Introduction
Throughout Europe, the dominant species of mussels present are the blue mussel Mytilus edulis, to the north, and the Mediterranean mussel Mytilus galloprovincialis, to the south. galloprovincialis was first reported to have expanded its range into Ireland in the 1970s (Gosling and Wilkins, 1977 In Ireland, M. edulis is predominantly found at sheltered sites (Gosling and Wilkins, 1981; Gosling et al., 2008; Lynch et al., 2020), and on wave-exposed Irish shores it intermixes with M. galloprovincialis in varying proportions, and this may also be seen worldwide (Gosling and Wilkins, 1981; Gosling, 2004). During the past three decades, reduced abundances of wild Mytilus have been observed, with total absence being reported along shorelines from countries in the North Atlantic, as well as local variation, however, some areas have experienced recovery after declining events(Baden et al., 22021) Until relatively recently, the distribution of Mytilus on the Norwegian coastline was identified as solely consisting of Mytilus edulis (L.), with an M. edulis/Mytilus trossulus (Lmk.) hybrid zone on the Swedish coastline, and Mytilus galloprovincialis within the Mediterranean (Gosling 1992). However, recent investigations have discovered a very different and often patchy distribution, with M. trossulus found on the west coast of Norway (Ridgway & Naevdal, 2004) and even as far north as the Arctic Circle in the Barents and White seas (Kijewski et al,2011; Va¨ ino¨ la¨ & Strelkov, 2011. The overarching aim of this survey was to determine the length and weight, and mussel species composition and abundance present in Ireland (Roaring Water Bay) and Norway (Rauhagen).
Methodology
A total of 600 mussels were collected from each location (Ireland and Norway) between April 2023 and June 2024 from Roaring Water Bay (Ireland) and Raundagen (Norway). At the laboratory, the length and weight of each mussel were measured prior to molecular analysis. Molecular analysis was performed to identify mussel species and were sampled at the laboratory. Length using a gill tissue (5 mm2) excised from each mussel, and DNA was extracted using the chelex-100 extraction method (Walsh et al., 1991). PCR was carried out to detect the nuclear DNA markers Me15/Me16 (Inoue et al. 1995). The PCR mastermix was modified slightly to include 0.5 microliter of the primer Me15 (5’CCAGTATACAAACCTGTGAAG-3’), 0.5 microliter of the primer Me16 (5’-TGTTGTCTTAATGGTTTG TAAGA-30), 10 microliter of the master mix, 14 microliters of ultrapure water and 2 microliters of total DNA. The following conditions were used for the PCR in a thermocycler: 94 °C for 30 s, 55 °C for 45 s and 70 °C for 90 s (40 cycles). The amplified products were electrophoresed, showing specific bands for M. edulis (180 bp), M. trossulus (168 bp), and M. galloprovincialis (126 bp), with the presence of 2 bands suggesting Mytilus hybrids.
Results
In Roaring Water Bay (Ireland), across all sampling times, M. edulis consistently dominated the mussel population, maintaining a high percentage (approximately 70–80%). M. galloprovincialis appeared in much lower proportions, typically around 5–10%, while hybrids made up the remainder. In Norway (Raudhaugen), Mytilus edulis and Mytilus galloprovincialis were present at all sampling times. The high proportion of M. edulis suggests it remains the dominant species in Raudhaugen Norwegian mussel populations, with M. galloprovincialis maintaining a consistent minority presence, and no hybrids were detected. There is no statistically significant difference in mussel lengths and weights between the two locations.
Discussion
The present study aimed to characterise the mussel species composition at two distinct geographic sites: Roaring Water Bay, Ireland, and Raudhaugen, Norway. Our findings revealed significant site-specific differences in species presence. In Roaring Water Bay, Mytilus edulis consistently dominated the mussel population throughout the sampling period. Mytilus galloprovincialis maintained a relatively low presence, while hybrids were also detected, making up the remainder. Importantly, a minor but noticeable increase in hybrid proportions was observed in October 2023 and June 2024. This seasonal fluctuation may indicate an environmental influence on hybridization rates, potentially linked to variations in sea surface temperature, reproductive timing, or changing population dynamics. In contrast, the Norwegian site at Raudhaugen demonstrated a different pattern. Although both M. edulis and M. galloprovincialis were present across all sampling times, M. edulis remained dominant, with no hybrid individuals detected. The absence of hybrids suggests strong species boundaries in this region, possibly due to lower environmental pressures favouring hybridization, reduced M. galloprovincialis reproductive success in colder conditions, or geographical isolation limiting gene flow between species..Overall, the dominance of M. edulis at both sites underscores its resilience; however, the observed hybridization in Roaring Water Bay signals potential genetic shifts within populations that warrant further long-term monitoring.
References
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