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 nonindigenous 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.