Matching field-based ranges in brackish water gradients with experimentally derived salinity tolerances of Conrad’s false mussel (Mytilopsis leucophaeata cochleata) and zebra mussel (Dreissena polymorpha)

The invasive alien false mussels Mytilopsis leucophaeata cochleata and Dreissena polymorpha (Dreissenidae) have established populations in the North Sea canal in the Netherlands that connects the harbours of Amsterdam with the North Sea. The favourable and unfavourable salinity ranges of both species were earlier studied in long-term outdoor mesocosm experiments. Their occurrence in salinity gradients in estuaries or canals connecting seaways to freshwater harbours provides information on their salinity tolerance under field conditions. By the combination of laboratory experiments and field data using the same source population a high predictability can be expected for establishment of the gradients facilitated by constructions. The reliability of experimentally derived salinity-tolerance limits for both dreissenid species was tested using data on their distribution in a salinity gradient of the littoral zone along the North Sea canal. The mussels used for the survival experiments in mesocosms were also collected from this canal. Favourable salinity ranges for adult survival in the mesocosms were 0.2 – 17.5 for M. leucophaeata cochleata and 0.2 – 6.0 for D. polymorpha. Unfavourable salinities were outside these ranges and led to high and fast mortality of these species. Mytilopsis leucophaeata cochleata was present over nearly the whole length of the North Sea canal with the highest densities close to the sea sluices where also the highest salinities and water temperatures were measured. Their densities in the canal decreased gradually at larger distances from the sea. Dreissena polymorpha co-exists with M. leucophaeata cochleata at the east end of the canal with low salinity due to the influence of freshwater of the river Rhine. The occurrence of D. polymorpha was restricted to a salinity below 4 and M. leucophaeata cochleata only occurred at a salinity above 1.5 (maximum value measured in the canal 9.2). Shorter salinity gradients with lower salinity ranges provided additional information on the co-existence of both species. Co-existence was observed at a salinity range of 1.5–3.3 (own data), 1.0–3.5 (Van Couwelaar and Van Dijk 1989), both in the North Sea canal, and 0.2–2.8, in the Canal through Voorne (Janssen and Janssen-Kruit 1967). These data correspond with studies of both species by Walton (1996) in the Hudson River (salinity range 0–3). Found salinity ranges in the North Sea canal for both species match with the tolerance results obtained by mesocosm experiments. A new invading dreissenid mussel Dreissena rostriformis bugensis and a mytilid Ischadium recurvum occur in the North Sea canal since 2006 and 2012, respectively. Competition between recent and earlier invaders is likely when salinity tolerances are similar. It has already been observed that D. rostriformis bugensis outcompetes D. polymorpha under freshwater conditions (Bij de Vaate et al. 2014; Matthews et al. 2014). Ischadium recurvum has the potential to colonize large parts of the canal and to be a strong competitor of M. leucophaeata cochleata (Goud et al. 2019). Since January 2022, the new ‘Zeesluis IJmuiden’ with the biggest locks in the world is in use, affecting the probability of population establishment of new introduced and invasive alien mussel species.