This long-lived bivalve is endemic to the Mediterranean Sea, where it has a range from Spain to Turkey along the northern and southern coasts and coasts of the Mediterranean islands. Since 2016, a devastating and geographically widespread mass mortality event (MME) has impacted P. nobilis populations throughout the Mediterranean Sea. Previous to the MME, the species was widespread and locally abundant in some locations. The mortality is caused by a pathogen (H. pinnae) and the associated die-offs have rapidly spread from the western (starting in Spain) to the eastern Mediterranean Sea in less than three years, causing mortality rates of 80-100% of the individuals in most locations, including those with long-term P. nobilis monitoring programmes. There are a few populations (less than ten subpopulations) that are known to remain pathogen-free and these are geographically isolated and located in sites characterized by very specific environmental conditions (lagoons with little access to the sea and differing salinities).

The presence of the pathogen throughout the environment hinders potential population recoveries through recruitment, which opens a highly worrying scenario. Fan mussels strongly rely on the survival of adults for the maintenance of populations and the slow population dynamics and low recruitment could seriously hinder recoveries following catastrophic events. In the past, major threats were very localised and came from illegal fishing, habitat loss, boat anchoring, invasive species and most recently climate change. However none of these threats had led to the extremely widespread and rapid population declines in the species. The percentage of population size reduction over the last ten years is ≥80%, and the pathogen that has caused the MME is still present in the environment, with continuing declines expected. Therefore, this species is listed as Critically Endangered, mainly supported by criteria A2be and A4be.

Continuous monitoring of the species populations is mandatory, as well in those sites where the species has recently disappeared in order to detect potential recruitment in the future. This assessment should be re-evaluated in five years to include additional information and particularly related to the evolution of the disease and the potential occurrence of resistant individuals and recruitment.

This species is endemic to the Mediterranean Sea, where it is distributed throughout the basin, occurring from the mediolittoral zone from the low tide level (Russo 2012) to approximately 60 m depth (Butler et al. 1993). A list of the recorded localities in the Mediterranean Sea is available in the attached file.

The majority of populations are in sharp decline since the outbreak of a mass mortality event starting along the Spanish coasts in early autumn 2016 (Vázquez-Luis et al. 2017a). These events have been spread rapidly eastwards through the range (Carella et al. 2019, Katsanevakis et al. 2019, Panarese et al. 2019, Čižmek et al. in prep.). Within a short time-scale of 18 months, the mass mortality has been reported to extend to many populations throughout the known range from the western populations in 2016/7 (Spain, France, Italy, Tunisia) into the eastern mediterranean in 2018 (Malta, Greece, Cyprus and Turkey) (IUCN 2018). In 2019 the mass mortality extended throughout the Adriatic Sea with reports along the coast of Albania and Croatia (IUCN 2019, Čižmek et al. in prep). Initially the causes were unknown, but histological and molecular evidence pointed out to a parasite Haplosporidium pinnae as very likely responsible for the mass mortality (Catanese et al. 2018). In terms of the impact of these events, Basso et al. (2015b) noted that prior to the first mass mortality event the average population density of P. nobilis was 9.78 ± 2.25 ind/100 m² (± SE), with maxima up to 130 ind/100 m², but since the mass mortality events, most of the populations used to calculate those density values have now disappeared (IUCN 2019).

The parasite does not impact the other species Pinna rudis, that lives in the same habitats in the western Mediterranean (Catanese et al. 2018, D. Kersting and D. Moreno pers. comm. 2019).

This species is a long-lived bivalve which occurs in coastal areas, between c. 0.5 and c. 60 m depth. Posidonia oceanica meadows are described as the main habitat of P. nobilis (Vázquez-Luis et al. 2014a, Basso et al. 2015b, Deudero et al. 2015), although it is also found to inhabit Zostera marina and Cymodocea nodosa meadows (e.g. Russo 2012, Kersting and García-March 2017). Additionally, the species is also known to form extensive populations on bare sand (Richardson et al. 1999, Katsanevakis 2007, Rabaoui et al. 2007), rhodolith and detritic beds (Kersting and García-March 2017), pebbly bottoms (Zavodnik 1967, Richardson et al. 1999), among boulders (Kersting and García-March 2017) or even in sediment-filled crevices and spaces in shipwrecks (Jiménez et al. 2017). It is generally absent from muddy sediments and in areas of severe sediment disturbance (Butler et al. 1993, Katsanevakis 2005, Tsatiris et al. 2018).

Reproduction of the fan mussel has been reported to occur mainly between May and August (de Gaulejac 1995, Deudero et al. 2017, Kersting and García-March 2017). Even though earlier studies estimated a larval period of about 10 days (Butler et al. 1993, de Gaulejac 1995), recent research has suggested that larval stages could last at least one month (Deudero et al. 2017, Kersting and García-March 2017, Trigos et al. 2018). In the western Mediterranean, larval settlement concentrates mainly between July and October, peaking in August–September (Cabanellas-Reboredo et al. 2009, Kersting and García-March 2017).

Recruitment is a key component of the population dynamics of P. nobilis, and hence also key to the recovery potential of impacted populations. The use of larval collectors has proven to be a useful tool for assessing recruitment potential in P. nobilis, providing insights into larval supply and recruitment prior to the exposure to pressures such as predation or dislodgement which act on benthic individuals (de Gaulejac et al. 2003, Cabanellas-Reboredo et al. 2009, Kersting and García-March 2017, Wesselmann et al. 2018). During field surveys significant differences in recruitment in the larval collectors have been found in those populations occurring in well-preserved ecosystems (e.g., MPAs), where predators are abundant (Kersting and García-March 2017). In these environments where the threats are due to predatory pressures, survival is enhanced where the refuge size is 45 cm (Kersting and García-March 2017).

Pinna nobilis populations have been described as a meta-population with source-sink dynamics, i.e., with areas like the Ebro Delta (Spain) acting as larval source and others like Alicante (Spain) acting as sink populations (Wesselmann et al. 2018). Wesselmann et al. (2018) showed that P. nobilis populations have high genetic diversity and low inter-population differentiation and suggested that this discovery has strong consequences for the conservation of the species, as it allows to reject the highest concern hypothesis of small isolated populations. Connectivity patterns of the species are strongly influenced by oceanographic currents, highlighting the importance of ocean currents and pelagic larvae transport in shaping the population connectivity of P. nobilis (Wesselmann et al. 2018). Ongoing genetic studies will provide in short further fine scale genetic information on P. nobilis populations from Spain, France and Montenegro (N. Vicente pers. comm. 2018). Additionally, the genome sequencing of P. nobilis has been recently undertaken (Bunet et al. submitted, 2019).

The population dynamics of P. nobilis in populations occurring in well-conserved sites are described as parsimonious, characterized by low mortality and recruitment rates e.g. Columbretes Islands Marine Reserve (Kersting and García-March 2017) and Cabrera National Park (Vázquez-Luis et al. 2017b). At these sites, fan mussels strongly rely on the survival of adults for the maintenance of populations and the slow dynamics could seriously hinder recoveries in front of catastrophic events (Kersting and García-March 2017). Mortality rates are negatively correlated to the size of the P. nobilis individuals (Katsanevakis 2007, 2009).

Growth rates in this species are quite high during the first years of life (Hendriks et al. 2012, Kožul et al. 2012, Kersting and García-March 2017), slowing down as the fan mussels grow. Hendriks et al. (2012) estimated growth rates of 0.18 mm day^-1 (in the laboratory) and 0.28-0.32 mm day^-1 (in the field), Kersting and García-March (2017) reported growth rates in the field of 0.31-0.32 mm day^-1, during the first year of live and of 7.8-7.9 mm month^-1 during the first two years of life. Katsanevakis (2007) reported a peak in growth rates in Lake Vouliagmeni (Greece) at an age ~9 to 11 months followed by declining growth rates with age. In the same study, a seasonality in growth rates was observed, with the lowest growth rates during the cold season (November–mid-March) and during the temperature peak in August (with temperatures exceeding 29 ºC), and the highest growth rates during late spring-early summer (Katsanevakis 2007). Trigos et al. (2015), reported growth rates in the laboratory of 0.26 mm day^-1, and after nine months of rearing they reached an average size of 96.7±14 mm, thus gaining 70.4±10.6 mm, which is similar to that registered for the same species in natural conditions (Kožul et al. 2012).

Several growth equations have been proposed for the species, with a large variability in reported growth patterns among fan mussel populations (Šiletić and Peharda 2003, Rabaoui et al. 2007, Katsanevakis 2007, Hendriks et al. 2012, Kersting and García-March 2017). García-March et al. 2019 proposed three general growth models related to broad environmental conditions (exposed open-sea, sheltered open-sea and lagoons) to be used when no specific model could be calculated for a population.

Hydrodynamics seem to be a determinant factor in the ecology of P. nobilis, affecting population parameters by influencing food availability and reducing survival by dislodging and killing individuals due to wave action (Combelles et al. 1986, García-March et al. 2007b, Hendriks et al. 2011, Coppa et al. 2013).

The pen shell has been described as a successive and asynchronous hermaphrodite (i.e., male and female gametes are released sequentially in the same spawning period and the development of both sexes is not synchronous since one sex is always in a more advanced stage of development, de Gaulejac 1995, Deudero et al. 2017). However, Trigos et al. (2018) found that, in aquaria, a high percentage of the studied specimens released almost simultaneously male and female gametes. The same study suggested the possibility of internal fertilization of oocytes as a mechanism enhancing the survival of larvae. Recovery after disturbances by the reproduction of surviving individuals would be enhanced if the reproductive traits described by Trigos et al. (2018) (i.e., simultaneous gamete release and internal fertilization) are to be found as well in their natural habitat. However, Deudero et al. (2017) found that autofecundation is generally avoided, although it cannot be completely discounted.

Currently, the ongoing mass mortality event is the most worrying and widespread threat to P. nobilis throughout the Mediterranean Sea (Vázquez-Luis et al. 2017a, Katsanevakis et al. 2019, Panarese et al. 2019, Čižmek et al. in prep.). In early autumn 2016, a mass mortality event impacting P. nobilis was detected across a wide geographical area of the Spanish Mediterranean Sea (Vázquez-Luis et al. 2017a). Underwater visual censuses revealed high mortality rates reaching up to 100% in the central and southernmost coasts of the Iberian Peninsula including the Balearic Islands (Vázquez-Luis et al. 2017a). The first histological examinations already revealed the presence of a haplosporidan-like parasite within the digestive gland of affected pen shells (Darriba 2017, Vázquez-Luis et al. 2017a), a following study described the haplosporidan parasite as a new species, Haplosporidium pinnae (Catanese et al. 2018). After its first detection, the mass mortality has rapidly spread throughout the Mediterranean Sea (Vázquez-Luis et al. 2017a, IUCN 2018, Katsanevakis et al. 2019, Panarese et al. 2019, Cabanellas-Reboredo et al. in press, Čižmek et al. submitted).

The eastward spreading of the mortality was characterized by similar mortality rates linked to the presence of H. pinnae. Panarese et al. (2019) reported up to 100% mortality rates in three months in the Ionian Sea. While in the northern Aegean Sea (Lesvos Island) Katsanevakis et al. (2019) reported mortality rates of 93% in October 2018, however by July 2019 mortality rates were 100% everywhere along the coastline except in the inner Kalloni Gulf where mortality rates remained low (<25%) (Katsanevakis et al., unpubl. data). Mass mortalities have been also reported in many Turkish sites, reaching up to 100% of mortality rates in some locations (B. Öztürk and S. Tunçer pers. comm. 2019). Similar trends have been recorded in Cyprus, where only a few individuals (<3-5%) in several large populations around the island were affected in 2016/2017. However, inspections in 2018 confirmed that the mortality rate was ~100% at least until 40 m depth (C. Jiménez and L. Hadjioannou, unpubl. data). Around the Habibas Islands (Oran, Algeria), mortality rates of 82% were reported in December 2018 (M. Benabdi unpubl. data) and since February-May 2019 the mass mortality has extended to the Adriatic Sea, with mortalities reaching 100% in many sites (Cizmek et al. submitted, H. Cizmek pers. comm. 2019) and reports along the coast of Albania (IUCN 2019).

However, there are some locations within the affected zone and with characteristic environmental settings that have remained unaffected since the first outbreak of the mortality, i.e., Fangar Bay (Spain), Mar Menor (Spain), Rhône Delta (France), Etang de Thau (France), Diana and Urbino pools (Corsica), inner Kalloni Gulf (Greece) and in the Venice lagoon (Italy) (Catanese et al. 2018, García-March et al. submitted, S. Katsnevakis, L. Tunesi, N. Vicente pers. commm.). The reasons behind the resistance of these populations to the disease are unknown but could be closely related to the particular environmental conditions in these sites, e.g., low or high salinity conditions as occurs in the Fangar Bay or Mar Menor, respectively, and which could act as a barrier against the haplosporidan parasite (Cabanellas-Reboredo et al. 2019).

In relation to these sites, it must be taken into account that some of these locations are however subjected to strong anthropogenic pressures (García-March et al. submitted). In fact, the Mar Menor lagoon (Spain) collapsed in spring 2016 due to eutrophication and most organisms, including P. nobilis, died at depths below 1.5 m (Garcia-Ayllon 2018, Quintana et al. 2018). Because the mortality events arrived in the eastern Mediterranean Sea and to the Adriatic more recently, further unimpacted locations maybe identified in these areas in the near future. The evidence from Alfacs Bay (Ebro Delta, Spain) suggests that the lagoons may not be shielded completely, as whilst initially Alfacs Bay showed no declines, nearly two years later the bay is partially impacted by the mortality events (Prado 2019). Elsewhere P. nobilis populations are reported to be unstable in these coastal lagoons and estuaries sites, hence the long term prospects are not good (García-March et al. 2019).

The results obtained by Cabanellas-Reboredo et al. (2019) strongly suggest that the parasite has probably dispersed regionally by surface currents, and that the disease expression seems to be closely related by temperatures above 13.5 ºC and to a salinity range between 36.5-39.7 psu.

Although H. pinnae has been identified in most sites, Carella et al. (2019) reported a mycobacterial disease as a causal agent of the P. nobilis mass mortality recorded along the Tyrrhenian coastline (Italy) in 2017-2018. However, Fanelli et al. (2018) pointed out an Haplosporidium sp. as the causal agent in very close Italian localities. It must be considered that mixed infections of these two pathogens have also been reported (Čižmek et al. in prep.).

Currently, most of the information on the extent of the MME comes from depths <50 m, where the species is most abundant. The data from, at least one deep population (50-60 m) suggests that this species has also been impacted by the MME in Cyprus (C. Jiménez pers. comm. 2019), but in other regions the subpopulations between 50mbsl and 60mbsl have not been surveyed, although this represents a small % of the known population. Similarly data on decline rates for P. nobilis populations along the southern Mediterranean coasts is very scarce, however given the rapid spread of the parasite throughout most of the Mediterranean and the lack of any barriers to dispersal it is considered that equivalent MME's are highly likely in these regions.

There are a few surviving individuals in some populations impacted by the mass mortality event: Embiez Island (France, N. Vicente pers. comm. 2019), Columbretes Islands Marine Reserve (Spain, D. Kersting pers. comm. 2019) and the Balearic Islands (Spain, M. Vázquez-Luis pers. comm. 2019).

Before the mass mortality event, according to the review by Basso et al. (2015b) the main threats to the species were described as:

Climate change
Warming could affect processes potentially influenced by temperature, like reproduction and recruitment (Basso et al. 2015a, Kersting and García-March 2017). Warming may induce as well a decrease in the survival rate of P. nobilis juveniles (Basso et al. 2015a). However, Basso et al. (2015b) underline that there is still limited understanding of the thermal niche of P. nobilis as well of its temperature dependence in terms of key processes, such as growth, metabolism or survival across their life cycle. Although P. nobilis juveniles have shown resistance to hypoxia interacting with warming for short exposures (Basso et al. 2015c), the pen shell is a high oxygen consumer, which could cause quick depletion of oxygen for instance in shallow coastal lagoons (Basso et al. 2015b). P. nobilis is likely to be particularly vulnerable to ocean acidification, as it supports the fastest shell growth reported for any bivalve (Richardson et al. 2004). However, the experiments conducted so far on pen shell juveniles, did not show a negative influence of ocean acidification (pCO₂ levels expected for the end of the century) on the performance of juveniles of this species (Basso et al. 2015a).

Invasive species
The invasive alga Lophocladia lallemandii has been reported to alter food sources (Cabanellas-Reboredo et al. 2010), antioxidant responses (Box et al. 2009) and growth (Kersting and García-March 2017) in P. nobilis. The interaction of P. nobilis with invasive algae (i.e., Caulerpa cylindracea, L. lallemandii) has been reported in several locations (Cabanellas-Reboredo et al. 2010, Vázquez-Luis et al. 2014, Kersting and García-March 2017). Kersting and García-March (2017) alerted that special attention should be paid to this additional threat in front of the rapid spread experienced by these invasive algae in the Mediterranean Sea and the fact that the fan mussel is acting as a preferred substratum and could be therefore acting as a stepping stone (Vázquez-Luis et al.2014).

Attention must be paid to potential invasive predators, like the crab Callinectes sapidus, this voracious crustacean could become a threat to P. nobilis juveniles, especially in unimpacted sites (García-March et al. 2019).

Contaminants
Bioaccumulation of polycyclic aromatic hydrocarbon levels and an increase in the antioxidant enzymes have been detected in P. nobilis after an oil spill (Sureda et al. 2013). Also, heavy metals concentrations have been found to be high in some cases (e.g., Vázquez-Luis et al. 2016). While it has been found that organotin paints (tributyltin) induce lamination and chambering in P. nobilis shells (Vicente et al. 2003).

Habitat loss and boat anchoring
Posidonia oceanica meadows, the primary habitat of P. nobilis, have experienced widespread degradation across the Mediterranean Sea (Marbá et al. 2014). Pinna nobilis is highly vulnerable to direct anchor damage (Hendriks et al. 2013, Vázquez-Luis et al. 2015).

Food web alterations
Pinna nobilis is predated mainly by Octopus vulgaris, Hexaples trunculus and Sparus aurata (e.g., García-March 2007a, Addis et al. 2009, Zhakama-Sraieb et al. 2011, Kersting and García-March 2017) but also by sea turtles (Caretta caretta) in Cyprus, Lebanon and Turkey (M. Draman and M. Bariche pers. comm.). However, it is not clear if the turtles are preying on the epibionts on the P. nobilis shells and in consequence accidentally ingesting the bivalve. It has been hypothesized that predation pressure on P. nobilis may have increased through a cascade of food web impacts, where the dusky grouper, Epinephelus marginatus, the main predator of octopus in the Mediterranean Sea, has declined greatly due to overfishing (Basso et al. 2015b). However, it has been described as well that in protected sites with well-conserved fish populations, including E. marginatus but as well S. aurata among other sparids, P. nobilis juveniles are also subjected to high predatory pressure (Kersting and García-March 2017). In this case, predatory pressure is not exerted by O. vulgaris (which is predated by the abundant E. marginatus), but by sparids and gastropods.

Exploitation
Furthermore, in some countries, P. nobilis is still illegally fished and even served in restaurants. In Greece, poaching by both recreational and professional fishers is an important source of mortality. In Lake Vouliagmeni (Greece), fishing mortality was much higher than natural mortality (Katsanevakis 2007) and substantially reduced the life expectancy of the species, affecting its population dynamics and limiting the potential of the population for growth (Katsanevakis 2009). It was found that Pinna nobilis was served regularly or occasionally in 16.4% of Greek seafood restaurants (in a survey of 219 restaurants) (Katsanevakis et al. 2011). Fan mussels occasionally appear in Greek fish markets, and many recipes with fan mussels still appear in the media (Katsanevakis pers. obs. 2018).

Today this species is protected though the range in northern parts of the Mediterranean, and it was listed on the EU Species and Habitats Directive in part due to overexploitation of the populations as described below. Pinna nobilis has been a valued resource for human exploitation since early Egyptians and Roman times. The "silk-like" byssus threads produced by Pinna nobilis that attach the bivalve to the substrate have been used and traded for millennia to produce textiles of the highest value (Campi 2004, Maeder et al. 2004). Precious and expensive fabrics were obtained from the byssus threads which were thought to be used to decorate fabrics and clothes which were ostentatiously made, making them real status symbols valued by the most influential figures of the Babylonian, Assyrian, Phoenician, Jewish, Greek and finally Roman societies (Maeder et al. 2004). The byssus threads also had therapeutic properties well known to fishermen because, thanks to its powerful hemostatic properties, it was used for the dressing of wounds that fishermen frequently obtained with fishing tools. In the past the byssus threads were obtained fishing Pinna nobilis, but today very few people, have the skills to follow this millenary tradition, and are able to work the byssus, to produce this texture material, similar to silk, applying a method that does not harm the single specimen treated (Lavazza 2012, 2014).

As a food source, P. nobilis has been of common use in traditional cooking in some Mediterranean regions and despite its currently protected status, the pen shell is still being illegally traded in Greek seafood restaurants (Katsanevakis et al. 2011) and occasionally in Cyprus in the domestic/family environment of fishermen and divers (C. Jiménez unpubl. data 2018). Similarly, Öztürk et al. (2004) reported the use of the posterior adductor muscle as seafood and bait in Turkey.

In Algeria P. nobilis is also traded to be used as decoration (as floor lamp or to serve seafood in restaurants) (M. Benabdi pers. obs. 2018).

Prior to the mass mortality events across the Mediterranean, conservation actions for P. nobilis were mainly based on the protection measures applied in relation to its inclusion in lists of protected species under several national and international laws and directives (e.g., European Council Directive 92/43/EEC and ANNEX II of the SP/BD Protocol of the Barcelona Convention) and by protection of populations inside existing marine protected areas.

After the onset of the mass mortality event (MME) in Spain, the Spanish authorities upgraded the status of P. nobilis in Spanish waters from “Vulnerable” to “Critically Endangered” (Orden TEC/1078/2018 del Ministerio para la Transición Ecológica) and immediately started a conservation translocation to move 215 individuals from populations believed to be non-impacted into ex situ aquaria in different Spanish research institutes in response to the MME (see publication).

Follow the spread of the mortality, other countries have developed action monitoring programmes to map and assess the status of the P. nobilis populations along their coasts (e.g., France, Italy and Tunisia).

Conservation recommendations:

1. Research on the pathogens and vectors: A deep study of the origin, epizootic dynamics and the complete lifecycle of H. pinnae is needed to find ways to mitigate the disease, gain knowledge of its longevity, and to limit its spread. Furthermore, the development of new tools for the rapid detection of the parasite (as qPCR, López-Sanmartín et al. 2019) and the combination of PCR with in situ hybridisation techniques (in development) will allow us to detect the presence of H. pinnae in other organisms that could act as vectors and/or reservoirs. As well as studies based on the transcriptomes of P. nobilis (RNA-seq), which represents a very useful technological tool in the knowledge of the factors that regulate the expression of genes of ecological and resistance to pathogens interest. Further studies are also required to clarify the pathogenicity and potential virulence of the reported mycobacteriosis.

2. The natural recovery of impacted populations will depend exclusively on the ability for survival of resistant individuals and future recruitment ability. Therefore, information on this is urgently needed in order to evaluate the recovery potential of the species. Currently it is anticipated that the recovery will be low, as the number of individuals that survive are limited and the age to first reproduction is 3-4 years. It is crucial to assess the occurrence and density of resistant individuals in the impacted areas, the levels of presence and longevity of the parasite and possible presence of unknown populations in unexplored suitable deeper habitats (e.g., rhodoliths and detritic beds >50 m depth, Kersting and García-March 2017).

3. Larval settlement and the occurrence of resistant recruits should be monitored as well in order to assess recruitment potential in the impacted areas. Currently, there is enough information on how to assess larval settlement and recruitment in P. nobilis both in the field and by means of larval collectors (e.g., García-March et al. 2007a, Cabanellas-Reboredo et al. 2009, Kersting and García-March 2017). Additional information on the construction and installation of P. nobilis larval collectors can be found in Kersting and Hendriks (2019).

4. Increasing site and habitat protection: Given the other lesser threats, it is now imperative that we ensure the protection of any remaining survivors in areas impacted by the MME as well as ensuring protection for the species in the few locations harbouring unimpacted populations. Protection measures in these locations should include prohibition of anchoring and fishing with gears that may cause incidental mortality. Extreme care should be taken with any proposals for translocations of fan mussels from areas of high mortality (e.g., shallow waters affected by high wave intensity) to more protected sites, as whilst they may be secure increasing probabilities of survival (Katsanevakis 2016), each case must be reviewed in the context of unintentional spread of pathogens and possible stressing of the animals.

5. Use of ex situ breeding and conservation translocations: In the current emergency situation throughout the Mediterranean, all restoration actions and translocations of P. nobilis must be adequately assessed before putting into practice, in order not to worsen the critical status of the species. It is feasible to obtain juveniles from larval collectors and reinstate them in the field. Kersting and García-March (2017) developed the methodology to grow in situ juveniles that settled previously in larval collectors and to transplant them into naturally occurring populations (where they survived over 10 yr. prior to the MME). Additionally, efforts are underway to reproduce P. nobilis in aquaria. In this sense, artificial culture of P. nobilis has been assessed by Trigos et al. (2018), testing different environmental conditions to establish the bases for the optimal rearing of the species and provide a source of individuals for restoring populations. However, these authors stated that the closure of P. nobilis biological cycle in captivity appears to be rather difficult.

6. Conservation breeding: It is important to continue research on the biological cycle of P. nobilis in order to develop a selective breeding program in the event that any survivors are found within the affected areas. However, using survivors in captivity should only be done if cultured conditions are optimized in order to diminish mortality and the long-term captivity stress detected (Prado et al. 2019).

7. Monitoring systems: There is an urgent need to set up a regional monitoring programme ensuring information and observation exchange throughout the Mediterranean Sea. It is important to provide support in those locations lacking financial or technical support, especially those in the southern mediterranean, as well as in those locations where there are data gaps.