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Zonation of algae and sessile invertebrates in the Gl6nan Archipelago. Biologic definition of bathymetric zones. The GI6nan Archipelago was investigated, from the surface to a depth of 60 m, by SCUBA diving. Four distinct communities ("peuptements") have been determined that fit into the system of zonation defined by the Genoa Symposium (1957). The results are compared with observations made in the English Channel (CA~IOCH 1968) and in the Atlantic Ocean (GLEMAI~C 1971). Since illumination depends upon the angle of the rocky face, photophilic species exhibit a maximum range when the rock surfaces are horizontal. The sublittoral zone is dominated by laminarians. They form two distinct communities with the animal and vegetal species they overshadow. The upper community is characterized by Laminaria digitata (LINNI~) L~ouRovx (horizontal rocky face from 0 to 6 m; vertical ro&y face from + 0.5 to --3 m). The lower cummunity, characterized by L. hyperborea (Gv~,mER) FOSLIE has a greater vertical extension (horizontal: --6 m to --26 m; vertical: --3 to --30 m). The circalittoraI zone is colonized by several animal species growing in a ramified manner: the upper community is characterized by Axinellidae (--30 m), the lower by DendrophyIlia eornigera (LAMAgCX) (--55 tO --60 m). Upright algae disappear somewhere between--40 and --47 m.

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Abatmct : A comparative study of isodidemnine-1 and dkkmrdn B ia presental using apuWmw@cmethods, partial degradation and partial synthesis. Tbia leads to the conch&on of the presence of a sin& depsipeptide, namely didem& B, with (3S,4R,SS) isxtatine instead of the previous staline residue. An attempt to determjne the whole conformation in solution of didemninBbyusingZD-NMRisahodesa'ibed.Over the last decade, the widely-distributed order of Aplowobranchiata