A new species of sponge without skeleton, Thymosiopsis conglomerans, is described from the Mediterranean. It differs from the other species of the genus, T. cuticulatus Vacelet & Perez, which lives in close sympatry although under different ecological conditions, in its morphology and in sterol composition. The new species is massive and incorporates large amounts of foreign materials. D7 sterols are present in both species of Thymosiopsis, but they differ significantly in the side-chains. The 28S rRNA sequences clearly confirm the close affinities of Thymosiopsis with Thymosia, a North-East Atlantic genus which is distinctive in having a skeleton made up of nodulose spongin fibres. The rRNA sequences also support the allocation of these two genera to the family Chondrillidae. The cytology of the new species is compared with that of T. cuticulatus and of Thymosia guernei from Portugal using transmission electron microscopy.
Porifera, taxonomy, sterol chemistry, rRNA sequences, Chondrosida.