# Trepenoids in Marine Heterobranch Molluscs
## 4 Diterpenoids
### Tricyclic Diterpenoids  
Tricyclic diterpenoids are present in sea slugs, pleurobranchoids, and sea hares. The Patagonian doridacean _Anisodoris fottaini_ contains anisodorrins 1-5 (58) (Figure 8), which are said to be _de novo_ biosynthetic [207]. Within nudibranchs, _Chromodoris_ species (now called _Flemida_ in Eastern Pacific, Atlantic, and Mediterranean species, and different genera for other geographical areas) [149], usually present a wide variety of diterpenes obtained from their sponge preys accumulated into MDFs situated in their mantle border [15]. The South African chromodorid _Chromodoris hamiltoni_ presents four unusual chlorinated homoditerpenes, the hamiltonins A-D, among other compounds [208]. _Chromodoris africana_ from the Red Sea, instead, contains the furanoterpene kurospongin (59) (Figure 8) and a macrolide. Kurospongin was also found in an Okinawan sponge, _Spongia_ sp., displaying ichthyotoxicity and feeding deterrency [209; 210; 211]. Tyrinnal (60), an unusual diterpene seco-11,12-spongiane (Figure 8), is found together with some known sesquiterpenes in the Patagonian chromodorid _Tyrinna nobilis_[89]. _Goniobranchus albopunctatus_ (previously known as _Chromodoris albopunctata_) from Australia presents 12\(\alpha\)-acetoxyspongian-16-one, 20-acetoxyspongian-16-one, (+)-spongian-16-one, and 7\(\alpha\)-acetoxyspongian-16-one, together with some propionate and (+)-isoagatholactone [90]. The same study reports that _Chromodoris splendida_ from Australia presents aplysulphurin (61) (Figure 8), probably from its diet, the demosponge _Darwinella tango_, together with gracilin metabolites, such as gracilin C 20, not found in the sponge. A recent study on 19 species of Chromodoridid nudibranchs from Australia, mentioned above, reported many diterpenoids, among other compounds, found in mantle and internal glands [154]. In that work, mixtures of compounds are described in _Goniobranchus_ nudibranchs, containing spongian diterpenes, norditerpenes, and rearranged diterpenes. _G. titorius_ contains two rearranged diterpenes, aplysulphurin and aplyviolacene (62) (also named Macfarlandin E; Figure 8) in the viscera and four spongian diterpenes in the mantle. _G. tasmaniensis_ contains the spongian diterpene aplyroseol-2 (63) (Figure 8), together with two related dialdehydes in the mantle, and other terpenes in the viscera, of the aplyroseol/dendrillol type. _G. collingwoodi_ possesses a series of 12 diterpenes, including 10 spongian diterpenes, and the furanone diterpene luffarin-X [154; 212]. Moreover, _G. splendidus_ presents 31 different terpen in mantle and viscera, 22 of which are regular spongian diterpenes, including chromoculatimine C (64) (Figure 8), 6\(\alpha\),7\(\alpha\)-diacetoxydendrillol-3, and methyl 15,17-epoxy-7,17\(\alpha\)-diacetoxy-ent-isocopalan-16-oate, while six compounds, aplysulphurin, membranonolide, aplyviolene, tetrahydroroaplysulphurin-1, tyrinnal, and splendidalactone-1 present a rearranged diterpene skeleton, and two compounds present a rearranged norditerpene (C19) skeleton of the gracilane type [154; 213]. Thus, _G. splendidus_ presents spongian diterpenes, rearranged diterpenes and norditerpenes, especially in the mantle rim, but populations differ in abundance, type, and richness of these compounds, also displaying a high individual variation between specimens from the same population, perhaps related to variability in dietary sponge communities among geographic locations [214]. _G. daphne_ presents the nordietpenes gracilin A (65) and daphnelactone (66) (Figure 8) in mantle and viscera, while the rearranged diterpene aplysulphurin is only present in the viscera [154]. _G. hunterae_ also contains aplysulphurin, as well as aplyviolene, and aplyviolacene (Macfarlandin E), while _G. verrieri_ presents the spongian norditerpene verrielactone (67) (Figure 8) and the rearranged diterpenes dendrillolide A and aplyviolacene, as well as gracilin A [154; 213].