Sponges associated with stylasterid thanatocoenosis (Cnidaria, Hydrozoa) from the deep Ross Sea (Southern Ocean)

In the Antarctic seas, where hard substrates are scarce, the presence of secondary bio-substrates formed by calcareous organisms is an essential condition to increase the epibiosis and therefore the diversity of sessile benthic fauna. The aggregations of stylasterid hydrozoa, with their branched carbonate structures, are an example of a secondary habitat defined as a 'deep marine animal forest'. The three-dimensional habitat made by these corals supports a high biodiversity of associated organisms, usually invertebrates. Recently, deep remotely operated vehicle (ROV) exploration of the Iselin Bank and the Hallett Ridge (Ross Sea, Antarctica) documented wide areas characterised by large thanatocoenosis of stylasterid skeletons lying on flat muddy substrates, with scattered living colonies generally made of few short branches. In our study, sponges associated with 54 dead colonies of two stylasterid species recorded in these areas were investigated. The analysis led to the discovery of a remarkable number of specimens (127) ascribed to 38 sponge species (31 encrusting and 7 massive). Two of these sponges, Asbestopluma (Asbetopluma) sinuosa and Lissodendoryx (Ectyodoryx) inferiolabiatae, are new. In light of the present data, we can assume that, in Antarctica, stylasterid skeletal remains, due to their three-dimensional structure, play an important role in maintaining sponge biodiversity. This is also due to the ability of sponge specie to produce miniaturised specimens able to colonise these peculiar substrata.