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Until the late 2010s the spiny fan-mussel Pinna rudis shared habitat in many Mediterranean sites with Mediterranean endemic Pinna nobilis, which dominated in abundance but that has gone locally extinct in most of its distribution area due to a recent widespread disease outbreak. In the Columbretes Islands Marine Reserve (NW Mediterranean) both species coexisted until 2017, when P. nobilis populations completely disappeared. In spring 2021 we revisited 7 permanent plots covering 1,485 m2 that had been previously monitored during the period from 2005 to 2009. We found that although previous studies described P. rudis populations as stable with low recruitment rates, recruitment after 2017 increased in comparison to 2005-2009. At least two cohorts of recruits (~3 and ~1 year in age) were to be found both within the plots as well as in other areas throughout the Columbretes archipelago, colonizing areas previously occupied by P. nobilis. We hypothesize that P. rudis has been benefited by the local extinction of the sibling species P. nobilis, most probably as a result of reduced interspecific competition. The ecological role once played mainly by P. nobilis as the dominant species, might now be taken over by P. rudis, whose populations could grow during the oncoming years.

The genus Pinna includes two species in the Mediterranean Sea: Pinna nobilis and Pinna rudis. Both these species are under threat from multiple stressors. Pinna nobilis, in particular, has been exhibiting mass mortality events (MMEs) since 2016. The population and distribution of these species have never been comprehensively explored in the Maltese archipelago, and in this work, we collate information collected between 2006 and 2019 through a number of SCUBA underwater visual census monitoring programs. The logistical barriers surrounding SCUBA-based sampling techniques and the low-density distribution of these species constitute significant obstacles to an extensive conventional population assessment. Citizen science was thus also deployed in this study to supplement the data collected through SCUBA surveys: recreational SCUBA divers worked as citizen scientists, providing data on the distribution of these two endangered species from areas never explored before. This information can be used for assessing the conservation status of P. nobilis and P. rudis in Maltese waters, whilst contributing to the next generation of ocean-literate citizens.

Recently, Pinna nobilis pen shells population in Mediterranean Sea has plummeted due to a Mass Mortality Event caused by an haplosporidian parasite. In consequence, this bivalve species has been included in the IUCN Red List as “Critically Endangered”. In the current scenario, several works are in progress to protect P. nobilis from extinction, being identification of hybrids (P. nobilis x P. rudis) among survivors extremely important for the conservation of the species. Morphological characteristics and molecular analyses were used to identify putative hybrids. A total of 10 individuals of each species (P. nobilis and P. rudis) and 3 doubtful individuals were considered in this study. The putative hybrids showed shell morphology and mantle coloration intermingled exhibiting both P. nobilis and P. rudis traits. Moreover, the analyses of 1150 bp of the 28S gene showed 9 diagnostic sites between P. rudis and P. nobilis, whereas hybrids showed both parental diagnostic alleles at the diagnostic loci. Regarding the multilocus genotypes from the 8 microsatellite markers, the segregation of two Pinna species was clearly detected on the PCoA plot and the 3 hybrids showed intermediate positions. This is the first study evidencing the existence of hybrids P. nobilis x P. rudis, providing molecular methodology for a proper identification of new hybrids. Further studies testing systematically all parasite-resisting isolated P. nobilis should be undertaken to determine if the resistance is resulting from introgression of P. rudis into P. nobilis genome and identifying aspects related to resistance., SI

Este artículo contiene 4 páginas, 1 figura, 1 tabla., Until the late 2010s, the spiny fan-mussel Pinna rudis shared its habitat in many Mediterranean sites with the Mediterranean endemic Pinna nobilis, which used to be dominant, but has recently become locally extinct in most of its distribution areas due to a widespread disease outbreak. In the Columbretes Islands Marine Reserve (NW Mediterranean), both species coexisted until 2017, when P. nobilis populations completely disappeared. In spring 2021, we revisited seven permanent plots covering 1,485 m2 that had been previously monitored from 2005 to 2009. We found that although previous studies have described P. rudis populations as stable with low recruitment rates, recruitment increased after 2017 in comparison to 2005-2009. At least two cohorts of recruits (~3 and ~1 year in age, respectively) were found in the plots as well as in other areas throughout the Columbretes archipelago, colonizing areas previously occupied by P. nobilis. We hypothesize that P. rudis has benefited from the local extinction of P. no bilis, most probably as a result of reduced interspecific competition. The ecological role once played mainly by P. nobilis as the dominant species might now be played by P. rudis, whose populations could grow in the coming years., D.K. Kersting was supported by a Juan de la Cierva-Incorporación contract (Spanish Min istry of Science, Innovation and Universities; IJCI-2017- 31457) and by the postdoctoral fellowship programme Beatriu de Pinós funded by the Secretary of Universities and Research (Government of Catalonia) and the Horizon 2020 programme of research and innovation of the Eu ropean Union under the Marie Sklodowska-Curie grant agreement No 801370.

The fan mussel, Pinna nobilis, represents the largest bivalve endemic to the Mediterranean Sea. Since 2016, dramatic mass mortality of this species has been observed in several areas. The first surveys suggested that Haplosporidium pinnae (currently considered species-specific) was the main etiological agent, but recent studies have indicated that a multifactorial disease may be responsible for this phenomenon. In this study, we performed molecular diagnostic analyses on P. nobilis, P. rudis, and bivalve heterologous host species from the island of Sardinia to shed further light on the pathogens involved in the mass mortality. The results support the occurrence of a multifactorial disease and that Mycobacterium spp. and H. pinnae are not necessarily associated with the illness. Indeed, our analyses revealed that H. pinnae is not species-specific for P. nobilis, as it was present in other bivalves at least three years before the mass mortality began, and species of Mycobacterium were also found in healthy individuals of P. nobilis and P. rudis. We also detected the species Rhodococcus erythropolis, representing the first report in fan mussels of a bacterium other than Mycobacterium spp. and Vibrio spp. These results depict a complicated scenario, further demonstrating how the P. nobilis mass mortality event is far from being fully understood.

A two-year growth study of 80 Pinna rudis individuals was conducted in offshore cages in the western Mediterranean Sea. A Von Bertalanffy growth model was fitted to monthly measured data of 40 individuals (Group 1), whereas length-dry weight relationship was established with the other 40 individuals (Group 2). Oceanographic data were sampled bimonthly. The individuals showed the fastest growth reported for a bivalve (1.32 mm day−1). Temperature acted as the main factor controlling growth, which showed strong seasonality, but phytoplankton availability acted as a limiting factor during the warmest periods of year. These data will be useful to understand P. rudis ecology. Furthermore, the length-dry weight regression is proposed as a tool for captivity diet confection of the critically endangered species P. nobilis. Natural mortality was 0% during the study period. info:eu-repo/semantics/acceptedVersion

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., SI

A mass mortality event (MME) impacting the bivalve Pinna nobilis was detected across a wide geographical area of the Spanish Mediterranean Sea (Western Mediterranean Sea) in early autumn 2016. Underwater visual censuses were conducted across several localities separated by hundreds of kilometers along the Spanish Mediterranean coasts and revealed worrying high mortality rates reaching up to 100% in the center and southernmost coasts of the Iberian Peninsula including Balearic Islands. Populations on the northern coasts of the Spanish Mediterranean Sea seemed to be unaffected (Catalonian region). Histological examination of affected individuals revealed the presence of a haplosporidan-like parasite within the digestive gland being probably the pathogen that causes this mortality. The present MME has spread rapidly, causing high mortality rates in infected populations. Taking into account the degree of impact, the geographic extent, and the high probability that the infection is still in a spreading phase, this might be considered the largest MME ever registered for P. nobilis up to date, forcing this emblematic bivalve into a critical viability status over hundreds of kilometers of coast, SI

Coastal ecosystems are being extensively degraded by human activities. Benthic, slow-growing and long-lived species are highly vulnerable to these impacts. Marine protected areas may avoid biodiversity losses through habitat protection. The pen shell Pinna rudis is a protected species, but scarce data are available on its ecology and biology. The present study is a comprehensive ecological study encompassing several unknown aspects of the growth and inner record in relation to habitat types, density and size distribu¬tion. During the summers of 2011, 2012 and 2013, a total of 418 strip transects were conducted by scuba diving in the Marine Pro¬tected Area of Cabrera National Park (39.14° N, 2.96° E). Samples were conducted across different habitats and depths, exploring 152,146.35 m2 in total. A large range of sizes and ages were recorded within the park with densities ranging from 0 to 6.89 ind./100 m2. Most pen shells were patchily distributed and concentrated mainly in caves. Two hotspots represented the highest densities ever recorded worldwide, showing a potential link to high larval accumulation and settlement. The population size structure showed a unimodal distribution with shell width ranging from 6.2 to 25.0 cm, with an average shell width of 16.0 ± 3.4 cm. The absolute growth was asymptotic, with a maximum age of 28-31 years and length of 45 cm. This study on the biology and ecology of a well-established population of Pinna rudis in the Western Mediterranean could set a baseline for the conservation of this species in other areas, SI

Spatial distribution, sizes and densities of Pinna rudis have been studied in the Cabrera National Park. Densities varied spatially within the park (from low 0-0.16 to high 6.89 ind./100 m ) corresponding to a wide range of sizes. Most pen shells were patchily distributed and mainly concentrated in caves. High densities were observed in two hotspots, and represented the highest densities recorded worldwide, possibly linked to retention processes through high larval accumulation. The population size structure showed a unimodal distribution with individuals ranging from 6.2 to 25.0 cm shell width, with an average shell width of 16.0 ± 3.4 cm. Given the scarce data on this species, the present study provides valuable information for the spatial management and conservation of this threatened species 0

Age and growth of Pinna rudis were studied in Cabrera National Park MPA. To determine age and growth, the shells were processed to study the growth records across the posterior adductor muscle scar. The absolute growth of studied population was asymptotic, with a maximum longevity and length of 28-31 years and 45 cm respectively. This is the first study on age and growth determination of this protected species 0

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For many bivalved animals, shell damage creates a temporary breach through which metabolites signalling the location of an injured animal might pass even when the valves are shut. Thus, shell injuries that expose soft tissues are generally thought to increase susceptibility to other causes of death, especially predators, resulting in low frequencies of shell repair. An exception to this generalization is hypothesized to be species with a highly retractable mantle edge and a thin and flexible valve margin, such as many pinnoidean and pteriodean bivalves. When the valve edges of species with these traits are broken the mantle may be withdrawn without injury. Moreover, a broad seal is created when the flattened flexible valve margins are squeezed together as the shell is shut. These traits might also permit large parts of the shell near the growing edge to be broken off without injury to the mantle. If mantle retraction and flexible margins are advantageous in increasing survivorship of marginal injuries (however caused), species with these traits should have relatively high frequencies of shell repair compared with species from similar environments that lack them. Anecdotal accounts suggest that the Pinnidae, or pen shells, which have the capacity to withdraw the mantle and squeeze the shell edge together, are often heavily scarred from breakage at the growing edge of the shell. But quantification of repair frequency and the extent of breakage in members of this group are lacking to test the prediction of a survivorship advantage: do the Pinnidae have higher incidences and severity of shell repair than coexisting bivalves that lack flexible valve margins and the capacity for deep mantle withdrawal? To evaluate the advantage of flexible valve margins and deep mantle withdrawal in damage control, we examined the pinnid species Atrina rigida, Servatrina serrata, Pinna carnea and Pinna rudis for evidence of breakage-induced shell repair. Shells of A. rigida (n 1⁄4 73) and S. serrata (n 1⁄4 9) were collected in situ from an intertidal muddy sand flat near Masonboro Inlet, North Carolina. A second sample (n 1⁄4 24) of A. rigida was collected from within seagrass beds in St. Joseph’s Bay, Florida. Samples of P. carnea (n 1⁄4 16) and P. rudis (n 1⁄4 23) from the Florida Keys were obtained from the collections of the Academy of Natural Sciences, Philadelphia. We defined repair frequency as the number of repairs in a sample divided by the number of specimens. For comparison with the semi-infaunal Pinnidae, we also calculated repair frequency for 29 other co-occurring shallow infaunal and semi-infaunal bivalve species lacking flexible valve margins and deep mantle withdrawal (Table 1). We categorized repairs as scalloped if the dorsal–ventral width of the scar exceeded its anterior–posterior length (e.g. Fig. 1A, C), or embayed if the scar was shaped like a piewedge, with the anterior–posterior dimension of the scar greater in length than the dorsal–ventral dimension, or vice versa depending on the species (e.g. Fig. 1B, D); gradations between these scar types exist (Fig. 1E). When the designation of shape was borderline between these categories, scars were scored as the less severe damage type. Our results support the hypothesis that flexible valve margins and deep mantle withdrawal are advantageous traits to survive breakage-induced shell injury. Repair frequencies (range 1⁄4 0.31–1.14) for the pen shell species exceed values for all other species we examined that lack flexible valve margins and deep mantle withdrawal, except for two venerid species (Fig. 2; Table 1). Over 60% of species with inflexible valve margins and non-retractable mantle edges had repair frequencies less than 0.1. A Mann–Whitney U-test indicates that the mean rank of repair frequencies for pinnids exceeds the mean rank value for species with inflexible valve margins and nonretractable mantle edges (U0 1⁄4 152, P, 0.01). The percentage of embayed breaks (Fig. 1B, D), the most severe type of shell repair, ranged between 38% and 100% among the pinnid species (Fig. 3; Table 1). The lowest of these values is nearly 10 times the maximum percentage (4%) found for species with inflexible margins and non-retractable mantle edges, suggesting a poor capacity to sustain and repair extensive marginal damage (Table 1). A high incidence of embayed repairs is associated with the capacity for deep mantle withdrawal and flexible valve margins (Pearson chi-square test of independence; x 1⁄4 124.7, df 1⁄4 1, P 0.001). Scars may extend more than half the anterior–posterior length (Fig. 1B); such breaks encroached near the posterior adductor muscle, but do not extend into the nacreous layer surrounding the adductor muscle scar (Fig. 3). Some scalloped breaks meander across the entire dorsal–ventral width of the shell (Fig. 1A), which suggests removal and re-secretion of the entire posterior margin of the shell. We conclude that the highly retractable mantle edges of the Pinnidae, coupled with the flexible nature of their shell, contribute to their unusually high tolerance of extensive shell breakage. We believe that this interpretation is robust and unlikely to change if a more constrained test standardized comparisons of repair frequency between species better, for instance by size, age or habit. Despite the wide variation of these factors in the current dataset, results within pinnids and species lacking flexible valve margins and deep mantle withdrawal are consistent with each other, suggesting that additional data are unlikely to alter our conclusions. An avenue of study that might prove fruitful is to evaluate repair frequency in species with similar traits as the Pinnidae, such as members of the Pteriidae, Malleidae and Isognomonidae. Our results predict these groups should also display higher frequencies of shell repair than co-occurring species lacking flexible margins and deep mantle withdrawal. From an evolutionary perspective, the high frequency and severity of injuries in the Pinnidae indicates that the evolution of breakage resistance is not constrained by the bivalved condition. An evolutionary change that might be interpreted as a response to increased predation pressure is a reduction in the number of nacreous layers of the pinnid shell. This reduction (or regression towards the valve anterior) and consequent increase in area of the calcitic outer shell layer not Correspondence: G.P. Dietl; e-mail: gregory.dietl@yale.edu *Present address: Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA RESEARCH NOTES

Pinna nobilis is the largest endemic Mediterranean marine bivalve. During past centuries, various human activities led to the regression of its populations. As a consequence of stringent standards of protection, demographic expansions are currently reported in many sites. We designed a set of four mitochondrial- and two nuclear- specific PCR-primers with the aim to provide molecular tools to gather new insights into the genetic variability of this species. A total of 54 specimens were sampled from 25 Mediterranean localities in order to evaluate the level of polymorphism of these markers in P. nobilis. The most variable molecular markers identified were the mitochondrial Cytochrome c Oxidase subunit I (COI), NADH dehydrogenase subunit 3 (nad3), and 16S ribosomal DNA (16S). Positive results, in terms of good amplifications and scorable sequences, were also obtained in the co-generic Pinna rudis. The molecular markers identified in this study, and the PCR-protocols provided, represent a useful tool for future researches devoted to infer the genetic variability of P. nobilis populations thus allowing the development of effective conservation measures.