Didemnum vexillum (D. vexillum) is a colonial ascidian, commonly known as carpet sea squirt or sometimes sea snot. It usually grows as a yellowish mass (but colour is highly variable) with many holes in it, covering whatever it is growing on. The holes are used for siphoning water to feed. It will grow as a covering over surfaces but may get quite thick (10cm), producing 'arms' or tendrils up to 1m long that hang below the main mass. It is characterised by a rubbery outer tunic made of polysaccharide cellulose (Turrell et al., 2018). D. vexillum is a filter feeder, feeding on phytoplankton and suspended organic matter. It is known to grow rapidly and aggressively on all manner of substrates. It can smother other sessile species including farmed molluscs, modify habitats it colonises and foul man-made structures, infrastructure and vessels (Turrell et al., 2018).

D. vexillum can restrict water exchange through nets, bags or ropes used to contain shellfish or salmon, reducing food supply and decreasing internal water quality (Turrell et al., 2018). Buoys and ropes can become weighed-down with this and other fouling species. Infestations lead to increased maintenance and management costs for aquaculture, as well as loss of income associated with reduced growth or increased mortality of the aquaculture species (Fletcher et al. 2013b).