|Recent population expansions of non-native ascidians in The Netherlands|In: Journal of Experimental Marine Biology and Ecology. Elsevier: New York. ISSN 0022-0981; e-ISSN 1879-1697, meer
Botrylloides Milne Edwards, 1841 [WoRMS]; Didemnum Savigny, 1816 [WoRMS]; Lamellaria Montagu, 1816 [WoRMS]; Molgula complanata Alder & Hancock, 1870 [WoRMS]; Perophora japonica Oka, 1927 [WoRMS]
Botrylloides; Didemnum; Invasive; Lamellaria; Molgula complanata; Perophora japonica
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Over the last 50 yrs seven non-native ascidians have settled in The Netherlands, concentrated in the two periods 1974–1977 and 1991–2004 (i.e., Styela clava, Aplidium glabrum, Diplosoma listerianum, Didemnum sp., Botrylloides violaceus, Molgula complanata and Perophora japonica). The year of the introduction of B. violaceus remains a matter of dispute because many of the Botrylloides specimens that are recorded in western Europe, have been identified as the closely resembling species B. leachi. Only Didemnum sp. has become a true invasive species and has become a threat to native ecosystems, especially in the province of Zeeland, by its ability to overgrow virtually all hard substrata present. This includes rocks, stones, sand, algae and almost all sessile marine animals. The sudden population expansion of the didemnid from 1996 onward, coincided with the cold winter of 1995–1996, which caused decreased population sizes of many marine animals. The resulting increase in the availability of suitable substrates for settlement and the strong decrease of grazing sea urchins, may have triggered the population expansion. Studying its population dynamics, the optimal growing temperature for Didemnum sp. appears to be 14–18 °C. Virtually all colonies die when the water temperature gets colder than 5 °C. Colonies growing on live marine animals seem to be more resistant to the cold, than those growing on rocks, stones and plants. Two potential predators of the didemnid have also been recorded in Dutch waters: the gastropods Trivia arctica and Lamellaria sp.