By Vivian Husa and Beate Hoddevik Sunnset
The Asian sea squirt has gradually become quite common on ropes and wharves in the Stavanger area, while in southern Norway it has been found in several marinas in the Grimstad and Arendal area during the past two years.
It is unclear how long the species has been in Norway. In 1990 the underwater photographers Rudolf and Erling Svensen took a picture of a sea squirt on a cage line off Stavanger, but they were unable to identify the species. The specimen has now definitively been identified as a Styela clava. The species was first recorded in Europe as long ago as in 1954, and it was expected that it might also establish itself in Norway.
Consequences of the Styela clava establishment
Currently we know very little about the distribution of Styela clava in Norway, or about its impact on local communities. It is likely that the species can thrive in the southern part of Norway, where summer temperatures easily exceed 15 °C and gives conditions for a successful reproduction of the species. In areas where the species thrives, it is not uncommon to find densities of 500-1500 individuals per square metre, which means that it can outcompete local species. Styela clava may have a negative impact on shellfish farming, as it can grow in large numbers on shellfish, cultivation cages for shellfish and ropes.
In the important mussel grounds of Prince Edward Island (Canada), the establishment of the species has resulted in a significant decrease in mussel production. The sea squirt competes with the shellfish for food, and reduces water flow through the farm. Removing sea squirts represents a large additional cost for farmers, moreover the animals contain a poison that can cause respiratory problems in humans if ventilation is inadequate when the sea squirts are removed.
We have not received any reports from Norwegian shellfish farms on large problems with sea squirts overgrowing equipment. However, we do ask shellfish farmers to check whether they have this species on their farms, and to get in touch with us if they find it.
Can spread further
Styela clava's ability to spread naturally is very limited, as the larvae settle close to their parents within 24 hours. Their rapid worldwide dispersal is therefore most likely the result of human activities.Leisure boats and movement of shellfish and equipment are considered to be important dispersal vectors.
Although Styela clava’s ability to spread naturally across large areas is limited, it will probably continue to spread from port to port on boats and other equipment, as has been observed in other parts of the world.
Styela clava is quite easy to recognise on account of its banded siphons (tubes, funnel) and the leathery surface with conspicuous bumps in the upper part of the animal. Adult individuals also have an conspicuous stalk, and can reach up to 20 centimetres in length. It cannot be confused with any Norwegian species.
Introduced across the globe
The Styela clava was first recorded in Europe in 1954, when it was discovered in the British Isles. Probably it had arrived there on a warship that had taken part in the Korean War. Subsequently it has spread along the European coastline, and it has also been introduced to the US, Canada, New Zealand and Australia. In 1978 it was discovered in Denmark, and it is now quite common in Limfjorden and Kattegat.