Toward a resolution of the cosmopolitan Botryllus schlosseri species complex (Ascidiacea, Styelidae): mitogenomics and morphology of clade E (Botryllus gaiae) See https://doi.org/10.1093/zoolinnean/zlaa023. Identification of clade E of Botryllus schlosseri as a new species Botryllus gaiae

Botryllus schlosseri, a model colonial ascidian and a marine invader, is currently recognized as a species complex comprising five genetically divergent clades, with clade A globally distributed and clade E found only within Europe. This species has been also recently re-described by designation of a clade A specimen as neotype. To clarify the taxonomic status of clade E and its relationship with clade A, here we examine the entire mitochondrial genome and the morphology of clade E. The clade E mitogenome has identical gene order to clade A, but substantially differs in the size of several non-coding regions. Remarkably, the clade A-clade E nucleotide divergence is incompatible with the intra-species ascidian divergence, but very similar to the congeneric one and almost identical to the divergence between species once considered morphologically indistinguishable (e.g., Ciona intestinalis-Ciona robusta and Botrylloides niger-Botrylloides leachii). As for the morphology, clade E differs from the B. schlosseri neotype mainly for: the number and appearance of the stomach folds; the shape of the anal opening, the first intestinal loop, and the typhlosole. Our integrative taxonomy approach clearly recognizes clade E as a species distinct from B. schlosseri, with unique morphological and molecular characters distinguishing it from the B. schlosseri neotype. Therefore, we here describe Botryllus gaiae n. sp.