Dias et al., (2021) suggested that Didemnum perlucidum may have been introduced into Australia in multiple introduction events (e.g. Northern Territory and Western Australia) with subsequent admixture and spread via anthropogenic vectors. There is a discontinuous distribution along the coastline of WA. WA Department of Primary Industries and Regional Development – DPIRD continue to monitor D. perlucidum as part of the introduced marine pest monitoring program.

D. perlucidum was also detected (October 2019) in Queensland. At present it is possible that this is a separate introduction to the region.

Didemnum perlucidum, recorded from Gove in the Northern Territory in 2007 and the Swan River Estuary in Perth in 2010, is a colonial sea squirt comprising many small individual sea squirts (<2 mm long) or zooids, in a single mat or colony. D. perlucidum colonies are generally thin (<2 mm) and encrusting. This species colonises and overgrows artificial and natural habitats (Bridgwood et al., 2014). It encrusts organisms, such as mussels, barnacles or tube worms. Colonies that attach to floating structures (e.g. floats and pontoons) can grow down in drooping tendrils. It can reach a diameter of many cm in size as a continuous sheet or in winter as colonies senesce (deteriorate), thinner discontinuous sheets.

Colony morphology and colour can vary depending on environment, season and reproductive state (Muñoz and McDonald, 2014). Colonies are usually white but can vary from grey/brown to an orange tinge. Due to morphological variations, particularly in cooler waters of temperate WA, colour as a morphological feature is not recommended for positive field identification (Bridgwood et al., 2014). Colonies appear thicker when they are reproductive with larvae (Bridgwood et al., 2014). Where it is introduced, this species is commonly associated with disturbed habitats such as marinas, harbours and aquaculture structures (Kremer et al., 2010). It is a common fouler on shellfish farms in Brazil, and is known to grow heavily over mussels and oysters and farmed mussel spat in Western Australia (Muñoz and McDonald, 2014). It also grows on pearl farm infrastructure in the Northern Territory, and buoys and mooring lines in Queensland.