Invasive Amphisteginids conquer the Sicily Channel and knock on the door of the Western Mediterranean Sea

This study reports an updated overview of the current distribution and establishment status of the invasive species Amphistegina lobifera Larsen, 1976 and of other non-indigenous foraminifera in the Sicily Channel. The dispersal of marine non-indigenous species (NIS) by human activities is redefining the biogeography of the oceans and is one aspect of global change. Understanding the role of NIS in altering the structure of marine communities requires accurate information on their temporal occurrence, spatial distribution and effect on native ecosystems. However many NIS, particularly those belonging to small-sized unicellular taxa, such as benthic foraminifera, are largely unrecognized and undetected, and the lag time between their first occurrence in a new area and their first record may be several years or even decades. Moreover, little is known about their potential impact on indigenous communities and how these small-sized NIS could affect habitats. Most of the non-indigenous foraminifera known from the Mediterranean Sea are native to the Red Sea or Indo-Pacific region, having crossed the Suez Canal and established along the eastern Mediterranean coasts, progressively spreading westwards and northwards. The most widespread and successful taxon belongs to the large algal symbiont-bearing genus Amphistegina and particularly to the species A. lobifera. Amphisteginids have so far been reported from the Levantine basin and the Central Mediterranean Sea (Tunisia, Malta and the Pelagian islands). Here, we report new data showing a notably increased distribution range in the Central Mediterranean. In summer-autumn 2017, we collected algae and sediment samples from shallow coastal habitats along the shores of the Maltese archipelago and Southern Sicily, as well as from Pantelleria and Favignana islands. The analysis of the foraminiferal content showed that A. lobifera is effectively established in Malta and Sicily and has reached the Pantelleria and Favignana islands which are at the oceanographic boundary between the Central and Western Mediterranean, suggesting that its spread in the coastal waters of the western Mediterranean is imminent. Our results also show that the thermophilic A. lobifera is at an advanced stage of invasion in the Sicily Channel, having probably been favoured by the recent sea surface temperature increase and isotherm shifts recorded in the Mediterranean basin, which has implications for other thermophilic non-indigenous foraminifera in the region.