The taxonomic significance of the short-shafted mesotriaene reviewed by parsimony analysis: validation of Pachastrella ovisternata Von Lendenfeld (Demospongiae: Astrophorida)

The spicule complement of a demosponge specimen collected from the Atlantic coast of Spain was noted to be similar to that of Pachastrella monilifera, but particularly characterized by the presence of short-shafted mesotriaenes. After detailed examination, it was concluded that the specimen undoubtedly belonged to the species Pachastrella ovisternata Von Lendenfeld, 1894. For a long time, this species has been regarded a synonym of Pachastrella monilifera Schmidt, 1868. Examination of this new specimen showed that Pachastrella ovisternata was a valid species of the genus Pachastrella. Moreover, it denoted the real existence of mesotriaenes in this genus. The parsimony analyses indicated that the species Yodomia perfecta, traditionally assigned to the genus Yodomia by the presence of mesotriaenes, ought to be reclassified in the genus Pachastrella, close to P. ovisternata. These analyses also showed that mesotriaenes are a product of a convergent evolution in shape of tetraxon spicules in Calthropellidae and Pachastrellidae. At the species level it is suggested that short-shafted mesotriaenes have arisen more than once in the family Pachastrellidae. Thus, the presence of mesotriaenes itself cannot be regarded as a diagnostic characteristic at generic level. However, it was noted to be a reliable feature in order to distinguish taxa at specific level.