Looks can be deceiving: Didemnum pseudovexillum sp. nov. (Ascidiacea) in European harbours

Abstract

A strongly divergent lineage, putatively a new cryptic species, of colonial ascidian was first detected as an anomalous sample in a population genomics study of the well-known worldwide invasive species Didemnum vexillum Kott, 2002. This putative new taxon, found in a marina in Roscoff, France, is indistinguishable from Didemnum vexillum in external aspect and coexists with it in syntopy. However, morphological characters such as spicules and larvae allow a clear-cut distinction. In accordance with the preliminary results based on genome-wide analyses, morphological traits and mitochondrial sequences of the Cytochrome Oxidase I gene both support the establishment of a new species Didemnum pseudovexillum sp. nov. Previous unidentified sequences in public databases showed that the new species is also present in NW Mediterranean marinas. Didemnum pseudovexillum sp. nov. is assigned for the time being a cryptogenic species status, although its presently known disjoint distribution across two biogeographic regions and its presence in ports are suggestive of an introduced species. Further studies should be performed to ascertain its current distribution and putative natural range and settle its native vs. non-native status. This finding casts doubts on previous reports of Didemnum vexillum and also calls for caution when performing fast field surveys of non-indigenous species such as Rapid Assessment Surveys (RAS) or BioBlitz surveys, based solely on external characters.

Morphology, genetics, and historical records support the synonymy of two ascidian species and suggest their spread throughout areas of the Southern Hemisphere

“…This study stresses the usefulness of traditional morphology coupled with DNA analysis for species identification, because they strengthen the taxonomic diagnoses, particularly in cases of cryptic complexes or in species with a high morphological variation (see, e.g., Nishikawa et al 2014;Rocha et al 2019;Turon et al 2020;Viard et al 2019). Also, we emphasize the need to make detailed diagnoses and to generate species barcodes to avoid confusion or misidentification in the future.…”

Cryptic genetic lineages of a colonial ascidian host distinct microbiomes

“…With the increasing use of genetic sequencing technologies, numerous instances of cryptic speciation have been detected among morphologically indistinguishable organisms (Florio et al., 2012; Thongtam na Ayudhaya et al., 2017), including some native (Aron & Sole‐Cava, 1991; Tarjuelo et al., 2004) and invasive (Turon et al., 2020) ascidian species. However, genetic analyses have simultaneously determined phenotypic colour variation may indeed represent intraspecific variability among some colour morphs of other ascidian species (e.g.…”

Clogging nets-Didemnum vexillum (Tunicata: Ascidiacea) is in action in the eastern Mediterranean

“…Didemnum vexillum was originally described from New Zealand (Kott, 2002) and regarded as a global alien invader in temperate waters (Turon et al ., 2020). It has a high growth rate that enables it covering large areas quickly, overgrowing almost every other sessile species (Lambert, 2009).…”