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Compound sea squirt

Diplosoma listerianum

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If you think you have found an aquatic invasive species:

  1. take photos
  2. note:
    • the exact location (GPS coordinates)
    • the observation date
    • identifying features
  3. contact us to report it
Compound sea squirt.

Compound sea squirt. © R. Groeneveld

Compound sea squirt is an invasive colonial tunicate primarily seen in tropical and temperate waters.

Identifying features

  • Dense colonies that are soft, gelatinous and translucent;
  • Forms fragile crusts which are hard to remove without tearing;
  • Colour : darkish grey;
  • Openings sometimes spotted with white dots.

Similar species (native)

Compound sea squirt may resemble certain types of sponges, but their texture is gelatinous rather than spongy.

Compound sea squirt may also be mistaken for Jelly Bryozoan (Alcyonidium gelatinosum), but this native bryozoan does not have the characteristic openings of tunicates.

Where it has been found

The compound sea squirt has been observed for the first time on the west coast of Canada, in 2005. In 2008, it was found in Atlantic Canadian waters for the first time, off the Magdalen Islands.


Compound sea squirt grows below the level of the tide up to 80 m depth. It often attaches to algae and eelgrass, as well as any hard substrates, including rocks, vessels, and pontoons.

Compound sea squirt.

Compound sea squirt. © M. Desraspes

Compound sea squirt.

Compound sea squirt. © R. Groeneveld

Compound sea squirt.

Compound sea squirt. © M. Desraspes

Ecological and economic impacts

The potential impacts fallowing the introduction and establishment of the Compound sea squirt are widespread. Coastal ecosystems are particularly vulnerable for many reasons including high shipping and recreational traffic and disturbance events. It may outcompete other organisms for food and space, thereby altering the natural community dynamics, in a short period of time. This species threaten aquaculture, fishing and other coastal and offshore activities. It Increase the weight of the aquaculture cultivation gear, causing work to be more demanding as well as increasing the operating costs for shellfish producers and processors.

Origins and mode of arrival

This species probably originates from northern Europe and has spread along the American east coast.

Mode of dissemination

The most likely vector for introduction of this species is hull fouling.

The increases of the aquaculture facilities area that happen in the last two decades, induced an expansion of the surfaces available for colonization by the Compound Sea Squirt. It has been observed that the growth of invasive ascidians, in temperate regions, gets easier as the waters get warmer.

The most plausible vector of introduction of the Compound Sea Squirt in North America is through hull fouling, as the larvae has a short planktonic stage and thus is unlikely to survive in ballast water.

Government action

Scientific research

Fisheries and Oceans Canada is studying the compound sea squirt population to improve its understanding of how it reacts and adapts to Canadian conditions.

For further information


  • Bullard, S. G., Lambert, G., Carman, M. R., Byrnes, J., Whitlatch, R. B., Ruiz, G., & Pederson, J. (2007). The colonial ascidian Didemnum sp. A: current distribution, basic biology and potential threat to marine communities of the northeast and west coasts of North America. Journal of Experimental Marine Biology and Ecology, 342(1), 99-108.
  • Mackenzie, A.B. 2011. Biological synopsis of the compound sea squirt (Diplosoma listerianum). Can. Manuscr. Rep. Fish. Aquat. Sci. 2966: v + 18 p.
  • Therriault, T. W., Herborg, L. M., & Department of Fisheries and Oceans, Ottawa, ON (Canada); Canadian Science Advisory Secretariat, Ottawa, ON (Canada). (2008). Risk assessment for two solitary and three colonial tunicates in both Atlantic and Pacific Canadian waters. Canadian Science Advisory Secretariat. Secrétariat canadien de consultation scientifique.
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