European Ascidian

Overview

The European ascidian (Ascidiella aspersa) is a solitary sea squirt native to the northeastern Atlantic, ranging from Norway to the Mediterranean and Black Seas. Often found in dense, unfused aggregations, this species is highly adaptable and has been introduced to coastal waters worldwide, including parts of North America, Asia, and the Southern Hemisphere, primarily via ship hulls.

Appearance

This species features an oval-shaped body that can reach up to 130 mm in length. Its firm, semi-transparent test (outer covering) is typically greyish, brownish, or whitish and often appears "dirty" due to accumulated detritus and silt. The oral siphon is located at the top of the body, while the atrial siphon is positioned about one-third of the way down the side. Both siphons may exhibit lighter-colored ridges or a frilled appearance.

Distribution & habitat

Ascidiella aspersa thrives in sheltered, low-energy environments such as estuaries, harbors, and semi-enclosed bays. It frequently colonizes hard substrates, including rocks, shells, pier pilings, and artificial structures like fishing nets or boat hulls. It is known for its tolerance to varying salinity levels and its ability to rapidly dominate available space in disturbed or artificial habitats.

Biology & behavior

As a filter feeder, it plays a role in local fouling communities. While it is a solitary organism, it often forms dense clusters that can compete with native species for space, sometimes impacting aquaculture operations by overgrowing cultivated mollusks. It is typically attached to the substrate by the left side of its body.