Many didemnid ascidians have calcareous spicules in the tunic. Since the spicules of each species have a specific shape and size-range, they are often regarded as an important character for taxonomy. To evaluate the taxonomic significance of tunic spicules, a quantitative survey of spicule size and shape was combined with a molecular phylogeny inferred from the partial sequence of the cytochrome c oxidase subunit I (COI) gene in some groups of didemnid species that are supposedly closely related. This study revealed the presence of substantial intraspecific variations in the shape and size of tunic spicules. The spicules are, therefore, not always crucial features discriminating species, particularly among related species. Although tunic spicules are potentially valuable features for didemnid taxonomy, their intraspecific variation should be carefully considered before they are used as a key character for species identification.

The rayed trough-shell Mactra corallina Linnaeus 1758 is a surf clam that inhabits the Atlantic Ocean, Black Sea and Mediterranean Sea and represents a commercially important bivalve. This species is present with two different and well-defined sympatric morphotypes, which differ mainly for the colour of the shell (white in the corallina morph, and brown-banded in the lignaria morph). The aim of this work is to resolve the confused and contradictory systematics of the bivalves belonging to M. corallina putative species by analysing molecular and morphological features. Fifteen specimens of M. corallina corallina (white variant) and 19 specimens of M. corallina lignaria (brown variant) were collected in the North Adriatic Sea and analysed by four molecular markers (12S, 16S, 18S and COI genes, partial sequences). Genetic analyses clearly support the presence of two different species, which were previously ascribed to M. corallina. In addition, 35 specimens identified on a morphological basis as M. c. corallina and 28 specimens identified as M. c. lignaria collected in the same area were used for a morphometric analysis. A positive correlation was found between the maximum width of shell (W), antero-posterior length and between W and the height of specimens from umbo to ventral margin, thus adding to molecular data.