Introduction
Aquaculture, particularly mollusc farming, is a major pathway for the introduction and spread of non-native species (NNS) in the Mediterranean Sea (Katsanevakis et al., 2014). Numerous NNS are mollusc pests and parasites that can negatively affect aquaculture production (Mikac et al., 2025). The accidental transport of these hitchhiking species is facilitated by transfer of adult and juvenile molluscs among farms. Tracking their presence and spread remains challenging since many are cryptic species and require molecular analyses for reliable identification. While traditional monitoring approaches face several limitations, innovative methods have recently emerged that leverage the knowledge of local communities (Azzurro, 2018). Limited information on the occurrence of NNS in aquaculture systems, together with insufficient legislative measures at both national and European levels, hampers the development of effective management strategies to contain their spread across the Mediterranean basin. Here, we present the results of a joint Italian-Croatian-Slovenian survey on non-native fauna associated with mussel and oyster farms in the Adriatic Sea, combining morphological and molecular analyses with farmers Local Ecological Knowledge (LEK).
Materials and Methods
Samples were collected in 2024 and 2025 from 11 mussel and 8 Pacific oyster farms along the Italian coast, 5 European oyster farms along the Croatian and one Pacific oyster farm along the Slovenian coast of the Adriatic Sea. Macrofaunal organisms fouling mussels and oysters were identified through morphological analyses, while molecular identification using COI, 16S, 18S and 28S markers was performed on selected species of interest. In parallel, farmers' ecological knowledge was explored through a semi-structured questionnaire administered at 24 Italian mussel farms and 11 oyster farms. Questions addressed farmers' perceptions, observations, and perceived impacts of non-native and pest species, as well as their practices related to translocation of juvenile and adult molluscs.
Results
A total of 22 non-native and 12 pest species were identified in Italian mollusc farms, several of which fall into both categories. Predatory flatworms were prevalent in mussel farms, while distinct non-native Polydora worm species were found boring into oyster shells along the three surveyed coasts. Polydora caeca and P. websteri were reported for the first time in the Mediterranean. Farmers' observations supported these findings, reporting increases in flatworms, ascidians, barnacles, hydrozoans and Polydora polychaetes, which they perceive as negatively affecting molluscs and reducing their marketability. While most farmers were aware of NNS and reported their presence, only a few were able to accurately identify them. Farmers also reported translocation practices involving both seed and adult mollusks between Italy, Croatia, Greece, France and Spain, both within and beyond the Mediterranean basin.
Discussion
The high number of NNS recorded in the Adriatic mollusc farms confirms the central role of aquaculture in their spread. Several species are recorded for the first time in association with farmed mussels and oysters in the Mediterranean highlighting that these environments remain understudied and require further research efforts (Mikac et al., in revision). The first record of P. websteri and P. caeca in the Mediterranean suggests an ongoing expansion of their distribution ranges; however, their presence may have previously been overlooked due to misidentifications (Mikac et al., 2025). In this context, molecular approach proves to be essential for accurate species identification and should be implemented in future studies. Farmers' knowledge proved valuable in detecting the presence of NNS and pests and in understanding their introduction pathways (Fossi et al., 2026). However, targeted educational programs are needed to improve farmers' taxonomic skills and to promote active reporting. The frequent transfer of molluscs between aquaculture facilities, both within the Mediterranean and with Atlantic farms, likely contributes to the spread of non-native and pest species in the region. These findings highlight the need for continuous monitoring of NNS in mollusc aquaculture, stronger cross-border cooperation, closer collaboration among farmers, researchers, and authorities, and the implementation of transparent traceability systems to better manage the movement of molluscs and limit the spread of non-native species.
Acknowledgment
This research was partly supported by the Biodiversity Genomics Europe project, funded by the Horizon Europe programme (Grant Agreement No. 101059492).
References
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