Ascidians (Chordata: Tunicata) from circalittoral and upper-bathyal soft bottoms sampled by experimental trawling in the Iberian Mediterranean Sea

Abstract Ascidians form aggregations in soft bottoms, thereby creating a complex habitat and increasing the biodiversity by providing new substrates, food, and refuge. As sessile and filtering organisms, they are extremely vulnerable to bottom trawling, which exerts a strong pressure on the circalittoral soft bottoms and could be used as bioindicators of this impact. During the MEDITS surveys of 2010 and 2017, the ascidiofauna of the Iberian Mediterranean Sea was studied from Alboran Sea to Catalonia at depths ranging from 37.5 to 693 m. A total of 3810 specimens belonging to 10 families and 38 species were assessed. Taxocenosis demonstrated that Southeast (SE) and Northeast (NE) Iberian Peninsula host a high diversity, specific richness, and abundance of both colonial and solitary species; however, in the Gulf of Valencia, the rates were lower than those previously recorded, and the colonial strategies were scarce. This may have occurred as a result of the continental shelf being steeper in the Alboran Sea and the SE and NE zones coasts than in the shallow shelf of Gulf of Valencia, which is more prone to bottom trawling. Moreover, differences were found in terms of depth, with the highest values of the previous rates being lower than 50 m, where Spanish legislation prohibits bottom trawling at depths lower than that mentioned. For the results obtained, both the taxocenosis of the ascidians and their strategies could be good indicators of the condition of the circalittoral soft bottoms of the Mediterranean Sea impacted by bottom trawling.