Molecular and Morphological Discrimination Between an Invasive Ascidian,Ascidiella aspersa, and Its CongenerA. scabra(Urochordata: Ascidiacea)

Abstract

The solitary ascidian Ascidiella aspersa (Müller, 1776) has sometimes been regarded as conspecific with A. scabra (Müller, 1776), although previous detailed morphological comparisons have indicated that the two are distinguishable by internal structures. Resolution of this taxonomic issue is important because A. aspersa has been known as a notoriously invasive ascidian, doing much damage to aquaculture e.g. in Hokkaido, Japan. We collected many specimens from European waters (including the Swedish coast, near the type localities of these two species) and Hokkaido, Japan (as an alien population) and made molecular phylogenetic analyses using the mitochondrial cytochrome c oxidase subunit I (COI) gene, and found that in terms of COI sequences all the analyzed specimens were clustered into two distinct groups, one of which is morphologically referable to A. aspersa and the other to A. scabra. Thus, these two species should be regarded as distinct from each other.

The Earliest Japanese records of the invasive European ascidian <i>Ascidiella aspersa</i> (Müller, 1776) (Urochordata: Ascidiidae) from Mutsu and Ago Bays, with a brief discussion of its invasion processes

“…Using molecular data, Nishikawa et al (2014) confirmed the morphological differentiation (distinguishable morphologically by comparing the number of branchial tentacles with that of inner longitudinal vessels on each side) of Ascidiella aspersa from A. scabra (Müller, 1776), which had been previously doubted by some taxonomists. Therefore, the specific identification and differentiation of species of the genus is now possible, even for formalin-preserved immature specimens.…”

“…The specimens of Ascidiella aspersa living on the ropes at Nishikawa et al (2014) suggested that the previous records of "Ascidiella aspersa", including alien populations, might sometimes be unreliable, because of the lack of morphological or molecular information to distinguish A. aspersa from A. scabra, both of which are known as colonizers (for the latter species see Nishikawa and Otani, 2004). In the Pacific Ocean, Brewin's (1946) description of A. aspersa in New Zealand is informative, from which its introduction to the southern half of the Pacific Ocean about 1945 seems definitely to be the earliest.…”

“…The European ascidian Ascidiella aspersa (Müller, 1776) is notoriously known as the first invasive ascidian to severely damage aquaculture in Japanese waters (Nishikawa et al, 2014; for other alien ascidians in Japan see Nishikawa, 2017).…”

Centrosomes and spindles in ascidian embryos and eggs

“…Ascidiella aspersa eggs and early embryos are also transparent and also translate well microinjected mRNAs encoding proteins coupled to GFPs and could be used for this type of project when Phallusia is not available (Levasseur & McDougall, 2000). Ascidiella has a more widespread distribution than P. mammillata, e.g., it can be collected at Woods Hole, USA as well as in Europe, and northern Japan (Nishikawa et al, 2014) although its gametes are not as abundant as those of Phallusia.…”

Distribution of the invasive ascidian <i>Ascidiella aspersa</i> (M&uuml;ller, 1776) (Urochordata, Ascidiacea) in Hokkaido, Japan

“…A. senegalensis Michaelsen, 19143 Nishikawa & Otani 2004Nishikawa et al 2014 A. scabra Berrill 1928;Lindsay & Thompson 1930;Berrill 1950A. senegalensis Michaelsen 1915 2012 2013 2010 3 2013 8 20 Fig.…”