How do morphotypes and chemotypes relate to genotypes? The colonial ascidian Cystodytes (Polycitoridae)

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Intraspecies variability is widespread in marine invertebrates. Size, colour, texture, general shape and secondary chemistry can differ quite drastically from one individual to another. Cystodytes dellechiajei (Polycitoridae) is a cosmopolitan colonial ascidian with several morphotypes, most of which differ in colour and spicular composition. New molecular tools enable us to assess the taxonomic status of these morphotypes. To determine whether variation observed in Mediterranean Cystodytes has a genetic basis, we sequenced 45 specimens from eight locations of the western Mediterranean and one from Mayotte (Indian Ocean), and obtained a 617 bp fragment of the mitochondrial gene COI. Fifteen different colour morphs were recorded and four kinds of spicules were found: disk‐shaped, sphere‐shaped, star‐shaped and discoidal, thick spicules with a toothed margin. Zooid morphology was remarkably uniform in the whole sample set. Different tree construction methods (distance‐based, parsimony‐based, and maximum‐likelihood‐based) yielded consistent results, and recognized six major clades, which had no correspondence with spicule shape and were only partially consistent with colour morphs. Results are discussed in the light of previous knowledge of the chemistry of blue, green, brown and purple colour morphs. In spite of the different colour patterns and spicular variability we concluded, on the basis of chemical and genetic data, that the morphological traits analysed were not consistent enough to be used to differentiate between Cystodytes species. We point out the importance of genetics and chemistry in assessing the taxonomic status of species with variable morphology.