Styela clava is a large solitary tunicate with a tough, leathery, club-shaped body that reaches 80–120 mm in height. The outer surface (tunic) is rough, warty, and brownish-grey, with two siphons at the top. A short stalk anchors the animal to hard substrates. Native to the northwest Pacific (Korea, Japan), it was first recorded in European waters in 1953 and has since invaded coastlines from southern Norway to the Iberian Peninsula, spreading mainly via hull fouling and transfer of aquaculture stock.
Club tunicates are a serious nuisance for shellfish growers. On mussel ropes, individual Styela attached between and on top of mussels add dead weight that reduces the mussel-to-fouler ratio at harvest — sometimes by 30–40 %. On oyster bags and trestles, Styela compete for space and food with the cultured stock. Dense populations of 50–100 individuals per metre of rope are recorded at heavily affected sites in Ireland, Brittany, and the southern North Sea. Fouled equipment also looks unsightly at market, and extra handling labour is needed to strip tunicates before sale. The cost calculator can estimate the financial impact for your farm.
Manual removal is the primary control method — each animal must be pulled off individually. Air-drying for 12–24 hours kills the tissue, and brine dipping at 70–80 ppt for 15–30 seconds is effective for smaller stock. Freshwater immersion for 2–4 hours kills Styela without harming mussels or oysters. Timing treatments before the summer settlement peak (June–August in most regions) limits recruitment. Related tunicate species, including Ciona and Botryllus, are covered in the organisms database. For management strategies, see the methods comparison.
Control Methods
Manual removal Air-drying Brine dipping Freshwater immersion