Didemnum vexillum is an aggressive, rapidly growing colonial ascidian and regarded as a global alien invasive species in temperate waters. It has recently become established in the western Mediterranean and the vectors of its introduction were assumed to be shipping or oyster trade. A dense settlement of it was encountered on nets of the bluefin tuna (Thunnus thynnus) cages placed at 60–65 m depths off the İzmir Peninsula (eastern Aegean Sea, eastern Mediterranean) in December 2022. It had considerably clogged net's eye openings, hindering water circulations inside cages. It had a vertical distributional pattern on 35 m long-nets, occurring solely on depths from surface down to 15 m, around where a summer thermocline develops. It has entirely replaced the native black mussel Mytilus galloprovincialis on nets. This colonial ascidian changed the routine cleaning procedure of nets in the farming. Three possible ways of its introduction to the eastern Mediterranean were proposed, but the most reasonable one is its secondary transfer via nets or ships from Malta. Mechanisms of its invasion biology and behaviour should be studied and monitored in the region.

The European sea squirt, Ascidiella aspersa was first found as an alien species in 2008 from Funka Bay, Hokkaido, northern Japan, causing serious damage to the scallop aquaculture industry. We investigated A. aspersa on cultured scallops and larval occurrence from July 2010 to June 2014 to clarify life history traits and population dynamics, and consider the relation between the life history of A. aspersa and the process of scallop aquaculture. Larvae of A. aspersa were found from June to December, and recruitment on cultured scallops occurred mainly between July and October. The ascidians grew well and their weights increased until February. We found that 60–80% of A. aspersa that had settled in summer had eggs or sperm in autumn, and 90–100% of A. aspersa matured early the following summer. Maturity size in September was 17–20 mm as male, 22–24 mm as female. Scallops in Funka Bay are hung in the spring and harvested from winter to the next spring. Ascidiella aspersa settle as larvae in early summer, and grow well until winter, resulting in overgrowth on scallops in the harvest season. The linking of the process of scallop aquaculture and the life history of A. aspersa explains why this invasive ascidian has caused serious damage to the aquaculture industry in the bay. In comparison to the earlier descriptions of the native population, A. aspersa in Funka Bay has longer reproductive and growth periods, earlier initiation of reproduction, and possibly smaller maturity size.