The specimens collected during several recent oceanographic cruises in the tropical western Pacific, sponsored jointly by the MNHN and the IRD, consist of 53 ascidian species, and among them 16 new species. For others, the geographic distribution is increased in the western Pacific. The remarkably high diversity of these organisms between 50 and 1000 m in this part of the world is demonstrated. In all oceans at these depths the ascidian fauna is dominated by solitary organisms, whereas along the littoral fringe the majority of ascidian species are colonial. This systematic pattern is likely to be influenced by substrate: hard nearshore and soft offshore. In this study, among the new species, the solitary ascidians largely dominate, especially well represented by stolidobranchs with eight Styelidae of four genera, four Pyuridae with also four genera, and one Molgulidae. However the originality of this deep fauna is enhanced by the presence, in the typical Octacnemidae family, of a new genus Myopegma n. gen. with a very small species M. melanesium n. gen., n. sp. which has a very peculiar musculature justifying a new taxon.
Ascidiacea, western Pacific ocean, outer slope, bathyal, new genus, new species.