Ascidiella aspersa is a solitary tunicate with a semi-translucent, greyish body covered in fine bumps (papillae) that give it a rough texture — hence “aspersa.” Adults reach 50–80 mm in height and attach by a broad base to nets, ropes, shells, and cage frames. Two prominent siphons project from the upper end. The species occurs across the northeastern Atlantic, from Norway to Portugal, and is common in the North Sea and along the Atlantic coasts of France and the British Isles. It tolerates a wide salinity range (18–35 ppt) and temperatures from 4 °C to 20 °C.
Ascidiella frequently co-occurs with Ciona intestinalis on aquaculture structures, and the two species together can dominate the fouling community on cage nets in summer. Dense tunicate coverage — 40–70 % of net surface area — blocks water flow, reduces dissolved oxygen, and forces fish to the centre of the cage where conditions are worse. On shellfish lines, Ascidiella competes for planktonic food and adds dead weight that can sink poorly buoyed dropper lines. Farms experiencing mixed tunicate fouling often face the highest cleaning costs of any fouling type — estimate yours with the cost calculator.
Air-drying for 8–12 hours kills Ascidiella tissue. Freshwater immersion for 2–4 hours is equally effective and leaves cultured mussels and oysters unharmed. Wrasse deployed in salmon cages feed on small Ascidiella, reducing re-colonisation rates between scheduled mechanical cleans. Timing treatments to precede the main settlement window (typically May–July) improves results. For a side-by-side view of tunicate control options, visit the methods comparison page. Related tunicate species are profiled in the organisms database.
Control Methods
Air-drying Freshwater immersion Manual removal Biological control (wrasse)