Four diterpenoids with eunicellane carbon skeleton, namely ophirin (1), calicophirin B (2), 13-deacetoxyl calicophirin B (3) and 13-deacetoxyl-3-deacetyl calicophirin B (4), were isolated from the South China Sea nudibranch Dermatobranchus ornatus, whereas calicophirin B (2), 13-deacetoxyl calicophirin B (3), astrogorgin (5) and an unnamed analogue 6 were obtained from gorgonian Muricella sp., simultaneously collected from the same region as that of D. ornatus. Structures of these compounds 1～6 were elucidated on the basis of detailed spectroscopic analysis and by comparison with those reported data, and a complete ¹H and ¹³C NMR assignment of ophirin (1) was carried out for the first time. An X-ray diffraction analysis on a single crystal of ophirin (1) not only confirmed the established structure but also led to a revised stereochemical illustration for the above metabolites. This is the first report of chemical studies on the nudibranch of genus Dermatobranchus, which suggests that Dermatobranchus ornatus can protect themselves from predators by selecting appropriate diet, and consequently transforming and bio-accumulating some useful dietary metabolites into selected body parts as defensive substances. The discovery of similar compounds from both the molluscs and the gorgonian suggests a probable predator and prey relationship between them.

Key words: Dermatobranchus ornatus, nudibranchia, gorgonian, eunicellane diterpenoid, Muricella sp.