General Impact
When Styela clava populations explode they often out-compete many native species for food. S. clava can reach densities of 500-1500 individuals per square metre. These extreme densities can have negative impacts on native and aquaculture species through competition for space and food, as well as predation of larvae from the water column. S. clava invasiveness is enhanced through its hardy nature; capable of withstanding salinity changes and temperature fluctuations (JNCC, 1997; NIMPIS, 2002).It can also occur as fouling on vessels, aquaculture and fishing equipment and other artificial structures. Dense fouling on fishing equipment, moorings, ropes, etc. can be time consuming to remove and can result in tangling of fishing gear. Hull fouling increases drag on vessels, requires an increase in the frequency of hull cleaning, and increases fuel costs. In Japan it has been known to impact human health causing an asthmatic condition in oyster shuckers when hammering open Styela fouled oysters in poorly ventilated areas (NIMPIS, 2002).
Location Specific Impacts:Prince Edward Is. (Canada) (Canada) 
Economic/Livelihoods: The sheer weight of the tunicates poses challenges for harvesting and farm husbandry. This pest could have a devastating effect on the sustainability of the entire Prince Edward Island (PEI) mussel industry (Davidson et al UNDATED).
Human nuisance: Styela clava has especially had a negative impact on mussel culture in Murray River, attaching in high densities to mussel socks and equipment, competing for food resources and fouling equipment. Processing mussel socks with tunicates has proved challenging because of the necessity for physical removal and prevention of spread to other unaffected areas" (Davidson et al UNDATED). Cork Harbour (Ireland)
Human nuisance: Styela clava is a pest to oyster farmers due to its fouling of oyster beds (Parker et al 1999). New Zealand
Economic/Livelihoods: Styela clava is a threat to New Zealand's $300 million aquaculture industry as it competes for food with shellfish and eats their larvae (NZPA, 2005).
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