Fig. 1

General features of the hexactinellid sponge Oopsacas minuta. a Phylogenetic position of O. minuta: this species belongs to the phylum Porifera (sponges), one of the best candidates as sister group to all other metazoans. Free animal silhouettes were downloaded from PhyloPic (http://phylopic.org/). Among the four lineages of Porifera (> 9500 species), O. minuta belongs to Hexactinellida (glass sponges). b Hexactinellida (in red) have particular features (e.g., syncytial tissues) compared to the three other sponge classes, Demospongiae (in orange), Homoscleromorpha (in blue), and Calcarea (in green). c O. minuta adults inhabit Mediterranean canyons and shallow caves (photo credit Dorian Guillemain), where individuals are 1–7 cm long and have a clear basal–apical polarity (ap apical pole, bp basal pole) (scale bar = 1 cm)