Sustainable Management of High-Impact Non-Native Molluscs and Their Potential Commercial Importance in the Eastern Adriatic Sea

: Molluscs, especially bivalves, play an important role in food production and are the dominant group in mariculture worldwide. They are also an important component of the marine eco-system, influencing the food web and nutrient cycling, as well as creating and modifying habitats. In the Mediterranean Sea, about 578 non-native species have been reported, including 230 taxa of non-native mollusc species. In most regional European seas, including the Mediterranean, the socioeconomic impacts of high-impact mollusc species have generally been poorly studied, and only a few well-documented cases have been reported. The present study collects scientific information on non-native molluscs in Croatian waters that are classified as high impact according to the European Alien Species Information Network EASIN: one Gastropoda— Rapana venosa— and seven Bivalvia species— Anadara kagoshimensis, Anadara transversa , Arcuatula senhousia , Brachidontes pharaonis , Magallana gigas , Pinctada radiata, and Ruditapes philippinarum . This study aims to fill the gap in the literature on high-impact non-native molluscs in the eastern Adriatic Sea through reviewing the literature on examples from other European seas, focusing on the possibility of commercialization and sustainable management of high-impact non-native molluscs. The invasion of non-native molluscs must be managed appropriately, especially if commercialisation is chosen as a management strategy. Additional management measures must be selected and objectively evaluated, taking into account the specificities, location and feasibility of the proposed measures. The study could help researchers, decision-makers, and the public to address the problems with high-impact species in the Adriatic countries, but also in other regions where the monitoring of high-impact non-native species is still inadequate. In addition to Croatia, the monitoring and management of these species is also important for other countries in the eastern Adriatic: Slovenia, Montenegro and Albania.


Introduction
Marine coastal areas are diverse ecosystems with complex, highly productive habitats that are sensitive to human impacts but also of great importance for human well-being [1][2][3][4][5]. Over the past century, anthropogenic influences have impacted the vulnerability of these ecosystems, and human-induced species introductions and subsequent species invasions also contribute to increasing this vulnerability. The growing worldwide awareness of bioinvasion as a major cause of global biodiversity change and its associated negative impacts on the environment, economy, and human well-being has attracted the attention of scientists, policy makers, and the general public. Research on bioinvasion into the marine environment has increased in recent decades as the marine biota in many regions has undergone rapid changes due to the introduction of non-native species [6][7][8][9][10]. In European seas, the number of newly introduced marine non-native species has increased from 6 to 21 per year since 1970 [10]. According to [11], the largest group of marine non-native species in the European seas is invertebrates (63%), and molluscs are the most abundant of them. The list of non-native species in the Mediterranean Sea includes 578 species (473 = 83% since 1970) [10]. Mollusca, with 230 taxa, have the highest diversity among established and causal non-native species in the Mediterranean Sea [12]. Besides their great role as marine invaders, Mollusca are an important marine animal group for the production of food and different bio-based materials [9].
In the list of the 100 worst invasive species in the Mediterranean Sea based on their impact on biodiversity and socioeconomics [13], 19 invasive mollusc species were included. The negative and positive impacts of 20 high-impact non-native molluscs in European seas on biodiversity and ecosystem services were assessed by [14], while the impacts of 17 high-impact non-native molluscs in the Mediterranean Sea were assessed by [5]. According to [14], the "native good, non-native bad" view is a misconception, and scientific publications are dominated by descriptions of negative impacts, while positive impacts are also underestimated [15].
Research studies on high-impact non-native molluscs in the eastern Adriatic Sea are mainly related to specific projects, such as BALMAS, a ballast water management system for Adriatic Sea protection (2013-2016); ALKA, for the identification and distribution of the invasive species Crassostrea gigas in the Adriatic Sea (2014-2015); or COCOA, which monitors competition between native Ostrea edulis and invasive Crassostrea gigas oysters in the Adriatic Sea-including their effects on the ecosystem, fisheries, and aquaculture (2021)(2022)(2023)(2024)(2025)(2026). Following the inventory of high-impact non-native mollusc species in Croatian waters by [71], published papers mostly deal with the distribution or ecology of highimpact non-native molluscs [49,50,52,56,57,[73][74][75][76]. However, research on the management and continuous monitoring of high-impact non-native mollusc species in Croatian waters is still lacking.
The present study collects scientific information on non-native molluscs in Croatian waters that are classified as high impact according to the European Alien Species Information Network [77]. The listed species in Croatian waters according to EASIN are: one Gastropoda-Rapana venosa (Valenciennes, 1846)-and seven Bivalvia species-Anadara kagoshimensis (Tokunaga, 1906), Anadara transversa (Say, 1822), Arcuatula senhousia (Benson, 1842), Brachidontes pharaonis (P. Fischer, 1870), Magallana gigas (Thunberg, 1793), Pinctada radiata (Leach, 1814), and Ruditapes philippinarum (Adams & Reeve, 1850). The information on these species is fragmented in the literature and is a possible source of confusion. Except for Croatia, the monitoring and management of these species are also important for other eastern Adriatic countries: Slovenia, Montenegro, and Albania.
This study aims to address the gap in the literature on high-impact non-native molluscs in the eastern Adriatic Sea through reviewing the literature on experiences from other regions (European Seas), with a focus on the possibility of commercialisation and the sustainable management of invasions. The study can be a useful tool not only for managers and policy makers in the eastern Adriatic but also for scientists in setting research and management priorities.

Material and Methods
A list of mollusc species relevant to the study (Table 1) was compiled using EASIN Combined Criteria Species Search. Main search criteria: main criteria-high-impact, species status-alien, environment-marine, occurrence in specific EU locations-Croatia [77,78]. The list was compared with published reports on species distribution and status [42,79] and species impact [5,13,14]. Cryptogenic and questionable species were not included in the list.
The most likely pathways of introduction for species were identified by [77] (general) and [80] (to the Adriatic) in accordance with the Convention on Biological Diversity (CBD) Pathway Classification Framework [81]. The species overview was updated with data from databases (WoRMS, AquaNIS, CABI, GISD, NIMPIS, NEMESIS, NOBANIS) and peer-reviewed journal articles, as well as grey literature selected through literature searches on Google Scholar. The veined rapa whelk or Asian rapa whelk Rapana venosa is a predatory large marine gastropod in the order Neogastropoda, family Muricidae [101]. It is native to the temperate western Pacific Ocean [17]. Because of its cryptic nature, it is difficult to detect individuals until they constitute a large portion of the community, and the likelihood of early observation of initial introductions is low [17].
Rapana venosa is one of the most successful invaders worldwide due to its high fecundity, rapid growth, high ecological plasticity, especially high tolerance to temperature and salinity, as well as pollution and oxygen depletion [102,103]. The first record of R. venosa outside its native distribution was in the Black Sea in 1946 [104]. The first record in the Mediterranean was in 1973 in the Italian part of the Adriatic Sea [82], where it moderately spread. In 1979, it was found in Slovenia [105], in 2011 in Albania [68], and in 2017 in Montenegro [106]. The first record of R. venosa in Croatia occurred during fisheries monitoring in 2017, when a fisherman reported seven adult live specimens caught in May 2004 in northwestern Istrian waters at a depth of 22 m using a trammel net [83]. This is the only report of R. venosa in Croatian waters. In Croatia, the status is unknown, but the species is expected to become invasive in Slovenia and northern Croatia due to the spread of the Italian population in the northern Adriatic [102].
Because of its life history, R. venosa can spread easily through ballast water, ship hull fouling, aquaculture products, or via tidal currents [18,102]. According to [77], the primary pathways of R. venosa introduction/spread are release in nature through fisheries in the wild and transport as a contaminant on animals, while secondary pathways are transport via contaminants and as stowaways in ship/boat ballast water as well as unaided natural dispersal. Although the original vector for the introduction into the Black Sea is unknown, molecular studies have shown that the Japanese population is the source of the Black Sea population and the source of the Adriatic Sea populations [107]. The introduction into the Black Sea is possibly related to the introduction of Magallana gigas [108], but could also be related to ballast water [16] or ship hull fouling [109]. According to [110], the Adriatic population was probably established through natural dispersal from the Black Sea or via shellfish introduced for aquaculture. According to [80], the pathway in the Adriatic Sea is 100% through transport as stowaways on ships or in boat ballast water.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed R. venosa as one of the 100 worst invasive species in the Mediterranean. The FAO protocol classifies R. venosa as the highest-risk-category species whose introduction should be prohibited [19], has the highest ecological impact in The Nature Conservancy database [8], and is a high-risk species for the EU [102]. In 2018, IUCN prepared an EU non-native species risk analysis-risk assessment for Rapana venosa (Valenciennes, 1846) for the European Commission [102]. Due to its negative impacts on ecosystem services and biodiversity, R. venosa has been classified as one of the most impactful non-native species in European/Mediterranean seas [5,14], with severe impacts documented in the Black Sea and moderate impacts in the Adriatic Sea. It poses a serious threat to shellfish fisheries, especially in estuarine waters of coastal regions [111]. Rapana venosa feeds on bivalves, including commercial species such as oysters and mussels [112,113]. Small specimens bore a hole in the bivalve shell, but large specimens attack bivalves without drilling [108]. In the Adriatic Sea, despite an initial rapid spread, R. venosa does not pose a problem to local fisheries or native bivalve populations, while negative impacts on farmed mussels can be reduced through suspending ropes out of reach of whelks [113].

Commercial Importance and Management
Although R. venosa was originally considered a marine pest in non-native areas, the invasion of R. venosa may have positive socioeconomic effects. Rapana venosa is a delicacy and commercially valuable living resource in its native area (Japan, Korea, China). The commercial size is 9-16 cm, and the market price in China is USD 7-9 per kg [114]. It could replace other muricids consumed in Mediterranean countries [115]. In the Black Sea, R. venosa encouraged the development of a lucrative fishing industry [103]. In the 2000s, Türkiye exported about 2100 tonnes of R. venosa meat per year, and Bulgaria exported about 900 tonnes of R. venosa meat per year (together, about 3000 tonnes of R. venosa meat per year or about 18,000 tonnes of R. venosa with shell) with a total export value of EUR 12,839,446 per year [20]. Due to unsustainable dredge and beam trawl fisheries, especially in Türkiye, both the population and catch have declined from 10,000 tonnes in 1988 to 1000 tonnes of meat per year in the early 2000s [116]. To protect R. venosa from overfishing, sustainable fishing practices have been introduced in the Black Sea [117], as well as licences and size limits for catches and a common management model for the Black Sea [22]. Nevertheless, the whelk fishery has increased pressure on R. venosa but reduced pressure on highly sensitive fish species in the Black Sea [21].
The meat of R. venosa has relatively high protein, low fat content [118], and a high content of omega-3 essential fatty acids [119]. It contains proteins that can be used as food preservatives and antibiotics [120], vitamins [121], and antioxidants with anti-inflammatory, antitumor, and antiviral effects [122][123][124]. Rapana spp. (operculum) are traditionally used in Chinese medicine [125]. The shells of R. venosa are studied for various tissue engineering applications (production of nano-phases of calcium phosphate biocompatible bioceramics for biomedical coatings) [126]. A good marketing strategy for whelk meat as an aphrodisiac and for decorative shells as tourist souvenirs and raw material for local craftsmen has significantly increased the demand for R. venosa in the Black Sea region [22]. Occasionally, R. venosa is sold at fish markets in the northern Adriatic [127], but Gastropoda are not commercially important as Bivalvia in the Adriatic Sea. However, commercial fishing remains the most effective method of controlling R. venosa in conjunction with a public education program [17]. The demand for whelk meat in the world market and an income rate of 79% [116] make this species a good candidate for commercial fisheries. In the case of a population explosion in the eastern Adriatic, R. venosa could be marketable in the Asian market.

Summary of Invasiveness
The subcrenate ark shell A. kagoshimensis is a marine bivalve in the order Arcoida, family Arcidae [101]. Anadara kagoshimensis is native to the temperate North Pacific [128].
Like other members of Anadarinae, the competitive advantage of A. kagoshimensis is that it can tolerate fluctuating physical variables (tides, salinity, temperature, oxygen concentration) and, for suspension feeders, difficult trophic conditions [129]. Because of its life history, it can spread easily through ballast water, aquaculture products, or via tidal currents.
The first record of A. kagoshimensis in the Adriatic (and the Mediterranean) dates back to 1966 [84], where it was probably introduced through shipping [128]. It spread rapidly to the west and north, forming large banks along the Italian coast, from coastal brackish water to a depth of 30 m on different types of bottoms [46]. According to [62], the first record of A. kagoshimensis in Slovenian waters was in 1996 [105]. The first record of A. kagoshimensis in Croatian waters was during the SoleMon survey with modified beam trawls off the coast of Istria in 2008 [48]. The abundance of the species increased in subsequent years, probably naturally due to larval transport with the current or supported by commercial fishing due to discarding practices [48].
According to [77], the primary pathway of A. kagoshimensis introduction/spread is Transport -Stowaway: Ship/boat ballast water and secondary pathways are Transport -Stowaway: Ship/boat ballast water, Transport -Stowaway: Other means of transport and Unaided: Natural dispersal. According to [80], the pathway in the Adriatic Sea is 33 % Transport -Stowaway: Angling/fishing equipment, 33 % Transport -Stowaway: Ship/boat ballast water and 33 % Transport -Stowaway: Ship/boat hull fouling.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed A. kagoshimensis on the list of the 100 worst invasive species in the Mediterranean. Refs. [5,14] summarised the biodiversity impacts of A. kagoshimensis in terms of the negative impacts on multiple species and positive and negative impacts as an ecosystem engineer, while its impacts on marine ecosystem services include negative impacts on food provision (fisheries and aquaculture), positive impacts on climate regulation, and both positive and negative impacts on ocean nourishment.
Ref. [130] explained the mass development of A. kagoshimensis in the Black Sea in terms of consuming the excess amount of organic matter in eutrophic waters in the Black Sea and through physiological and biochemical adaptations to hypoxic conditions that often occur during eutrophication. In the second half of the 20th century, A. kagoshimensis stimulated adaptive change in the Black Sea shelf ecosystem as it positively affected biodiversity through promoting the growth and diversity of filter feeders [131]. The rapid expansion of A. kagoshimensis populations in the Adriatic Sea has displaced commercially important autochthonous bivalves in soft-bottom habitats such as Chamelea gallina (Linnaeus, 1758) and Cerastoderma glaucum (Bruguiere, 1789) [13,128,132]. However, after the population explosion phase, number of individuals of the species has declined in the Adriatic Sea [133]. Although A. kagoshimensis negatively impacts food provisions due to its competitive nature and poses a threat to commercial shellfish aquaculture [134], harvest of this species can also have positive economic impacts.

Commercial Importance and Management
Bivalves of the order Arcoida (ark shells or blood cockles) are an economically important group of bivalves and are used for akagai, red clam, or surf clam sushi. The global fishery production of ark shells is about 591,000 tonnes per year and has a value of about USD 600,000,000 [135]. Anadara kagoshimensis is an economically important species in China (maohan in China), Korea, and Japan (mogai in Japan) [136]. After World War II, it was introduced in some Asian coastal areas as an additional source of food and has been cultivated in shallow waters in South Korea and Japan since the early 1960s [129]. The tissues of A. kagoshimensis also contain a large amount of carotenoids with antioxidant properties [137]. Its rapid growth, great ecological plasticity, stress tolerance, and welldeveloped market make it a good candidate for commercialization in the event of an invasion in the Adriatic Sea.

Summary of Invasiveness
The transverse ark shell A. transversa is a suspension-feeding marine bivalve in the order Arcoida, family Arcidae [101]. It is native to the western Atlantic Ocean [138]. Anadara transversa is an opportunistic species capable of living in highly polluted ecosystems [139], even under hypoxic conditions due to respiratory pigments with high oxygen affinity [86]. The species is a strong competitor for space and its invasive potential is enormous because of its ability to choose between a burrowing and a non-burrowing strategy [73].
In the Mediterranean Sea, the first record of A. transversa was in Izmir, Türkiye, in 1972 [85]. In the Adriatic Sea, the first record was in Italy in 2000 [140]. Ref. [141] found a rapidly growing population in the eastern Adriatic consisting of individuals of all size classes and dominating the outer belt of the sand biocoenosis. Using subfossil records preserved in sediment cores, [86] found that A. transversa was introduced to the Adriatic in the 1970s and is now found along most of the Italian Adriatic coast. Anadara transversa was first recorded in Slovenian waters in 2003 [46].
In Croatian waters, [142] reported two empty shells found at the mouth of the Jadro River in the central Adriatic, but the first record of living organisms occurred in 2011 when six juvenile specimens were found in the northern Adriatic [47]. As part of the SoleMon project [48], A. transversa was recorded at one station in the middle of the open Adriatic Sea at 70 m depth. The first record in Montenegrin waters was in the Boka Kotorska Bay in 2015 [64].
According to [77], the primary pathway of A. transversa introduction/spread is transport as stowaways in ship/boat hull fouling, and the secondary pathway is Transport as a Contaminant on animals. The species was introduced into the Mediterranean Sea from the Gulf of Mexico via ballast water or ship hull fouling, and secondary dispersal occurred via aquaculture [59]. The selective distribution in the Mediterranean suggests colonization through different introduction vectors [143]. Aquaculture was excluded as the initial vector of introduction, and shipping was the most likely vector. According to [80], the pathway in the Adriatic Sea is 33% "Transport-Contaminant: Contaminant on animals", 33% "Transport-Stowaway: Ship/boat ballast water", and 33% "Transport-Stowaway: Ship/boat hull fouling".

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed A. transversa on the list of the 100 worst invasive species in the Mediterranean region. Refs. [5,14] summarised the impacts of A. transversa on biodiversity in terms of positive and negative impacts on multiple species as well as positive and negative impacts as an ecosystem engineer (bioturbator, structural, chemical, and light engineer), while impacts on marine ecosystem services include negative impacts on food provision, positive impacts on climate regulation, and positive and negative impacts on ocean nourishment.
Anadara transversa has become very abundant in many Mediterranean sites, especially in degraded ecosystems [59,144,145]. Anadara transversa lives as an epibiont on species, including species of commercial value such as oysters, and negatively affects these species through restricting their movements and habitat use or competing for food.
Ref. [146] found large numbers of A. transversa on installed collectors of Pinna nobilis (Linnaeus, 1758) in Brijuni National Park. This result indicates the negative impact of A. transversa on keystone species or species of high conservation value such as the endemic Mediterranean bivalve P. nobilis. The species P. nobilis is of highest conservation value as it is listed as Critically Endangered in the IUCN Red List [147], in Annex IV of the Habitat Directive as a Species of Community Interest in Need of Strict Protection [148], and in Annex II of the Barcelona Convention Protocol concerning Specially Protected Areas and Biological Diversity in the Mediterranean as endangered or threatened species that all parties shall manage with the aim of maintaining them in a favourable state of conservation [149].

Commercial Importance and Management
Anadara spp., including A. transversa, are good candidates for commercialisation in case of a population explosion in the eastern Adriatic Sea.

Summary of Invasiveness
The Asian date mussel or green mussel A. senhousia is a small suspension feeder marine bivalve in the order Mytilida, family Mytilidae [101]. The distribution range of the species is the western Pacific Ocean [150]. As an opportunistic, fast-growing, fouling organism with a small body, high fecundity, short life span (up to 24 months), and long planktonic dispersal phase (14 to 55 days), it is a very successful invader [103,151,152].
The first record in the Mediterranean was in Israel in 1964 [87], and it may have been introduced via the Suez Canal. The first record in the western Mediterranean in lagoons along the French Mediterranean coast in 1978 was associated with the farming of Pacific oysters imported from Japan [153]. The first record in the Adriatic was from the Ravenna Lagoon, Italy, in 1992, and may have been introduced with a large quantity of imported bivalve R. philippinarum for aquaculture in 1986 [88,89]. The first record in Slovenian waters dates from 2005 by [45]. The first record in Albanian waters was in 2011 [69]. It was first detected in Croatian waters by local fishermen in the Savudrija Bay in 2003 [46]. Ref. [154] found A. senhousia in aggregations of the non-native polychaetes Ficopomatus enigmaticus (Fauvel, 1923) in the delta of the Neretva River in the Croatian waters of the southern Adriatic in 2010, as did [56] in 2019.
According to [77], the primary pathway is Transport -Contaminant: Contaminant on animals, Transport -Stowaway: Ship/boat ballast water and Transport -Stowaway: Ship/boat hull fouling. A. senthousia is easily transported in ballast water and via biofouling due to its potential to survive and life history. In the Mediterranean, the invasion of A. senhousia has been associated with shellfish farming and trade, but other possible mechanisms include shipping, since the planktonic larval stage is long enough for transport in ballast water, or hull fouling [103]. According to [80], the pathway in the Adriatic Sea is 33 % Transport -Stowaway: Angling/fishing equipment, 33 % Transport -Stowaway: Ship/boat ballast water, and 33 % Transport -Stowaway: Ship/boat hull fouling.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed A. senhousia on the list of the 100 worst invasive species in the Mediterranean region. Refs. [5,14] summarised the impacts of A. senhousia on biodiversity and ecosystem services. Arcuatula senhousia is a structural engineer and alters sedimentary properties through building up byssal mats on the surface of soft sediments [14]. Because A. senhousia can grow above aggregations of other non-native species [154], it increases environmental complexity [54]. Mussel mats provide protection from predators, contribute to nutrient cycling, and allow enrichment with organic matter that promotes the growth of some species [155,156]. The extent and direction of these effects depend on density, as byssus mats smother seagrasses (Zostera marina Linnaeus, 1758) and species that live in seagrass beds [156][157][158]. Extensive mussel mats can be harmful to native bivalves as they compete for space and food with other suspension-feeding bivalves including commercially important species Ruditapes decussatus, Ostrea edulis, and Mytilus spp. [54,159], but they also indirectly increase predation [160]. However, A. senhousia has been associated with killing cultivated clams in China and Japan [161]. Ref. [162] reported no significant effect on cultured R. decussatus and R. phillipinarum in the Adriatic. Ref. [14] assessed the impacts of the species on marine ecosystem services in terms of negative impacts on food provision, water storage and provision, and ocean nourishment in addition to positive impacts on water purification, while [5] added positive and negative impacts on climate regulation [163].

Commercial Importance and Management
Besides negative impacts on food provisions, A. senhousia can also have positive impacts, as it is an important food source for predators [164,165]. Arcuatula senhousia is commercially harvested in China, Taiwan and Japan as fish bait, feed for poultry, and in crustacean farming [150,159]. It is also used as a fertiliser, and in China, where aquaculture experiments have been conducted, it is used for human consumption [159,161]. Commercial harvesting of A. senhousia is a possible control method, but cost-benefit analyses should be conducted and public acceptability should be investigated. Ref. [159] used a natural capital approach to determine the potential impact of A. senhousia on European habitats, fisheries, and aquaculture if the species continues to expand, and concluded that risk assessment and monitoring of this species is essential, particularly in habitats of conservation and commercial interest. The Australian National Control Plan for A. senhousia [158] highlighted that the research and development strategy to improve control and monitoring should include several research areas that will help improve the management of this species. In case of a population explosion in the eastern Adriatic Sea, A. senhousia can be a valuable commodity due to its biology and the well-developed Asian market.

Summary of Invasiveness
Brachidontes pharaonis is a small, fast-growing, suspension-feeder marine bivalve in the order Mytilida, family Mytilidae [101]. Brachidontes pharaonis is native to the Indian Ocean [166]. According to [167], genetic studies revealed that B. pharaonis is restricted to the Red Sea and the Mediterranean Sea. Ref. [168] claimed that B. pharaonis is not really a non-native species in the Mediterranean, but an undiscovered native species, because many haplotypes occur in the Mediterranean and northern Red Sea that cannot be traced to other populations.
Brachidontes pharaonis has high invasion potential because the species has high habitat plasticity and tolerates very stressful conditions (variable temperatures, air exposure, food availability, concentrations of hydrocarbons and other pollutants) and has a relatively long planktonic larval stage that facilitates natural dispersal [169]. Considering the warming trend in Mediterranean waters and the invasive potential of B. pharaonis, the species could expand its distribution [169].
In the Mediterranean Sea, B. pharaonis was first recorded in 1876 in Port Said, Egypt, at the northern entrance of the Suez Canal [90]. Brachidontes pharaonis entered the Mediterranean Sea via the Suez Canal and colonized the eastern Mediterranean Sea from Egypt to Greece [170], Sicily [169,171], and Malta. In the Adriatic Sea, [91] noted that several specimens were found in 1996 near Cape Savudrija in Croatian waters. The first record in Albanian waters was form Vlora Bay in 2006 [44,70]. The first record in Slovenian waters dates back to 2012 [53]. Ref. [53] stated that the records from Venice and Bari (Italy) and Split (Croatia) reported by [172] were incorrect or based on mere misidentifications, as the authors did not display the supposed specimens and did not provide additional details or the location of the sampled material (if any). Ref. [173] indicated the distribution of the species in Croatian waters from the Istrian Peninsula to Split, but the authors did not display any evidence [53]. Ref. [42] classified the status as established in Albania and Italy, casual in Slovenia, and unknown in Croatia.
According to [77], the primary pathways of B. pharaonis introduction/spread are Corridor: Interconnected waterways/basins/seas and Transport -Stowaway: Ship/boat hull fouling. The introduction of B. pharaonis in the Mediterranean is considered as true Lessepsian migrant [7]. Ref. [174] suggested that the introduction and spread of B. pharaonis in the Mediterranean was related to ship transport from outside the Red Sea, based on a molecular study that showed that the Mediterranean population has both genotypes from the Red Sea and genotypes from outside the Red Sea, with the latter becoming more abundant with increasing distance from the Suez Canal. At some sites (Croatia, Malta, and Sicily), B. pharaonis is mainly restricted to concrete port structures, suggesting that shipping is the main vector [91,172,175]. According to [80], the pathway in the Adriatic Sea is 50 % Transport-Stowaway: Ship/boat hull fouling and 50 % Unaided: Natural dispersal.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed B. pharaonis on the list of the 100 worst invasive species in the Mediterranean region. Ref. [14] summarised the biodiversity impacts of B. pharaonis in terms of positive and negative impacts on multiple species as well as being an ecosystem engineer (structural engineer), while the species impacts on marine ecosystem services include negative impacts on water storage and provision and ocean nourishment as well as positive impacts on water purification and climate regulation. Ref. [5] listed B. pharaonis as the fourth species on a list of ten worst invasive species based on its negative impact on biodiversity. According to [5], B. pharaonis did not exhibit alarming invasive behaviour until the late 1990s, when its populations exploded in the eastern Mediterranean, displacing native species (Mytilaster minimus) mainly through the mechanisms of competition and creation of new habitats.

Commercial Importance and Management
The negative effects of B. pharaonis on water storage and provision include the clogging of intake pipes of industrial plants [13]. As a fouling organism, B. pharaonis can have detrimental effects on aquaculture/fisheries as well as shipping, causing high operational and maintenance costs. According to [151], the species has no importance for humans. Nevertheless, B. pharaonis could be used similarly to A. senhousia (as fish bait, feed for poultry and in crustacean farming, fertiliser, etc.) if its population expands in the eastern Adriatic Sea. However, since B. pharaonis is a pest, the primary control method should be eradication.

Summary of Invasiveness
The Pacific oyster, Japanese oyster, M. gigas, formerly known as Crassostrea gigas, is a large suspension-feeder marine bivalve in the order Ostreida, family Ostreidae [101]. It is native to the Pacific Northwest, where it is traditionally cultured, but was introduced elsewhere for aquaculture in the late 1960s [103]. Magallana gigas inhabits nearly all temperate coasts of the Pacific and Atlantic oceans [15,26]. There is a high risk of M. gigas spreading into temperate regions as well as high latitudes, aided by global warming [176].
To compensate for the scarcity of flat oysters, the Pacific oyster has been introduced to Europe, mainly for aquaculture purposes [177]. For the first time, M. gigas was introduced into France from Japan in 1966 to support the depleted oyster industry, including oyster areas in the Mediterranean Sea [23]. It was hypothesised that M. gigas would not form self-sustaining populations because temperature is the main limiting factor, but the species has naturalised and expanded its range [178].
The first record in the Adriatic dates from 1964, when fishermen found long oysters in the northern Italian Adriatic [91,92]. In 1969, [93] recorded two specimens of M. gigas (141-170 mm). In 1970, M. gigas were introduced from California to Laguna di Varano (central Adriatic) and Puglia (southern Adriatic) [94].
Since these observations were made before Pacific oyster aquaculture introduction in the Adriatic Sea, they may be related to the introduction of M. angulata in 1966 [92] or to another vector (shipping). In the northern Adriatic, wild populations of M. gigas are established on rocky shores in Italy [46], Slovenia [62], and Croatia [49,52,57,74]. The first record of M. gigas in the waters of Slovenia dates back to 1971 [91]; in Montenegrin waters, to 1977 [179]; while in Albanian waters, it was first detected in 2001 [180].
In Italy, M. gigas farming is limited but is constantly developing [181][182][183]. According to [94], M. gigas spat was introduced for experimental purposes in Croatia (Ston) in the southern Adriatic Sea in 1969. In 1972, M. gigas was intentionally introduced into Lim Bay (Istria, Croatia) to study its potential for aquaculture [184,185], but it was never commercially farmed in Croatia [75]. In the central and southern Croatian Adriatic, the species is recorded only sporadically [74], probably due to the main Adriatic current [75]. Dense aggregations of M. gigas were found in the Lim Bay (Croatia), a nationally important shellfish farming area [57]. Since M. gigas was found mainly in the intertidal zone, while the native oyster Ostrea edulis was found only in the subtidal zone, there seems to be no spatial competition between M. gigas and O. edulis [52,57]. Ref. [76] found O. edulis larvae in the stomach contents of M. gigas. In the Adriatic Sea, the limiting factor for M. gigas is water temperature, which can reach 30 °C in summer [186].
According to [77], the primary pathway is Escape from confinement: Aquaculture/mariculture. Secondary spread can occur via both natural and human-mediated vectors: in ballast water or on ship hulls [15]. According to [80], the pathway in the Adriatic Sea is 50 % Transport -Stowaway: Ship/boat hull fouling and 50 % Escape from Confinement: Aquaculture/mariculture.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed M. gigas as one of the 100 worst invasive species in the Mediterranean region. Refs. [5,14] summarised the impacts of M. gigas on biodiversity, including competition for resources, creation of new habitats, and hybridization with local oyster species [187]. Magallana gigas has positive impacts on climate regulation and coastal protection through forming oyster reefs and preventing intertidal erosion [25], and negative impacts on ocean nourishment. Oyster reefs also serve as nurseries and spawning areas [188], providing good habitat for Z. marina [189] and shorebirds [190]. Magallana gigas has positive and negative impacts on symbolic and aesthetic values, as well as on recreation and tourism, contributing to the decline of mussel beds [191], but also forming important oyster reefs (good for snorkelers, but the sharp shells can injure swimmers).

Commercial Importance and Management
Magallana gigas has both positive and negative impacts on food supply because it has historically contributed to the food supply, but it can also compete with other commercial species. In terms of farming, it has many advantages, including relatively inexpensive and easy production, but it may also be associated with the introduction of hitchhiking nonnative species, parasites, and diseases [190,192]. Ref. [193] included M. gigas in the list of the top 27 non-native animals introduced for aquaculture in Europe (ranked third according to heterogeneity of impact). In Annex IV of Council Regulation (EC) No. 708/2007 concerning the use of non-native and locally absent species in aquaculture [194], M. gigas is listed as one of 10 species of economic value in the EU, unless a particular Member State decides otherwise. Production of M. gigas was 610,000 tonnes in aquaculture and 25,900 tonnes in capture fisheries in 2020 [195,196] (Magallana gigas production has positive economic impacts (over USD 1.2 billion in 2019) [197] as well as social impacts, providing direct income and employment for thousands of people and contributing to the sustainability of coastal communities [103].
In some areas (Australian New South Wales, the Wadden Sea), M. gigas is considered a pest or noxious species, and management programmes have been adopted, such as the Global Invasive Species Programme and the National Introduced Marine Pest Information System (NIMPIS) in Australia [15,198,199]. Eradication methods have been tried in the Netherlands (handpicking and experimental dredging) [26]; Denmark, where guided tours were available to hand-collect M. gigas for food [26]; and the UK (bashing with a hammer and hand-collecting) [27]. Manual removal is labour intensive but less destructive than mechanical removal (dredging) and is feasible in MPAs or important shellfish areas, justifying the use of volunteers [27,28]. Harvesting wild M. gigas is neither effective nor profitable, but harvested material can be used as fertiliser, food supplements, construction materials (for the creation of artificial reefs), etc. [26,200]. Future innovative regulatory methods should include large-scale handpicking and robotic oyster harvesting [28].
In some areas (France), M. gigas supports a large aquaculture industry and even wild spat is of economic importance to the industry, which is protected and carefully managed by fishery authorities [15]. Member states have to implement a surveillance programme and assess a programme of measures to reduce the impact of non-native species under the EU Marine Strategy Framework Directive (MSFD, 2008) [15].
Since M. gigas has great socioeconomic importance, management measures should be based on responsible aquaculture practices (the production of triploid seed to prevent escape and the quarantine of oysters to prevent spread of diseases and hitchhiking species) according to risk management protocols and codes of conduct (FAO Code of Conduct for Responsible Fisheries [201], ICES Code of Practise on the Introductions and Transfers of Marine Organisms [202], Council Regulation No 708/2007 [194], and other preventive measures in lieu of eradicating feral populations [203]). In the case of an invasion in the eastern Adriatic Sea, M. gigas is likely to become a valuable commodity in the Croatian market and possibly an export product.

Summary of Invasiveness
The rayed pearl oyster P. radiata is a small-to medium-sized suspension-feeder marine bivalve in the order Ostreida, family Margaritidae [101]. Pinctada radiata is native to the Indo-Pacific, including the Red Sea, and is one of the earliest Lessepsian immigrants to the Mediterranean, having first been recorded on the coast of northern Egypt in 1874 [95], five years after the opening of the Suez Canal [41]. It is a fouling species that attaches via byssus on hard substrates (natural or artificial) and may form aggregations consisting of pearl oysters, worm tubes, and algae [204]. The invasiveness of P. radiata is due to its long life span, rapid growth, high reproductive potential, adaptation to subtropical environments, and tolerance to pollution [205].
In the Adriatic Sea, P. radiata was first reported in 1996, when live specimens attached to an oil platform transported from Sicily were found in the Bay of Trieste (Italian northern Adriatic) [96]. Ref. [206] reported detections from the southern Italian Adriatic (Torre Guaceto). The first record in Albanian waters dates back to 2010, when [41] reported an individual in Saranda, and since 2014, it has been frequently observed in Vlora Bay in Posidonia meadows [53]. The first record in Montenegro dates back to 2016, when 15 individuals of P. radiata were collected [51]. The first record in Croatian waters dates back to 2006, when two juvenile specimens were found in the northern part of the Adriatic coast [97]. In 2015, six specimens were collected from an old fish cage on the island of Mljet (southern Adriatic), and in 2017, another thirty specimens were collected [50]. In 2016, [207] found one specimen on the Pelješac Peninsula (southern Adriatic) attached to a plastic collector at a depth of 11 m near farmed oysters in close association with O. edulis, M. galloprovincialis, Mimachlamys varia, and Pteria hirundo.
According to [77], the primary pathway of introduction for P. radiata is CORRIDOR: Interconnected waterways/basins/seas and Release in nature: Fishery in the wild, and the secondary pathway of introduction is Transport-Stowaway: Ship/boat hull fouling and Unaided: Natural dispersal. The distribution of P. radiata in the Mediterranean Sea indicates that its progressive spread is mainly due to natural dispersal [97]. According to [80], the dispersal pathway in the Adriatic Sea is 100 % Transport-Stowaway: Ship/boat hull fouling, while according to [103] in Croatian waters, breeding and propagation, and interbasin transfers are the cause.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed P. radiata as one of the 100 worst invasive species in the Mediterranean. Refs. [5,14] summarised the impacts on biodiversity of P. radiata as positive and negative multiple-species impacts, and as an ecosystem engineer. Pinctada radiata has been reported to dominate benthic communities, outcompeting other native filter feeders for resources [14]. The species impacts on marine ecosystem services include negative impacts on ocean nourishment, positive and negative impacts on food provisions, and positive impacts on water purification, climate regulation, lifecycle maintenance, and cognition. The positive effects of P. radiata on lifecycle maintenance are related to the ability of pearl oysters to form dense oyster beds and provide feeding and nursery grounds for many species [14]. Due to its ability to accumulate metals [208,209] and fat-soluble pollutants [210] in its soft tissues, P. radiata is an ideal species for pollutant biomonitoring (positive effects on cognitive benefits). According to [211], P. radiata is an ideal species for subregional comparisons of chlorinated hydrocarbon levels. The wide distribution of the species and the possibility of producing large numbers of genetically similar animals of known age are a major advantage of P. radiata as a bioindicator of environmental pollution [212].

Commercial Importance and Management
Pinctada radiata has a long tradition in pearl production and plays an important role in human nutrition [204]. The meat of P. radiata is a delicacy in many western cultures and in some Mediterranean countries such as Lebanon and Egypt [213,214], as well as in Qatar [215] and Saudi Arabia [216]. It has been cultivated in Japan, China, and India for decades [217]. The only negative economic impact of P. radiata is the fouling of aquaculture facilities, especially on mussel farms, and it should be removed from seed bivalves intended for mariculture [16]. Local public awareness campaigns and population monitoring are suggested prevention measures, as is research into their interaction with native species and their role in ecosystem functioning, while control measures include physical removal of new populations by hand, especially in aquaculture facilities and MPAs [16].
In Japan and Qatar, a billion-dollar pearl industry exists with P. radiata [16]. According to [218], Japanese and Chinese production of pearls and pearl shells (for souvenirs) has declined due to deteriorating water quality and disease, creating a gap in the supply chain. Pinctada radiata was imported to Greece for aquaculture purposes [219], cultivation was abandoned as unsuccessful, and for a long time, it was considered a species of little commercial interest [220,221]. Pinctada radiata is found on the Greek shellfish market under various names, and catches are often misreported or misidentified and recorded under other taxa [222]. Ref. [222] studied the marketing of P. radiata, a new edible bivalve product on the Greek market, and concluded that this species is a substitute for the main commercial species in times of scarcity. The authors emphasise the importance of market promotion campaigns for a new product, including promotion through regional festivals and labelling of non-native species to increase customer familiarity with non-native species and encourage consumption of novel bivalve species. In the case of an explosion of P. radiata in the Adriatic Sea, a scenario similar to that in Greece is likely. Pinctada radiata as a strong competitor for space could be a problem for mussel culture in the eastern Adriatic Sea.

Summary of Invasiveness
The Manila clam, Pacific palourde, or Japanese carpet clam R. philippinarum (Venerupis (Ruditapes) philippinarum (A. Adams and Reeve, 1850) as an accepted alternate representation) is suspension-feeder marine bivalve in the order Venerida, family Veneridae [101]. Ruditapes philippinarum is native to the subtropical and temperate coastal seas of the western Pacific and parts of the Indian Ocean and has natural populations distributed from the Philippines to the southern Kuril Islands [223].
Since the early 20th century, R. philippinarum has become established on the Pacific coast of North America, the Atlantic coast of Europe, and in the Mediterranean Sea as a result of human activities associated with the aquaculture and fishing industries. Due to its high fecundity and growth rate, it has become established in the most suitable habitats, such as coastal lagoons. The decline in fishing and aquaculture of Ruditapes decussatus (carpet clam) native to Europe led to the introduction of R. philippinarum into European waters (first in France for commercial farming in 1972) [224]. In France, intensive culture of this species began in the early 1980s [225,226]. As a result of the successful culture and establishment of R. philippinarum in northern France, the species was introduced into the Thau Lagoon on the French Mediterranean coast in 1980 [98]. Subsequently, aquaculture demand led to importation into Spain, the United Kingdom, Norway, Germany, Belgium, Israel, Tunisia, and Italy [99,227]. In 1983, R. philippinarum was introduced into the northern Adriatic (Venice Lagoon) to supplement local fisheries of the autochthonous R. decussatus [58,91,99] and colonized the lagoons of the northern Adriatic in a relatively short time [228][229][230].
In Slovenia, R. philippinarum was first introduced in 1993 in an abandoned salt pond near the mouth of the Dragonja River [231] and spread to the coastal wetland of the Škocjan Inlet [62]. The first record in Croatian waters occurred in 2013 in Zelena Laguna (on the western Istrian coast) [100], and this location is about 100 km in a west-easterly direction from the site of the first introduction in the Adriatic Sea. In Zelena Laguna, R. philippinarum colonized the intertidal sand substrate together with the native species R. decussatus, and R. philippinarum accounted for 12% of the total Ruditapes spp. collected [100]. In Albanian waters, it has been frequently observed in the Vlora Bay since 2013 [53], and the first record in Montenegro dates back to 2015 [64]. As the species has been observed in Italy since 1983 and in Slovenia since 1993, the late first detection in these Adriatic countries is probably due to the lack of monitoring or publication of data [58].
According to [77], the primary pathway is Escape from confinement: Aquaculture/mariculture and secondary pathway is Unaided: Natural dispersal. According to [80], the pathway in the Adriatic Sea is 100 % Transport-Contaminant: Contaminant on animals. A planktonic larval stage allows local dispersal once it is naturalised.

Impacts on Biodiversity and Ecosystem Services
Ref. [13] listed R. philippinarum as one of the 100 worst invasive species in the Mediterranean region. Refs. [5,14] summarised the impacts of R. philippinarum on biodiversity (including competition for resources, creation of new habitats, and hybridization) and concluded that it has positive and negative impacts on the multispecies level and on ecosystems as an ecosystem engineer (bioturbator). Ruditapes philippinarum increases bioturbation rates, sediment mixing, and oxygen and nutrient levels in the water column and can positively affect primary production [14]. The positive effects of R. philippinarum are related to its ability to serve as an abundant prey source for other species. Ref. [14] assessed the impacts of the species on marine ecosystem services in terms of positive impacts on food provision, water purification, and climate change, as well as positive and negative impacts on ocean nourishment, while [5] added positive impacts on cognitive benefits. In addition to food provision, R. philippinarum is also used as a bioindicator of pollution because it can accumulate large amounts of pollutants harmful to humans [103].

Commercial Importance and Management
Since the late 1980s, the Manila clam has been one of the most widely farmed marine species (over 4 million tonnes and about USD 7 billion per year) in the world and the most produced bivalve [232,233] due to its ease and low cost of farming, rapid growth, short production cycle, and high profit. Manila clams are farmed for consumption live, fried, or steamed; processed via drying, salting, or canning; or used to produce clam extracts, while the shell is used to produce calcium-rich ash for use in animal feed formulations [103]. China is the leading producer, but R. philippinarum is also important in European aquaculture. The culture of R. philippinarum has been preferred to the culture of R. decussatus because it has a high growth rate, can easily obtain seeds from controlled propagation, and is more tolerant of temperature, salinity, and substrate fluctuations [234]. In 2018, Ireland and Italy (about 31,000 tonnes and 20,000 tonnes, respectively) were the main producers of cultivated Manila clam in the EU [232].
In the lagoons of the northern Adriatic Sea, environmental conditions (abundance of benthic pinnate diatoms) [230] were favourable for the growth and natural reproduction of the introduced R. philippinarum [30]. Since early 1985, R. philippinarum has been harvested and reared in this area by local fishermen organised in cooperatives. In the Venice Lagoon, where rearing was spatially limited, R. philippinarum was intensively and illegally fished with illegal fishing gear (hydraulic dredges) [30]. This intensive illegal fishing is a source of social problems (conflicts between commercial and casual fishers), but also a source of environmental problems. In addition, illegal fishing also poses a risk to consumers because shellfish do not undergo the required sanitary inspections before reaching the market [29]. Food fraud often occurs in markets because R. philippinarum is easily confused as R. decussatus, and in Italy, both species are marketed under the same name, vongola verace, although the price of R. decussatus is much higher than that of R. philippinarum due to the better organoleptic characteristics of R. decussatus [235].
Because R. philippinarum tends to hybridize with native generic species [236], hybridization could lead to local extinction of R. decussatus in areas where R. philippinarum is intentionally introduced to increase commercial harvest [237]. Therefore, [31] suggested prohibiting the release of non-native R. philippinarum in natural waters and selecting unhybridized R. decussatus for the recolonization of overfished areas. According to [193], the introduction of R. philippinarum is also associated with the introduction of hitchhiking species.
In the northern Adriatic, repeated introductions of R. philippinarum have led to the depletion and local disappearance of the native R. decussatus [60,238]. In Croatia, where the annual production of R. decussatus is several tonnes per year [239] and demand is high, especially during the tourist season, there is a risk of intentional introduction of R. philippinarum by fishermen with the aim of increasing the productivity of the isolated sandymuddy sites along the Croatian coast. Since R. philippinarum can severely affect native populations of R. decussatus in the Adriatic Sea, it is of great importance to follow its possible spread along the Croatian coast. The similarity and plasticity of the shell morphology of R. philippinarum may make identification difficult if based only on simple visual examination [237]. Therefore, morphological, morphometric, and genetic identifications should be used when planning surveillance strategies and ecological studies on the invasion of R. philippinarum.
The best management option is prevention and involves avoiding the establishment of further wild populations through public education and awareness, together with a monitoring programme, especially in MPAs [16]. The high invasive potential combined with uncertainty about the extent of the risk points to the need for further research on the impact of naturalised populations, especially in the context of future climate change scenarios [103]. In 2000, [30] created a dynamic model of the growth of R. philippinarum in the lagoons of the northern Adriatic Sea. This model is a first attempt to model the growth of R. philippinarum as a function of a number of ecological parameters in a new environment. Ref. [240] coupled an eco-physiological model [30] with a population dynamics model to analyse different harvesting scenarios. Ref. [241] created management models for the sustainable harvest of R. philippinarum in invaded European coastal systems. In the case of a population explosion in the eastern Adriatic Sea, the above models can be a valuable management tool from both ecological and socioeconomic perspectives.

Discussion
There is a considerable time lag between when a listed non-native mollusc species is first observed in the field and when it is published, which can lead to uncertainty in the analysis of introduction rates and lead to inadequate management actions and their evaluation [242]. The low detectability is probably related to a limited monitoring effort. For example, after R. venosa was recorded in Italian waters in 1973, it was reported in Croatian waters in 2004 [83]. Anadara kagoshimensis was recorded in Italian waters in 1966 and in Croatian waters in 2008 [48], and A. senhousia was recorded in Italian waters in 1992 and in Croatian waters in 2003 [46]. These time lags vary and depend on the lifestyle or other characteristics of each species. Species such as R. venosa are difficult to detect because of their cryptic nature, while others are easily misidentified with other native or non-native species (Anadara spp., M. gigas, etc.) because of the wide variability in morphological features.
Ref. [243]  . The same authors indicated that the time lag in reporting non-native species depends on the available taxonomic expertise of personnel involved in research at the country level and that some taxa require extensive work to identify at the species level, which is true for all listed molluscs.
High-impact non-native mollusc species are very successful invaders due to their biological characteristics. These species are native to tropical or temperate seas, and are eurythermal species. Given the warming trend in the Mediterranean Sea and the invasive potential of the listed species, populations of this non-native thermophilic species (especially Lessepsian migrant B. pharaonis and cosmopolite M. gigas) could continue to grow, expand their geographical range, and threaten native bivalve species in natural habitats or in aquaculture. Rapana venosa and Anadara spp. have a high tolerance to oxygen depletion and can become very abundant, especially in degraded ecosystems, as demonstrated by their successful invasion of the Black Sea. These species can colonise different types of substrates and may compete with native species and/or commercially important species for space (A. transversa, B. pharaonis, P.radiata). A problem for both native and cultivated bivalve species can be invaders that are summer spawners, due to competition for space, and all listed high-impact non-native bivalve species are summer spawners, and some are even year-round spawners (A. transversa, B. pharaonis). Because of their life history, all of the listed species can spread easily via ballast water (B. pharaonis) or tidal currents (R. venosa) and via hull fouling (P. radiata) and aquaculture products (R. philippinarum), while some are intentionally introduced for aquaculture purposes (M. gigas).
Due to their negative impacts on biodiversity and ecosystem services, all high-impact non-native species reviewed in this study are included in the list of the worst invasive species in the Mediterranean by [13], as well as by [5,14]. The impact on the biodiversity of listed species includes a negative impact on native species or entire communities through competition for resources (B. 16haraonic and native Mytilaster minimus, M. gigas, and native O. edulis), predation (R. venosa), hybridisation (R. philippinarum and native R. decussata), disease transmission (M. gigas), introduction of hitchhiker non-native species (M. gigas), and ecosystem engineering. A striking example of non-native mollusc species impact on keystone species or species of high conservation value is large numbers of A. transversa found on installed collectors for the endemic Mediterranean bivalve P. nobilis in Brijuni National Park [146]. Since P. nobilis is already at high risk of extinction due to mass mortality, even in MPAs such as Brijuni National Park, and it is an obligation to ensure its maximum possible protection and recovery, A. transversa should be closely monitored and a programme of active eradication should be considered as this serious competitor may pose an additional threat to Mediterranean endemic P. nobilis. According to the EU Marine Strategy Framework Directive, member states are obliged to implement a surveillance programme and assess a programme of measures to reduce the impact of invasive species [244]. Thorough monitoring should help detect high-impact non-native molluscs early and initiate eradication or containment before they spread further, and it should be adapted to the specific site.
Some of the listed high-impact non-native mollusc species have negative impacts on food provision via causing declines in commercial stocks through direct predation (R. venosa feeds on bivalves) or through competition for resources with (other) commercially important species (A. senhousia, B. 17haraonic, M. gigas, P. radiata). However, some of them are simultaneously of great commercial importance for aquaculture (M. gigas, R. philippinarum), fishing as target species (R. venosa, Anadara spp., and A. senhousia), or support the fishing industry through serving as baitfish for commercially important species in fishing or aquaculture (A. senhousia).
Since the beginning of 1985, R. philippinarum has been harvested and cultivated in this area in the lagoons of the northern Adriatic Sea, but it is under intense pressure from illegal fishing, which causes social and environmental problems. Various models developed for the management of R. philippinarum in invaded European coastal systems [30,240,241] can serve as valuable management tools from both ecological and socioeconomic perspectives.
Next to R. philippinarum, M. gigas is the most important species in shellfish production. It is important to stress that M. gigas is a hardy species that has saved oyster production in France, which is not only important for gastronomy but is also part of tradition and culture. In some areas (Australian New South Wales, Wadden Sea), M. gigas is considered a pest or noxious species and management programmes have been adopted to eradicate it. In contrast, in France, even wild spat is protected and carefully managed. Therefore, management approaches should not only be adapted to the socioeconomic context but also to the respective habitat. It is obvious that best practises are not always suitable for other sites, as one solution does not fit all. Although M. gigas was introduced to Croatia for experimental purposes, it has never been grown commercially in Croatian farms. Considering the possible impact of M. gigas on native oyster species and the invasiveness of the species, it would be good to regularly monitor sites of major importance such as aquaculture facilities and MPAs. There have been some efforts to establish monitoring of the native flat oyster and Pacific oyster larvae in the plankton of Lim Bay (Croatia), but these have been sporadic initiatives for specific projects or research, rather than systematic, continuous monitoring that can provide a complete insight into the population in the area. It is important to emphasise that after the first detection of the species, there is a relatively large gap in the literature on the status and distribution of the species due to the lack of monitoring. Researchers are aware of this gap in the literature and are trying to fill it (especially for M. gigas).
In addition to R. philippinarum and M. gigas, Rapana venosa, Anadara spp,. and A. senhousia are good candidates for commercialisation in case of expansion due to their fast growth, great ecological plasticity, stress tolerance, and well-developed market. Rapana venosa and bivalves of the order Arcoida are delicacies in the Asian market. These species could potentially open up new markets in Mediterranean and European countries. Currently, there is no regular accumulation of data on these species in the eastern Adriatic. In the event of increased commercial interest, a comprehensive study should be carried out on the size of the total and commercial stocks, the period of active reproduction, the size and weight structure, and control and monitoring measures. Although R. venosa is classified as a high-risk species for the EU and was originally considered a marine pest in nonnative areas, the Black Sea case study shows that invasive species, once considered a pest, can become a valuable asset and a protected (sustainably managed) species (licences and size limits on catches) [245]. Commercial harvesting is an option for the management of high-impact non-native molluscs, but it is not always the best. If commercial harvesting is chosen, an appropriate collection method and potential market should be identified, and the potential risks associated with commercial use should be clarified. Collection methods should be sustainable (the example of unsustainable fishing of R. venosa in the Black Sea) and adapted according to taxa (the example of different eradication methods for M. gigas in the Wadden Sea).
In addition to human consumption, bivalves can also be harvested commercially as fish bait or feed in agriculture and as fertiliser (A. senhousia). Thus, for species that have no commercial importance for humans (B. pharaonis), new uses may be found, and the commercial harvest of these species may prove to be a possible control method. The shells of molluscs can be sold as souvenirs and raw material for local craftsmen (R. venosa), as building material for artificial reefs (M. gigas), or as biomaterial for research into potential pharmaceutical products or for the biomonitoring of heavy metal contamination (P. radiata). One example of successful management is how The Australian National Control Plan for A. senhousia highlighted the importance of a research and development (R&D) strategy in control and monitoring.
When the management of invasive species involves species with high impact potential, a risk assessment is always recommended to raise awareness and prioritise action. These should not only focus on the threat posed by high-impact non-native species, but also on the benefits of control measures, especially for biodiversity and commercial activities. The management of high-impact non-native molluscs is a complex and serious social, economic, and political challenge. The complexity of species interactions and their impacts makes environmental management decisions difficult and sometimes controversial. It is therefore crucial to use the best available science and to take into account the interests of all stakeholders.

Conclusions
The present study collects scientific information on non-native molluscs in Croatian waters that are classified as high impact according to the European Alien Species Information Network (EASIN). The study aims to address the gap in the literature on highimpact non-native molluscs in the eastern Adriatic Sea through reviewing the literature on experiences from other European Seas, with a focus on the possibility of commercialisation and sustainable management of high-impact non-native molluscs.
Invasion of non-native species can create new business opportunities, but it must be properly monitored and managed. Management approaches need to be prioritised in a targeted manner, taking into account the species, location, and feasibility of the proposed measures. More specific national or local measures need to be taken to protect vulnerable sectors or sensitive habitats in accordance with environmental factors and the ecological status of the habitat.
Although the management of targeted invasive species should include tailored approaches such as a species-based approach and a site-based approach, eastern Adriatic countries should also learn from other countries/regions-lessons learned from case studies of other countries are better than lessons learned by themselves! Unfortunately, the initiative to monitor non-native species is usually only taken into account when the invasion of non-native species poses an obvious ecological problem and/or has a clear impact on ecosystem services (fisheries or tourism) in a given area. This overview study could not only help researchers, decision-makers, and the public to address the problems of high-impact species in Adriatic countries, but also in other regions where the systematic and continuous monitoring of high-impact non-native species is still lacking.
Author Contributions: All authors contributed to the study conception and design. Conceptualization (the idea for the article) and writing-original draft preparation, G.J.M.; writing-review and editing (critically revised the work), V.N. and A.D.; resources (the literature search and data analysis), G.J.M., V.N., and A.D. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflicts of interest.