A new Antarctic association:: the case of the hydroid Sarsia medelae (new sp.) associated with gorgonians

Sarsia medelae sp. nov. is described and illustrated from material collected in the Weddell Sea during the R/V Polarstern cruises ANT XVII/3 (April 2000) and in the Ross Sea during the R/V Italica cruise XIX (March 2004). The specimens have always been observed in association with several gorgonians of genera Thouarella sp., Primnoisis sp., and Notisis sp. The polyps of Sarsia medelae are considered as mesobiont of the gorgonian host with a non-parasitic relationship. It is the first time that this kind of association, which is so far only known from the Indian and Pacific Ocean, is reported in the Southern Ocean. This finding is discussed in the framework of the hypothesis that a part of Antarctic fauna has its origin in species dating back to before the Cretaceous period, because this kind of association between a hydroid and a gorgonian host being only known in the actual Antarctic shelf and in regions belonging to the previous Thethys areas.