Modelling habitat range and seasonality of a new, non-indigenous polychaete Laonome sp (Sabellida, Sabellidae) in Parnu Bay, the north-eastern Baltic Sea

An as-yet-undescribed, non-indigenous polychaete species was found at very high densities in the eastern part of the Baltic Sea in Estonia in 2012. The species belongs to the sabellid genus Laonome Malmgren, 1866, but it could not be assigned to any of the previously described species. To date, the species has established a stable population after surviving a notably cold winter (2012/2013). To study the local distribution and abundance of the species, a spatial grid with some stations repeated seasonally and interannually was sampled in a quantitative manner. Based of the survey data and available environmental data, the variables that contributed significantly to explaining variation in the abundance of the polychaete were determined using the Boosted Regression Trees modelling approach. Molecular barcodes to characterize the identity of the species were also established. The abundance of Laonome sp. exhibited strong seasonal variation, peaking between July and November. Besides seasonality, the quantity of decomposed microalgae in the sediment and wave exposure best explained the variation in abundance. Laonome sp. is now well-established in the Baltic Sea and locally reached high densities in low salinity areas. This non-indigenous polychaete may potentially modify sediment morphology and chemistry and disrupt the natural infaunal communities. Laonome sp. could displace or even completely eliminate some species currently present in the study area and beyond if it spreads; however, it could also facilitate currently-present species through the provision of alternative substrate and/or food. Given its persistence and high abundance in Parnu Bay, colonization of other low-salinity areas of the Baltic Sea can be expected.Peer reviewe