Botryllus schlosseri is a colonial tunicate that forms flat, encrusting sheets on hard surfaces. Individual zooids — each 2–3 mm long — are arranged in distinctive star-shaped clusters of 5–12 around a shared exhalant opening. Colony colour varies widely: orange, purple, brown, or green. The species is native to the northeast Atlantic and Mediterranean and thrives in sheltered, low-energy waters at temperatures above 10 °C. A colony can double its surface area in as little as two weeks through asexual budding.
Star ascidians are a serious concern for shellfish growers. On mussel ropes and oyster bags, Botryllus colonies spread over the shells, block the valves, and compete for the same planktonic food the stock depends on. Heavily fouled mussels lose 15–30 % of their growth rate and require extra handling labour at harvest. On cage nets, Botryllus contributes to mixed soft-fouling communities alongside Ciona and Ascidiella, collectively restricting water flow. Use the cost calculator to model the financial impact on your production.
Control options include air-drying (effective after 4–8 hours in direct sun), brine dipping at 70–80 ppt for 10–20 seconds, and manual scraping. The colonial growth form means even a small surviving fragment can regenerate a full colony, so thorough removal matters. Anti-fouling coatings applied to nets and trays reduce initial colonisation, and timing treatments to the spring settlement window improves results. Compare all tunicate control strategies on the methods comparison page, or read more about related species in the organisms database.
Control Methods
Air-drying Brine treatment Manual removal Anti-fouling coatings