History of the introduction of a species resembling the benthic foraminifera Nonionella stella in the oslofjord (Norway): morphological, molecular and paleo-ecological evidences

Specimens resembling the benthic foraminifera Nonionella stella (Cushman and Moyer, 1930), a morphospecies originally described from the San Pedro Basin, California, USA, were observed for the first time in the Oslofjord (Norway) in 2012. This study investigates the Oslofjord Nonionella population in order to confirm its non-indigenous species (NIS) status and assess its introduction time. Morphological characterisation based on SEM imaging complemented by molecular identification using small subunit (SSU) rDNA sequencing and assessment of the recent past record (sediment core), were performed on material collected in the Oslofjord in 2016. Examination of the dead fauna showed that specimens resembling N. stella only appeared recently in the Oslofjord, confirming the NIS status of this population. Moreover, DNA results indicate that the Oslofjord specimens differ genetically from N. stella sampled in the Santa Barbara Basin (California USA). Hence, we propose to use the name Nonionella sp. T1 for the specimens sampled in the Oslofjord for the time being. In the southern part of the Skagerrak, specimens morphologically similar to Nonionella sp. T1 were reported as NIS in the Gullmar fjord (Sweden) in 2011 and in the Skagerrak in 2015. Molecular data indicate that the two populations from Gullmar- and Oslofjords are identical, based on their SSU rDNA sequences. In addition, analyses of foraminiferal dead assemblages suggest that the population from the Gullmar fjord settled prior to the Oslofjord population, i.e. ~ 1985 and about 2010, respectively. This implies that Nonionella sp. T1 may have been transported from Sweden to Norway by northward coastal currents.