Abstract
The magnitude of the worldwide invasions of Didemnum vexillum Kott, 2002 has taken a number of years to be comprehended. During the past 15 years, it has been identified as different species depending on its location—D. carnulentum on the U.S. west coast, D. lutarium or D. vestum in New England, D. lahillei or D. helgolandicum in France and the Netherlands, D. vexillum in New Zealand, D. pardum or D. moseleyi in Japan. A number of recent publications refer to it as Didemnum sp. or Didemnum sp. A. This paper presents a chronology of the steps in the development of our awareness and understanding of this species based on comparative morphology and genetics, and lists invaded regions and the approximate minimum length of time it has been known in each area. Evidence is presented that D. vexillum may have originated in Japan. The importation of vast quantities of Japanese oysters and spat into many countries prior to the 1960s is discussed but eliminated as a likely vector because there are no reports of a sudden didemnid ascidian appearance prior to the 1970s. Introductions (including to the type locality in New Zealand) are very likely due to shipping (either via hull or sea chest fouling), with subsequent local spreading by fouled recreational craft, barges, etc., drifting and reattachment of dislodged fragments, and movements of fouled aquaculture stock and gear. Based on morphological and genetic comparisons of hundreds of world-wide samples, museum type specimens, and anecdotal information on the presence of this species in various locations over several or many decades, the valid name is concluded to be Didemnum vexillum Kott, 2002 due to the lack of any pre-existing published description. D. vestum Kott, 2004 is synonymized under D. vexillum.
Highlights
Fundamental to understanding the scale and impacts of biological invasions in the sea is a clear and accurate resolution of the identification of invasive species
I tentatively identified the specimens as D. lutarium, a species described from New England by Van Name in 1910
Most specimens for morphological analysis were relaxed with menthol crystals in Unraveling the identity of Didemnum vexillum sea water, fixed and stored in 10% seawater formalin buffered with sodium borate, and examined in seawater, with subsamples fixed directly in 95% ethanol
Summary
Fundamental to understanding the scale and impacts of biological invasions in the sea is a clear and accurate resolution of the identification of invasive species. It was subsequently described as a new species, Didemnum vexillum, by Kott (2002), who declared it native to New Zealand rather than an introduction. A molecular biologist at the University of Aukland, New Zealand, was the first to sequence the 18S rDNA from New Hampshire, New Zealand, and Japan samples in 2005 She found 98% sequence identity among them and concluded that they were all the same species (unpublished data). Stefaniak (2009, this issue) decided to utilize the nuclear gene Tho, and this has proven to be diagnostic at the species level She succeeded in sequencing the coI gene, and has shown that all the populations of Didemnum sp. The final tasks, were to determine the valid species name and the region of origin, concluded to be Didemnum vexillum, most likely originating from Japan, based on the data presented below
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