From its appearance, carpet sea squirt (Didemnum vexillum) is difficult to define. The species forms structures which are plant-like in appearance and remain attached to surfaces much like plants remain attached to the soil. However, carpet sea squirt is in fact an animal.
To be more specific, carpet sea squirt is a species of tunicate – a marine filter feeder. There are over 2000 species of tunicate across the World’s oceans living a variety of different lifestyles from solitary to colonial to pelagic. However only species from the genera Didemnum and Styela have been found to invade coastal waters.
As an individual, carpet sea squirt is incredibly small with a barrel-like body structure. However, individuals live in large colonies creating the appearance of one uniform large structure. The species feeds of planktonic particles which it extracts by filtering sea water through its barrel body.
This squirt is capable of smothering native sessile communities by overgrowth – suffocating and starving the native colonies underneath. This is a particular concern for the aquaculture industry as carpet sea squirt readily overgrows aquaculture equipment and mussels, costing the economy. Furthermore, valuable aquaculture species such as the Bay Scallop avoid settling on carpet sea squirt where it is locally abundant. This could leave large areas of sea bed unsuitable for this favourable species and reduce the species diversity of our coastal habitats.
Carpet sea squirt is currently only found in nine marinas across the UK, as well as in the Netherlands, France, Spain and Ireland. Its distribution is concentrated around marinas and ports as it is transported on the hulls of boats or in ship ballast water. If you happen to visit any coastal habitats around Europe keep your eyes peeled for this species and report it using our Smartphone app
Here is a guide to identifying the species
Lengyel, N. L., Collie, J. S & Valentine, P. C (2009). ‘The Invasive colonial ascidian on George’s Bank – ecological effects and genetic identification’ Aquatic Invasions 4: 143-152
Morris, J. A., Carman, M. R., Hoaglands, E., Green-Beach, E. R. M & Karney, R. C (2009). ‘Impact of the invasive colonial tunicate on the recruitment of the bay scallop and implications for recruitment of the sea scallop on George’s Bank. Aquatic Invasions 4: 207-211
Photo Credits: Carpet Sea Squirt (CCW), Carpet Sea Squirt (CCW), Carpet Sea Squirt (CCW - Rohan Holt)