Morphological evidence that the molecularly determined Ciona intestinalis type A and type B are different species: Ciona robusta and Ciona intestinalis

• Published in: Journal of zoological systematics and evolutionary research Vol. 53; no. 3; pp. 186 - 193
• Main Authors: Brunetti, Riccardo, Gissi, Carmela, Pennati, Roberta, Caicci, Federico, Gasparini, Fabio, Manni, Lucia
• Format: Journal Article
• Language: English
• Published: Berlin Blackwell Publishing Ltd 01.08.2015; John Wiley & Sons, Inc
• Subjects: Biological research; Ciona intestinalis; Ciona robusta; Cryptic species; Geographical distribution; neotype; topotypes; Tunicata
• ISSN: 0947-5745; 1439-0469
• DOI: 10.1111/jzs.12101

Ciona intestinalis is considered a widespread and easily recognizable tunicate, the sister group of vertebrates. In recent years, molecular studies suggested that C. intestinalis includes at least two cryptic species, named ‘type A’ and ‘type B’, morphologically indistinguishable. It is dramatic to certify that two different species may be hidden under the name of a species widely used as a model species in biological researches. This raised the problem of identifying diagnostic morphological characters capable of distinguishing these types. We compared the morphology of specimens belonging to the two types and found that only type A specimens possess tunic tubercular prominences, allowing unambiguous discrimination. Remarkably, these structures were already described as distinctive of the Japanese species Ciona robusta, Hoshino and Tokioka, 1967; later synonymized under C. intestinalis (sensu Millar, 1953). In this study, we have confirmed that C. intestinalis type A corresponds to C. robusta. Based on the geographic distribution of C. intestinalis type B, and considering that the original C. intestinalis species was described from North European waters, we determined that C. intestinalis type B corresponds to C. intestinalis as described by Millar in 1953 and possibly to Linnaeus' Ascidia intestinalis L., 1767 for which we have deposited a neotype (from Roscoff, France) and for which we retain the name Ciona intestinalis (Linnaeus, 1767). Ciona intestinalis is a model chordate in various fields of biology. Molecular studies have identified two forms, named “type A” and “type B”, as putative cryptic species, leading to the hypothesis of a complex of species. We compared their morphology and found that only type A possesses tunic tubercular prominences, allowing unambiguous discrimination. We found that Ciona robusta Hoshino & Tokioka, 1967 corresponds to C. intestinalis type A and that Ciona intestinalis (Linnaeus, 1767) corresponds to C. intestinsalis type B.