# Trepenoids in Marine Heterobranch Molluscs
## 3 Sesquiterpenoids
### Bicyclic Sesquiterpenoids  
Bicyclic sesquiterpenoids are mainly present in sea slugs, with a couple of examples in sea hares, and none in the remaining heterobranch groups. Within nudibranchs, the Antarctic doridacean _Bathyoria hodgsoni_ contains the dimane sesquiterpene hodgsonal (20) (Figure 4) [104; 105]. Hodgsonal is found in the most exposed body parts (mantle and dorsal papillae) and is probably _de novo_ biosynthesized by the nudibranch. Hodgsonal is used as a feeding deterrent against sympatric predators, such as the sea star _Odontaster validus_ and the anemone _Epiactis_ sp. [105]. Furthermore, austrodorins A and B, and the two nor-sesquiterpenes austrodoral (21) (Figure 4) and austrodoric acid, were found in the Antarctic slug _Doris kerguelenensis_, but their ecological role is still unknown [106; 107; 108; 109; 110]. The widely studied nudibranch _Dendrooris_ possesses dimane sesquiterpenes distributed in different body parts [3; 15]. In this slugs, dimane esters are generally associated to the reproductive organs and eggs, while dimane sesquiterpenes are detected in the mantle. _D. arborescens_ possess the known sesquiterpene 7-deacetoxy-olepupuane (22) (Figure 4) [111], while _D. carbunculosa_ presents several cytotoxic dimane sesquiterpenes, the dendrocarbons A-N [112]. Moreover, _D. krebsi_ from Mexico also possess drimane sesquiterpenes and esters [113], allocated in the body in a way similar to that previously described for other _Dendrooris_ species [114]. _D. denisoni_ from New Zealand presents cinnamolide, olepupuane, and polygodial in its mantle [78]. The phylogenetically related genus _Doriopsilla_ also presents compounds related to those of _Dendrooris_. Pelseneeriols-1 (23) and -2, two furanosesquiterpene alcohols, are present in the mantle of _Doriopsilla pelseneeri_ from the Atlantic [115]. _De novo_ biosynthesis of drimane esters, sesquiterpenes, and 15-acetoxy-ent-pallescensin was demonstrated for _D. areolata_ and _Doriopsilla_ sp. through the mevalonic pathway [116; 117; 118; 119]. Two more diastereomeric acetates of pelseneeriol-1 and -2 (Figure 4) were further described in that study. Moreover, both _D. albopunctata_ from the Pacific and _D. areolata_ from the Atlantic also contain drimane sesququiterpenes and _ent_-pallescensin A [113], located in their bodies in a way similar to the _Dendrooris_ species [114]. _Doriopsilla pharpa_ contains polygodial, and their extracts are deterrent to two fish, the blenny _Chasmodes bosquianus_, and the mummichog _Fundulus heteroclitus_, which learned to avoid food items that contained extracts from _D. pharpa_[120], while extracts of the slug were also rejected by the crabs _Callinectes similus_ and _Panopeus herbstii_ in the field. Another well-studied nudibranch group is that of the Phyllidids, which are often colorful animals containing isocyanate compounds [15]. These isocyanates display a wide array of activities, such as antifouling, antibiotic, antifungal, and antitumor properties, and have been studied in depth during recent years [121; 122; 123; 124; 125]. Several studies on dietary sesquiterpene isocyanides suggest that the nudibranch's sequester them from different demosponges, showing a much broader feeding variability than previously reported [46]. One of the most studied species is _Phyllidella pustulosa_, where compounds are obtained from the demosponge _Acanthella cavernosa_[126]. Specimens from China and Vietnam also contain sesquiterpene isocyanides and related compounds, some of them found also in _Acanthella_ sponges [126; 127; 128; 129].