of phytoplankton, a component which is the basis of the pelagic food web (Sarhan et al., 2000; Granata et al., 2004). The deposition of particulate organic carbon connects surface primary production to benthic secondary production (Pfannekuche, 1993). Zooplankton diel vertical migration (DVM) contributes to the biological pump as a driver of carbon fluxes (Hernández-Leon et al., 2010; Ochoa et al., 2013; Ariza et al., 2015). Horizontal or vertical connectivity in marine ecosystems is essential for MSP as well as potential risk assessment of marine activities and discharges, however, there is currently a scarcity of knowledge on connection in these ecosystems (Sutton, 2013; Muñoz et al., 2015; Muñoz et al., 2017). Furthermore, in terms of connectivity direction, studies have primarily focused on horizontal or vertical one, but not both at the same time (Sutton, 2013; Schill et al., 2015; Muñoz et al., 2017). MSP and risk assessment necessitate the simultaneous consideration of all types of connectivity (physical, biological, horizontal, and vertical) (Muñoz et al., 2017).

There are, to the best of our knowledge, relatively few studies about the presence of EC in marine environments and among MPAs, while none including OECMs. Specifically, Ortiz-Lozano et al. (2013) studied EC across three coral reef systems in the southwest Gulf of Mexico. This study pinpointed that the found heterogeneity at a biogeographical and habitat level represents one of the main criteria to establish MPAs networks (Ortiz-Lozano et al., 2013). Pittman et al. (2014) provided direct evidence of ecological connectivity throughout an MPAs network in Caribbean reefs, considering movements of fish populations by using telemetric data. Subsequently, Friesen and coauthors (2019) incorporated connectivity within benthic habitats, as a proxy of adult movement due to the lack of information on population or individual mobility, into MPA planning in a case study conducted on Canada’s Pacific coast. In this paper, it was found a low interconnectedness among existing MPAs and the need to increase connectivity by prioritizing spaces that existing MPAs could use as steppingstones. In addition, Friesen et al. (2021) examined the ecological connectivity between MPAs considering two commercially important species in the Northern Shelf Bioregion in British Columbia (Canada). Lastly, even if not among MPAs, He et al. (2022) examined the importance of ECs to migratory species in the Yangtze estuary, the largest estuary in China, based on the variance in temporal and spatial density of three top fishery species to identify migratory ECs connecting optimum habitats that may be important in preserving population or community connectivity.

Common methodologies and tools for projecting and mapping ECs in terrestrial conservation planning have been established (Fenu and Pau, 2018; Liang et al., 2018; Bergès et al., 2020; Hilty et al., 2020). However, to our knowledge, in marine environments only two of these approaches were applied: least-cost theory and circuit theory (e.g.: McRae et al., 2008; McRae and Kavanagh, 2011; Pittman et al., 2014; Thomas C. J. et al., 2014; Friesen et al., 2021; Weeks, 2017). The two methods consider costs or resistance to migration based on species or habitat preferences (McRae et al., 2008; Correa Ayram et al., 2016). To run the lowest cost path, organisms need to be familiar with the landscape and its costs (Adriaensen et al., 2003; McClure et al., 2016). In this regard, least-cost path analysis calculates the single route between two regions with the lowest aggregate cost. On the other hand, circuit theory includes random walk theory similarly to random exploratory individual movements, assuming that organisms have not priori knowledge on the landscape (McRae et al., 2008; Dickson et al., 2019). Furthermore, the circuit theory technique examines the probability contributions of all feasible pathways in the landscape, allowing for the assessment of path redundancy and movement bottlenecks (Carroll et al., 2012; Dickson et al., 2019).

MSP is one of the most significant processes for determining spatial priorities for the conservation through ecological connectivity (Beger et al., 2010). It is well understood that connectivity varies in space and time, making measurement and modeling difficult for conservation planning (Fahrig, 2003). Nevertheless, demographic growth and climate change present new problems and possibilities for incorporating connectivity into ecosystem-supportive planning (Simberloff, 1992; European Commission, 2015; Muñoz et al., 2017). Climate change is also expected to have a considerable influence on marine biological connectivity patterns (Harley et al., 2006; Andrello et al., 2015; Bruno et al., 2018; Friesen et al., 2021). Data on current and future ocean conditions, and the distribution of the species can be integrated to deepen how hotspots of connectivity as well as MPA networks interconnectedness may change over time (Heyman and Wright, 2011; Hooker et al., 2011; Piquer-Rodríguez et al., 2012; Friesen et al., 2019). Therefore, to integrate all available information on the ecological connectivity into MPAs planning is crucial (Carr et al., 2017). However, as stated earlier, direct integration of connectivity into MPA network planning is rare (Magris et al., 2014). To enhance connectivity between existing MPAs, planners should prioritize areas that can serve as steppingstones between existing ones. Connectivity regards the spatial structure of a network of MPAs and the potential capacity of organisms to move within each MPA of the network and in other suitable habitats outside to maintain itself. In some cases, to evaluate connectivity, only habitat inside MPAs have been regarded (Allison et al., 1998; Jonsson et al., 2020).

Connectivity can also depend on the movement between habitat patches within an MPA during various life stages (Berkström et al., 2022). Furthermore, connectivity within MPAs is important for species with reduced dispersal ranges and living in fragmented habitats, while connectivity between MPAs and the surrounding area is important for dispersal and genetic exchange between populations for larger areas (Andersson et al., 2008). Therefore, if any climate refugia exist, locating and protecting them can assist to preserve sensitive ecosystems (Brito-Morales et al., 2018). In this context, future studies should examine shifts in connectivity patterns in relation to climate change (Magris et al., 2014; Andrello et al., 2015). To better understand connectivity patterns of species regarding possible climate change implications, studies should evaluate vulnerability and movements throughout all life stages (Moffitt et al., 2009; Álvarez-Romero et al., 2018; Friesen et al., 2021). For instance, it is known that climate change is predicted to modify larval dispersion patterns, whether owing to decreasing planktonic larval duration or larval sensitivity to changing environmental features (O'Connor et al., 2007).

To our knowledge, there are very few MSP research that include horizontal, vertical, physical, and biological connectivity (Muñoz et al., 2017). This might be due to the existence of barriers across disciplines (oceanography, biology, and management) as well as the difficulty of developing a sample strategy that allows to study physical and biological connectivity simultaneously (Muñoz et al., 2017). Typically, benthic habitat data may be the only information available in MSP procedures where data are scarce (Brooks et al., 2004). Because of limitations of time and resources, conservation planners frequently use limited data to guide their decisions (Ban et al., 2009; Hansen et al., 2011). The biology of ecologically significant species (spawning and nursery grounds, reproductive periods, migration patterns) should be deepened to determine potential connectivity and offer a firm foundation for reserve siting, planning, and zoning (Fraschetti et al., 2018).

Faced with a future of increasing susceptibility to human activities (Halpern et al., 2019), successful marine biodiversity conservation necessitates a strategic planning approach to identifying places where numerous anthropogenic hazards coexist with ecological components (Halpern et al., 2008; Crain et al., 2009; Micheli et al., 2013).

Investigating connectivity in a managed area is crucial for population management as well as possible pollutant spread derived from human activities (Muñoz et al., 2015). In this context, currents have a role in connecting coastal managed areas environmentally, but that result administratively disconnected because differently managed by various local governments or nations (Muñoz et al., 2015). In case good connectivity is found between areas resulting under the jurisdiction of different administrations and countries, MSP requires administrative cooperation in that area as well as connectivity estimation and identification of ECs and the time necessary to cross them (Muñoz et al., 2015).

Lastly, encouraging is that policymakers and practitioners increasingly recognize the importance of ecological connectivity, even there are significant challenges for integrating connectivity into policies, planning, and conservation (Lausche, 2011; Keeley et al., 2019).