Fan Mussel Pinna Nobilis Research Articles

Insights on Pinna nobilis population genetic structure in the Aegean and Ionian Sea.

The fan mussel Pinna nobilis Linnaeus, 1758 is an endemic species of the Mediterranean Sea, protected by international agreements. It is one of the largest bivalves in the world, playing an important role in the benthic communities; yet it has been recently characterized as Critically Endangered by the IUCN, due to mass mortality events. In this context, the assessment of the genetic variation of the remaining P. nobilis populations and the evaluation of connectivity among them are crucial elements for the conservation of the species. For this purpose, samples were collected from six regions of the Eastern Mediterranean Sea; the Islands of Karpathos, Lesvos and Crete; the Chalkidiki and Attica Peninsulas; and the Amvrakikos Gulf. Sampling was performed either by collecting tissue from the individuals or by using a non-invasive method, i.e., by scraping the inside of their shells aiming to collect their mucus and thus avoid stress induction to them. Conventional molecular techniques with the use of the COI and 16S rRNA mitochondrial markers were selected for the depiction of the intra-population genetic variability. The analyses included 104 samples from the present study and publicly available sequences of individuals across the whole Mediterranean Sea. The results of this work (a) suggest the use of eDNA as an efficient sampling method for protected bivalves and (b) shed light to the genetic structure of P. nobilis population in the Eastern Mediterranean; this latter knowledge might prove to be fundamental for the species conservation and hence the ecosystem resilience. The haplotype analyses reinforced the evidence that there is a certain degree of connectivity among the distinct regions of the Mediterranean; yet there is evidence of population distinction within the basin, namely between the Western and the Eastern basins. The combination of both genetic markers in the same analysis along with the inclusion of a large number of individuals produced more robust results, revealing a group of haplotypes being present only in the Eastern Mediterranean and providing insights for the species' most suitable conservation management.

Reproduction of the endangered fan mussel Pinna nobilis under natural and captivity conditions

Abstract A mass mortality event that started in 2016 has put the fan mussel Pinna nobilis close to the brink of extinction, leading to the species being classified as Critically Endangered in the Red List of the International Union for Conservation of Nature. Under current circumstances, with the persistence of the disease caused by Haplosporidium pinnae in the open sea, isolated populations in coastal lagoons and estuarine bays appear to be the main chance for the survival of the species. The low resilience of those populations highlights the importance of implementing additional measures to avoid the extinction of the fan mussel. One of these measures relies on completing its reproductive cycle in captivity. The reproductive window of the remaining fan mussel population in Alfacs Bay was explored to obtain gametes and to understand possible limitations in the reproductive activity in wild populations. For this purpose, during the reproductive season, several groups of individuals were relocated for short periods (~2 weeks) into the vivarium to obtain gametes under controlled conditions. Simultaneously, controlled temperature treatments and adapted food dosage for conditioning adult fan mussels out of their reproductive season in the wild were conducted in a recirculating aquaculture system. The reproduction of natural populations was mostly restricted to May, featuring temperatures around 20°C. A low success of spawning was obtained in the vivarium, with a 0% success in 11 out of 20 of the trials (six to nine individuals each) and an overall success of 16% (25 out of 156 individuals). A 100% success (14 out of 14) was obtained with individuals conditioned for ~2 months in the recirculating aquaculture system. A second conditioning of six of those individuals was attempted after a ~3‐month period, with a 33.3% success of spawning.

In situ mapping of biomineral skeletal proteins by molecular recognition imaging with antibody-functionalized AFM tips.

Spatial localizing of skeletal proteins in biogenic minerals remains a challenge in biomineralization research. To address this goal, we developed a novel in situ mapping technique based on molecular recognition measurements via atomic force microscopy (AFM), which requires three steps: (1) the development and purification of a polyclonal antibody elicited against the target protein, (2) its covalent coupling to a silicon nitride AFM tip ('functionalization'), and (3) scanning of an appropriately prepared biomineral surface. We applied this approach to a soluble shell protein - accripin11 - recently identified as a major component of the calcitic prisms of the fan mussel Pinna nobilis [1]. Multiple tests reveal that accripin11 is evenly distributed at the surface of the prisms and also present in the organic sheaths surrounding the calcitic prisms, indicating that this protein is both intra- and inter-crystalline. We observed that the adhesion force in transverse sections is about twice higher than in longitudinal sections, suggesting that accripin11 may exhibit preferred orientation in the biomineral. To our knowledge, this is the first time that a protein is localized by molecular recognition atomic force microscopy with antibody-functionalized tips in a biogenic mineral. The 'pros' and 'cons' of this methodology are discussed in comparison with more 'classical' approaches like immunogold. This technique, which leaves the surface to analyze clean, might prove useful for clinical tests on non-pathological (bone, teeth) or pathological (kidney stone) biomineralizations. Studies using implants with protein-doped calcium phosphate coating can also benefit from this technology. STATEMENT OF SIGNIFICANCE: Our paper deals with an unconventional technical approach for localizing proteins that are occluded in biominerals. This technique relies on the use of molecular recognition atomic force microscopy with antibody-functionalized tips. Although such approach has been employed in other system, this is the very first time that it is developed for biominerals. In comparison to more classical approaches (such as immunogold), AFM microscopy with antibody-functionalized tips allows higher magnification and keeps the scanned surface clean for other biophysical characterizations. Our method has a general scope as it can be applied in human health, for non-pathological (bone, teeth) and pathological (kidney stone) biomineralizations as well as for bone implants coated with protein-doped calcium phosphate.

The characterization of toll-like receptor repertoire in Pinna nobilis after mass mortality events suggests adaptive introgression.

The fan mussel Pinna nobilis is currently on the brink of extinction due to a multifactorial disease mainly caused to the highly pathogenic parasite Haplosporidium pinnae, meaning that the selection pressure outweighs the adaptive potential of the species. Hopefully, rare individuals have been observed somehow resistant to the parasite, stretching the need to identify the traits underlying this better fitness. Among the candidate to explore at first intention are fast-evolving immune genes, of which toll-like receptor (TLR). In this study, we examined the genetic diversity at 14 TLR loci across P. nobilis, Pinna rudis and P. nobilis × P. rudis hybrid genomes, collected at four physically distant regions, that were found to be either resistant or sensitive to the parasite H. pinnae. We report a high genetic diversity, mainly observed at cell surface TLRs compared with that of endosomal TLRs. However, the endosomal TLR-7 exhibited unexpected level of diversity and haplotype phylogeny. The lack of population structure, associated with a high genetic diversity and elevated dN/dS ratio, was interpreted as balancing selection, though both directional and purifying selection were detected. Interestingly, roughly 40% of the P. nobilis identified as resistant to H. pinnae were introgressed with P. rudis TLR. Specifically, they all carried a TLR-7 of P. rudis origin, whereas sensitive P. nobilis were not introgressed, at least at TLR loci. Small contributions of TLR-6 and TLR-4 single-nucleotide polymorphisms to the clustering of resistant and susceptible individuals could be detected, but their specific role in resistance remains highly speculative. This study provides new information on the diversity of TLR genes within the P. nobilis species after MME and additional insights into adaptation to H. pinnae that should contribute to the conservation of this Mediterranean endemic species.

Physiological responses of the fan mussel Pinna nobilis to temperature: ecological and captivity implications

The fan mussel Pinna nobilis is experiencing a mass mortality event throughout the Mediterranean Sea. The survival of the remaining isolated populations and the maintenance and reproduction of individuals in captivity could determine the future of the species. This paper examines the clearance rate (CR) and oxygen consumption (VO2) of fan mussel individuals measured under 5 different temperatures (8.5, 14, 18, 23 and 28ºC). The measurements of both physiological variables revealed a threshold limit of thermal tolerance at both extreme temperatures, indicating the critical situation of the remaining populations, which are located in coastal lagoons and shallow bays where these extreme temperatures usually occur. Besides, the high clearance rates observed highlight the significant ecosystem service provided by the species in terms of water filtration, especially in confined waters with low renewal rates. Routine control of the clearance rate as an early warning system is proposed for detecting ailing individuals. Such a system could also be used to improve captivity conditions.

Molecular characterization of accripin11, a soluble shell protein with an acidic C-terminus, identified in the prismatic layer of the Mediterranean fan mussel Pinna nobilis (Bivalvia, Pteriomorphia).

We have identified a novel shell protein, accripin11, as a major soluble component of the calcitic prisms of the fan mussel Pinna nobilis. Initially retrieved from a cDNA library, its full sequence is confirmed here by transcriptomic and proteomic approaches. The sequence of the mature protein is 103 residues with a theoretical molecular weight of 11 kDa and is moderately acidic (pI 6.74) except for its C-terminus which is highly enriched in aspartic acid. The protein exhibits a peculiar cysteine pattern in its central domain. The full sequence shares similarity with six other uncharacterized molluscan shell proteins from the orders Ostreida, Pteriida and Mytilida, all of which are pteriomorphids and produce a phylogenetically restricted pattern of nacro-prismatic shell microstructures. This suggests that accripin11 is a member of a family of clade-specific shell proteins. A 3D model of accripin11 was predicted with AlphaFold2, indicating that it possesses three short alpha helices and a disordered C-terminus. Recombinant accripin11 was tested in vitro for its ability to influence the crystallization of CaCO3 , while a polyclonal antibody was able to locate accripin11 to prismatic extracts, particularly in the acetic acid-soluble matrix. The putative functions of accripin11 are further discussed in relation to shell biomineralization.

Wide-Geographic and Long-Term Analysis of the Role of Pathogens in the Decline of Pinna nobilis to Critically Endangered Species

A mass mortality event (MME) affecting the fan mussel Pinna nobilis was first detected in Spain in autumn 2016 and spread north- and eastward through the Mediterranean Sea. Various pathogens have been blamed for contributing to the MME, with emphasis in Haplosporidium pinnae, Mycobacterium sp. and Vibrio spp. In this study, samples from 762 fan mussels (necropsies from 263 individuals, mantle biopsies from 499) of various health conditions, with wide geographic and age range, taken before and during the MME spread from various environments along Mediterranean Sea, were used to assess the role of pathogens in the MME. The number of samples processed by both histological and molecular methods was 83. The most important factor playing a main role on the onset of the mass mortality of P. nobilis throughout the Mediterranean Sea was the infection by H. pinnae. It was the only non-detected pathogen before the MME while, during MME spreading, its prevalence was higher in sick and dead individuals than in asymptomatic ones, in MME-affected areas than in non-affected sites, and it was not associated with host size, infecting both juveniles and adults. Conversely, infection with mycobacteria was independent from the period (before or during MME), from the affection of the area by MME and from the host health condition, and it was associated with host size. Gram (-) bacteria neither appeared associated with MME.

New insights about Haplosporidium pinnae and the pen shell Pinna nobilis mass mortality events

Since early autumn 2016, Mass Mortality Events (MME) have drastically impacted the population of the fan mussel Pinna nobilis in the Mediterranean Sea. Haplosporidium pinnae, a newly described Haplosporidian species, has been considered the causative agent of the mortality outbreaks in association to opportunistic bacterial pathogens. In the present study, we first reported a cytological description of H. pinnae in moribund specimens of P. nobilis which were collected in the Gulf of Taranto (Ionian Sea, Italy) during summer 2018. Different life-cycle stages of the parasite, including uni- and binucleate cells, small plasmodia, big multinucleate plasmodia and sporocysts with spores, were detected in all the examined animals and most of the parasite cells were present in gills, mantle and digestive gland, while the spores were found only in the latter organ. Histology and molecular biology were also performed, confirming the nature of the infectious agent, as already reported in the area. Additionally, molecular study revealed the presence of bacteria from the Mycobacterium ulcerans - M. marinum complex but no evident macroscopical or microscopical lesions, just as no bacteria referred to Mycobacterium were observed by histology. In conclusion, the present study aimed to provide further contributions to the understanding of the mortality of P. nobilis, pointing to the role of the cytological method of investigation both for diagnostic and epidemiological purposes, and discussing the current epidemic situation in the Adriatic sea.

Unexpected residual habitats raise hope for the survival of the fan mussel Pinna nobilis along the Occitan coast (Northwest Mediterranean Sea)

In 2019, the status of the Mediterranean fan mussel Pinna nobilis was elevated to ‘Critically Endangered’ on the IUCN Red List, in response to the pandemic caused by the parasite Haplosporidium pinnae. Identifying refuge habitats, free from parasites, is critical to the survival of the mussel. The distribution of P. nobilis was investigated along the Occitan coast (Northwest Mediterranean Sea) because of the presence of a unique lagoonal system that may provide potential refuges. Interviews with users and managers were conducted to identify target zones where the species was sighted. In situ surveys were carried out to define the main aggregations of fan mussels and characterize the habitat. Line transects were deployed to count and measure individuals to estimate density, abundance, and size distribution. Population densities were variable, ranging from 0.6 ± 0.2 (SE) to 70.8 ± 7.6 ind. 100 m-2, representing one of the highest densities reported in the Mediterranean Sea. The total abundance of individuals across the coast was extrapolated to 163000, with 87% located in Thau and Salses-Leucate, highlighting these lagoons as essential for the survival of the species. This study also revealed the diversity of habitats colonized by P. nobilis. In the context of the pandemic, only the lagoon populations remain unaffected and provide natural refuges that have disappeared from all open-water coastal areas. However, the conditions in these lagoons could become unfavorable, leading to the collapse of the last P. nobilis populations. We therefore propose that Thau and Salses-Leucate lagoons, which harbor the largest remaining populations of P. nobilis, should be declared as conservation priorities.

Investigating population dynamics from parentage analysis in the highly endangered fan mussel Pinna nobilis.

Understanding dispersal patterns is a major focus for conservation biology as it influences local survival and resilience in case of local disturbance, particularly for sessile species. Dispersal can be assessed through parentage analyses by estimating family structure and self‐recruitment. This study documents the family structure of a pelagic spawner, Pinna nobilis, which is facing a major crisis that threatens its survival as most of its populations have been decimated by a parasite, Haplosporidium pinnae. In this context, we focused on a single population (Peyrefite, Banyuls‐sur‐mer, France) where 640 individuals were sampled in 2011, 2015, and 2018 and genotyped for 22 microsatellite markers. Genetic diversity was high and homogeneous among years, with mean allele numbers ranging between 13.6 and 14.8 and observed heterozygosities (H o) between 0.7121 and 0.7331. Low, but significant, genetic differentiations were found between 2011–2015 and 2015–2018. A parentage analysis described 11 clusters, including one prevailing, and revealed that 46.9% of individuals were involved in half‐sib relationships, even between years, suggesting that source populations were recurrent year after year. There were few individuals resampled between years (30 in 2015 and 14 in 2018), indicating a rapid turnover. Considering the large number of half‐sib relationships but the low number of relations per individual, we conclude that P. nobilis exhibit homogeneous reproductive success. Self‐recruitment was not detected, making this population highly vulnerable as replenishment only relies on connectivity from neighboring populations. In the context of the pandemic caused by H. pinnae, these results will have to be considered when choosing a location to reintroduce individuals in potential future rescue plans.

Living under threat: Will one of the last Pinna nobilis populations be able to survive?

Abstract The critically endangered fan mussel Pinna nobilis, endemic to the Mediterranean Sea, faces a drastic population decline all over the Mediterranean Sea owing to an infectious disease. Coastal lagoons represent one of this species’ last reservoirs; thus, knowledge of the population conservation status in these ecosystems is essential. An extensive monitoring programme was conducted during 2014, 2017 and 2019 in the Mar Menor lagoon, a highly eutrophicated lagoon from Spain. On average, 32,535 m2 have been surveyed each year to determine population densities, distributions and size structures. The cumulative impacts produced by intensive agriculture activity around the lagoon, together with a catastrophic meteorological phenomenon that occurred in 2019, have triggered a significant decline in fan mussel population densities and in population mean shell sizes over the years. Additionally, some fan mussels were infected by the protozoan Haplosporidium pinnae. Therefore, the current P. nobilis population from the Mar Menor is mainly composed of young individuals in specific areas that are facing an extreme situation. Urgent management and conservation actions focused on restoring the ecosystem and protecting P. nobilis individuals are urgently needed, together with reinforcement projects to increase the species’ stock in one of its last reservoirs.

Noble fan-shell, Pinna nobilis, in Lake Faro (Sicily, Italy): Ineluctable decline or extreme opportunity?

The largest bivalve in the Mediterranean Sea, the fan mussel Pinna nobilis, is at risk of extinction due to mass mortality events (MMEs) caused by the spread of pathogens, Haplosporidium pinnae in particularly. In spite of being a protected area, Lake Faro (northeast Sicily, Italy) suffers from high anthropogenic pressures that affect the P. nobilis population that inhabits the lake and the two canals that connect it to the sea. In the present study, the population's long-term changes have been monitored in three distinct periods: before the MME (2010), at the beginning of the epidemic spread along the Italian coasts (2018), and after the MME (2020). The survey, carried out by visual census, showed that, relative to 2010, the population of P. nobilis halved in 2018 and disappeared from the canals in 2020; while in the lake, living specimens were only 27.69 % of the total at this time, without recruits. The disappearance of P. nobilis, allowed rapid colonization by the congeneric Pinna rudis and the invasive oyster Pinctada imbricata radiata, which had never been recorded in the Lake Faro system before 2020.

Marine restoration in the Mediterranean: red coral and fan mussel discourses, uncertainty and reaching restoration targets

Although there is a wide range of legislative support for marine environmental policy and management, existing and emerging pressures continue to threaten biodiversity and ecosystems. Recent ambitious policies have represented a paradigm shift, moving from the traditional hands-off preservation towards more active forms of intervention through ecosystem restoration, with explicit targets for restoration of degraded ecosystems (2020/2030 EU Biodiversity Strategy). This work analyses the discourses and uncertainties in the cases of the fan mussel Pinna nobilis and the red coral Corallium rubrum in the Mediterranean, through a literature review and expert interviews. The major discourses are placed within a restoration framework based on the degree of intervention and the underlying motivations. The major discourse for both of these emblematic threatened species is ecocentric-based ‘Putting Nature First’, set around passive restoration (low-level intervention with protective measures to reduce stressor impact). Emerging discourses resulting from coastal developments and mass mortality events for the mussel and overfishing for corals, has required active restoration, with on-going small-scale actions. Both species highlight comparable shortcomings and constraints concerning restoration, particularly facing unknown baselines (what to restore to) and multiple uncertainties (population status, diseases, co-stressors, and long-term success of restoration). The top-down policy restoration targets lack detail towards implementation and most restoration efforts to-date are local-scale through bottom-up personal motivations, that have not been meeting policy targets. Lessons learned include the need to scale up passive measures and to couple with increased levels of active intervention with clear ideas on what, why, how and where to restore.

Mass Mortality Report of Critically Endangered Fan Mussel (Pinna nobilis, Linnaeus 1758) from Cunda Island, Ayvalık (Aegean Sea, Turkey)

Fan mussel Pinna nobilis (Linnaeus, 1758) species has been critically endangered in the Mediterranean Sea because of Haplosporidium pinnae. Thus, the purpose of this study was to determine the current status of the fan mussel populations at three station (Patrica Beach, Biyikli Beach and Cataltepe Beach) in Cunda Island Ayvalik, Aegean Sea. Underwater observation carried out on July 2020. Mass mortality (100%) was observed at all station during the study. Length of fan mussel was ranged between 39.8 cm and 79.8 cm and no individuals were in small sized.

Population trends of the fan mussel Pinna nobilis from Portofino MPA (Ligurian Sea, Western Mediterranean Sea) before and after a mass mortality event and a catastrophic storm

Two Pinna nobilis populations thriving inside the borders of the Portofino Marine Protected Area (MPA) (Ligurian Sea, western Mediterranean Sea) were monitored before (2012) and after (September 2018) a dire mass mortality event that, since September 2016, spread through the whole Mediterranean Sea. In Portofino MPA, recorded mortality rates reached values of 91.29% and 43.94% in the two populations. The presence of a Haplosporidium protozoan parasite, considered to be the main cause of the mortality episodes, was confirmed from histological evidence: sporocysts and plasmodia were observed in all the tubules of the digestive glands of the collected specimens. Moreover, a catastrophic storm hit the Ligurian coasts at the end of October 2018, causing considerable damages both below and above the surface; a new survey conducted in November 2018 showed the complete annihilation of the two studied populations, as a probable combination of the continued parasite infections and the mechanical impacts caused by the storm. Finally, in June 2020 the sites were monitored again looking for traces of recovery, but no new specimens were recorded, indicating that P. nobilis became virtually absent from the MPA.

Reduced Antioxidant Response of the Fan Mussel Pinna nobilis Related to the Presence of Haplosporidium pinnae.

The endemic fan mussel (Pinna nobilis) in the Mediterranean Sea is at high risk of disappearance due to massive mortality events. The aim of the study was to evaluate the antioxidant response of P. nobilis collected in the Balearic Islands (Western Mediterranean) before and after the mass mortality event. Individuals collected before (between 2011 and 2012) and after (between 2016 and 2017) the event were analyzed by histological, molecular, and biochemical methods to compare pathogenic loads and biochemical responses. All the individuals collected during 2016–2017 presented symptoms of the disease and were positive for Haplosporidium pinnae, while acid-fast bacteria or/and Gram-negative bacteria were detected in some individuals of both sampling periods. The activities of the antioxidant enzymes catalase and superoxide dismutase in the gills were significantly lower in P. nobilis affected with the parasite compared to those in the asymptomatic ones, while levels of malondialdehyde, as an indicator of lipid peroxidation, were higher in infected individuals. When analyzing the differential effects of H. pinnae and Mycobacterium sp. on P. nobilis, it was observed that significant effects on biomarkers were only observed in the presence of H. pinnae. Co-infection of P. nobilis by H. pinnae with other pathogens such as Mycobacterium sp. constitutes a serious problem due to its high mortality rate in the Balearic Island waters. This concerning situation for P. nobilis is favored by a reduction in antioxidant defenses related to H. pinnae infection that induces oxidative stress and cell damage.

Pinna nobilis in the Greek seas (NE Mediterranean): on the brink of extinction?

The Mediterranean endemic fan mussel Pinna nobilis is suffering an ongoing basin-scale mass mortality event (MME) since 2016. As most Mediterranean populations have collapsed, the species has been declared as Critically Endangered in the IUCN Red List of threatened species. In an effort to track the progress of the MME and provide updated information on the status of the species in the Greek seas, we compiled data collected through dedicated surveys and opportunistic assessments during 2019 and 2020. A total of 14589 fan mussel individuals, of which 81.1% dead, were recorded in 258 site surveys. Of the remaining 2762 live individuals, 256 were juveniles. Two marine areas which still sustain living populations were identified, namely Kalloni Gulf (Lesvos Island), and Laganas Bay (Zakynthos Island). The inner part of Kalloni Gulf appears to maintain the largest surviving population of the species in the eastern Mediterranean, with an abundance estimate of 684000 individuals (95% confidence interval: 322000-1453000). Solitary, potentially resistant, scattered individuals were recorded in several sites. Other previously abundant populations that had been assessed in the past, specifically those of Lake Vouliagmeni (Korinthiakos Gulf), Souda Bay (Crete) and Gera Gulf (Lesvos Island), and which collectively summed up to ~350,000 individuals, have now been wiped out. Our results document the collapse of most P. nobilis populations throughout the Greek seas. The MME has substantially progressed between early 2019 and mid-2020, as indicated by the increase of mortality at sites consecutively monitored multiple times. This work highlights the urgent need for continuous monitoring of surviving populations and calls for the immediate implementation of an effective protection and management strategy that will ensure the persistence of surviving individuals and the production of resistant offspring.

Development of 26 highly polymorphic microsatellite markers for the highly endangered fan mussel Pinna nobilis and cross-species amplification.

The fan mussel, Pinna nobilis is a highly endangered bivalve species endemic to the Mediterranean Sea. During the last few decades, populations have been greatly reduced due to anthropic impacts and they are now under strict protection in most Mediterranean countries. Today, the species is facing a major crisis following the introduction of an haplosporidan parasite which is driving mass mortality in almost all P. nobilis populations throughout the Mediterranean Sea. Gathering additional knowledge regarding dynamics and connectivity patterns of P. nobilis populations is now more than ever critical. Here, we describe the development of 26 highly polymorphic microsatellite markers. Average allelic diversity of 10.9 alleles per locus was reported and heterozygosity ranged from 0.0294 to 0.9737. We tested cross-species amplification in four Pinna species for the new markers together with 10 already published markers, and analysed its success according to the genetic distances among species. Cross-species transferability success ranged from 3 to 38% and had a negative relationship with the genetic distance between the target species and the tested species. The establishment of this new set of high-resolution markers provides a useful tool to understand processes driving gene flow and genetic diversity in P. nobilis populations and the closest congeneric species.

Pinna nobilis in the Mar Menor coastal lagoon: a story of colonization and uncertainty

Populations of the Mediterranean fan musselPinna nobilishave progressively decreased over the last decades as a result of anthropogenic activities. The rate of decline has strongly increased since 2016, when a mass mortality event triggered by the parasiteHaplosporidium pinnaeoccurred, and evidence exists thatMycobacteriumspecies may also have played a major role in the event. Indeed, the epidemic has spread throughout the Mediterranean, although coastal lagoons seem to offer a degree of ‘resistance’ against the parasite. In the early 1980s,P. nobilisappeared in the Mar Menor lagoon and rapidly became an important component of the benthos. However, colonization of the lagoon by the fan mussel was cut short in 2016 when a massive mortality event occurred, possibly as a consequence of the environmental collapse that occurred in the lagoon, parallel to the mortality that the species suffered in the Mediterranean that same year. In this study, we estimated the spatial distribution ofP. nobilisin the Mar Menor for 3 periods: 2003-2004, 2013 and 2016. The first 2 periods use published data, and the last period uses data collected in a new campaign. The probability of occurrence for the 3 periods was estimated using random forest and random forest regression-kriging models. The main environmental variables that determined the dispersion and colonization of the bivalve in the lagoon before 2016 are also identified.

Geostatistical approach to investigate spatial patterns of the endangered fan mussel Pinna nobilis (Linnaeus, 1758)

The manta tow survey technique combined with the geostatistical approach was used to identify the reliability of these tools to identify the position of the banks of P. nobilis and evaluate the spatial distribution and abundance of its population. A large spatial scale study was carried out through a geostatistical approach to identify the spatial distribution and abundance of the endangered fan mussel Pinna nobilis populations in the Sant’Antioco Island, Sardinia, Tyrrhenian Sea. A smaller spatial scale approach, which was carried out by using the standard method of quadrates by scuba diving, was used to study the size distribution and to supply correction factors for the estimates obtained by geostatistics. Kriging mapping revealed the presence of a very dense population with values of abundance as high as up to 70 individuals 100 m−2, corresponding to a total number upscaling to 3.9 million individuals over the whole area. The small spatial scale study showed that the fan mussel population had a spatially heterogeneous size structure, with each location characterized by a different size/age structure mirroring the observed patterns of spatial autocorrelation. The comparison between the density of specimens directly observed by scuba diving and those estimated by the geostatistical technique showed that estimated value was overestimated when the density was low and underestimated when the density was high. We therefore conclude that the technique proposed in this study is a less time-consuming technique for prospection studies for the assessment of the presence/absence of P. nobilis population and their abundances in shallow waters, which are the first step for the development of conservation and management strategies of this endangered species.