Ascidians are the largest and most diverse class of the subphylum Tunicata, and are important components of marine benthic communities. They are also renowned worldwide invaders, of growing concern due to ease of spread and impacts on native biota. We hereby combined bathymetric and habitat mapping, extensive transect and photo-quadrat sampling, and a morphological and molecular study on selected specimens to evaluate taxonomic composition, cover, and distribution pattern of ascidians in a marine reserve of the central-western Mediterranean Sea, the Miseno Lake Bacoli, Naples, Italy.
Twenty-four taxa were found, of which 9 were non-indigenous species NIS , 5 cryptogenic, and 10 native, delineating the Miseno Lake as a major hotspot of NIS and cryptogenic ascidians.
DNA barcoding yielded sequences, confirming identification for 22 taxa but also revealing single to few mismatches per species.
Taxonomic insights were offered in the text and the Supplementary Material for several taxa, including the understudied Ascidia colleta and Ascidia malaca. The presence of a new clade was discovered in the Distaplia bermudensis species complex. Didemnum pseudovexillum and Polyclinum constellatum were respectively first recorded in Italy and western Mediterranean, and the same holds true for other taxa whose accurate identification was only possible through molecular evidence. The analysis of photographic archives allowed backdating and georeferencing to the Miseno Lake of the first records of Aplidium accarense and Botrylloides niger in Italy and of P.
Generalized Additive Models revealed that the abundance of NIS increased with availability of hard substrates rocks or litter , decreased with depth, and was the highest in the channels connecting to other water bodies, followed by the peripheral areas of the lake. No certain evidence of competitive restriction of native ascidians was found.
