Sponges are often dominant in marine benthic communities, especially on Mediterranean hard substrates. The
Homoscleromorpha group is a poorly known sponge clade, represented by a single family, the Plakinidae, with an
unresolved phylogenetic position at the basis of the Metazoan tree. For some of the representatives of this group, the
absence of skeleton, main morphological character for sponge taxonomy, explains also the very complicated history of
species statut. At present time, 77 species are listed in the World Data Base, with 22 valid species in the Mediterranean
and several representatives remaining to be described. All the genera of the family are represented in the
Mediterranean: one species from genus Corticium, one species of Pseudocorticium, one species of Placinolopha, one
Plakortis, ten species of Plakina and 8 Oscarella species. Most of these sponges only grow sparsely in the Mediterranean
with distributions often limited to dark submarine caves. The exceptions are Corticium candelabrum and several species
of the genus Oscarella, who seem to grow only on coralligenous substratum. In some places, Oscarella sp. can be
predominant and constitute specific facies. Therefore, they appear as strong competitors for space, overgrowing massive
sponges, sea fans and bryozoans. This is especially the case of a new species which seems to be highly dynamic. This
strong out-competing ability may be particularly due to an efficient secondary metabolism and the biochemical defense it
confers. This hypothesis is also supported by the absence of well-known predator or epibiotic organisms.