title: Morphological Differences between Larvae of the Ciona intestinalis Species Complex: Hints for a Valid Taxonomic Definition of Distinct Species.
abstract: The cosmopolitan ascidian Ciona intestinalis is the most common model species of Tunicata, the sister-group of Vertebrata, and widely used in developmental biology, genomics and evolutionary studies. Recently, molecular studies suggested the presence of cryptic species hidden within the C. intestinalis species, namely C. intestinalis type A and type B. So far, no substantial morphological differences have been identified between individuals belonging to the two types. Here we present morphometric, immunohistochemical, and histological analyses, as well as 3-D reconstructions, of late larvae obtained by cross-fertilization experiments of molecularly determined type A and type B adults, sampled in different seasons and in four different localities. Our data point to quantitative and qualitative differences in the trunk shape of larvae belonging to the two types. In particular, type B larvae exhibit a longer pre-oral lobe, longer and relatively narrower total body length, and a shorter ocellus-tail distance than type A larvae. All these differences were found to be statistically significant in a Discriminant Analysis. Depending on the number of analyzed parameters, the obtained discriminant function was able to correctly classify > 93% of the larvae, with the remaining misclassified larvae attributable to the existence of intra-type seasonal variability. No larval differences were observed at the level of histology and immunohistochemical localization of peripheral sensory neurons. We conclude that type A and type B are two distinct species that can be distinguished on the basis of larval morphology and molecular data. Since the identified larval differences appear to be valid diagnostic characters, we suggest to raise both types to the rank of species and to assign them distinct names.
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milano, Italy.
authors.forename: Roberta
authors.initials: R
authors.lastname: Pennati
authors.affiliation: Dipartimento di Scienze dell'Ambiente e del Territorio e di Scienze della Terra, Università di Milano Bicocca, Milano, Italy; Laboratoire d'Ecologie Alpine (LECA), Université Grenoble-Alpes, Grenoble, France.
authors.forename: Gentile Francesco
authors.initials: GF
authors.lastname: Ficetola
authors.affiliation: Museo di Storia Naturale, Venezia, Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Brunetti
authors.affiliation: Dipartimento di Biologia, Università degli Studi di Padova, Padova, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Caicci
authors.affiliation: Dipartimento di Biologia, Università degli Studi di Padova, Padova, Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Gasparini
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milano, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Griggio
authors.affiliation: Department of Zoology, University of Oxford, Oxford, United Kingdom.
authors.forename: Atsuko
authors.initials: A
authors.lastname: Sato
authors.affiliation: Humboldt-Universität zu Berlin, Institut fur Lebenswissenschaften, Vergleichende Zoologie, Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Stach
authors.affiliation: Department of Integrative Zoology, University of Vienna, Vienna, Austria.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Kaul-Strehlow
authors.affiliation: Dipartimento di Bioscienze, Università degli Studi di Milano, Milano, Italy.
authors.forename: Carmela
authors.initials: C
authors.lastname: Gissi
authors.affiliation: Dipartimento di Biologia, Università degli Studi di Padova, Padova, Italy.
authors.forename: Lucia
authors.initials: L
authors.lastname: Manni
mesh_terms: D000818:Animals; D002938:Ciona intestinalis / Q000033:anatomy & histology* / Q000145:classification* / Q000166:cytology / Q000648:ultrastructure; D007814:Larva / Q000033:anatomy & histology / Q000145:classification / Q000166:cytology / Q000648:ultrastructure; D008953:Models, Anatomic; D011984:Sensory Receptor Cells / Q000166:cytology