Radiocarbon dating of individual foram tests show that alleged Lessepsian species are of Holocene age

The Lessepsian invasion – the largest marine biological invasion – followed the opening of the Suez Canal in 1869 (81 years BP). Shortly afterwards, tropical species also distributed in the Red Sea appeared on Mediterranean shores: it was the dawn of what would become the invasion of several hundred tropical species. The time of the Suez Canal opening coincided with an acceleration in natural history exploration and description, but the eastern sectors of the Mediterranean Sea lagged behind and were thoroughly explored only in the second half of the 20^th century. Many parts are still insufficiently studied today. Baseline information on pre-Lessepsian ecosystem states is thus scarce. This knowledge gap has rarely been considered by invasion scientists: every new finding of species belonging to tropical clades has been assumed to be a Lessepsian invader.

We here question this assumption by radiocarbon dating seven individual tests of miliolids – imperforated calcareous foraminifera – belonging to five alleged non-indigenous species. Tests were found in two sediment cores collected at 30 and 40 m depth off Ashqelon, on the Mediterranean Israeli shelf. We dated one Cribromiliolinella milletti (core at 40 m, 20 cm sediment depth), three Nodophthalmidium antillarum (core at 40 m, 35 cm sediment depth), one Miliolinella cf. fichteliana (core at 30 m, 110 cm sediment depth), one Articulina alticostata (core at 40 m, 35 cm sediment depth) and one Spiroloculina antillarum (core at 30 m, 110 cm sediment depth). All foraminiferal tests proved to be of Holocene age, with a median calibrated age spanning between 749 and 8285 years BP. Only one test of N. antillarum showed a 2-sigma error overlapping the time of the opening of the Suez Canal, but with a median age of 1123 years BP. Additionally, a thorough literature search resulted in a further record of S. antillarum in a core interval dated 1820–2064 years BP in Turkey.

Therefore, these foraminiferal species are not introduced, but native species. They are all circumtropical or Indo-Pacific and in the Mediterranean distributed mostly in the eastern sectors (only S. antillarum occurs also in the Adriatic Sea). Two hypotheses can explain our results: these species are Tethyan relicts that survived the Messinian salinity crisis (5.97–5.33 Ma) and the glacial periods of the Pleistocene in the Eastern Mediterranean, which may have never desiccated completely during the Messinian crisis and which may have worked as a warm-water refugium in the Pleistocene; or they entered the Mediterranean Sea from the Red Sea more recently but before the opening of the Suez Canal, for example during the Last Interglacial (MIS5e) high-stand (125,000 years BP) when the flooded Isthmus of Suez enabled exchanges between the Mediterranean and the Indo-Pacific fauna. The recognition that some alleged Lessepsian invaders are in fact native species influences our understanding of the invasion process, its rates and environmental correlates.