# Trepenoids in Marine Heterobranch Molluscs
## 9 Concluding Remarks  
Heterobranch molluscs are rich in natural products; however, only a small proportion has been studied so far. As reported here, their terpenids are particularly abundant and diverse; they include monotereponoids, sesquiterpenoids, diterpenoids, sesterterpenoids, triterpenoids, tetraterpenoids, and steroids. This review discusses more than 330 metabolites isolated from ca. 70 species of heterobranch molluscs. The monotereponoids reported here may be linear or monocyclic, while sesquiterpenoids may include linear, monocyclic, bicyclic, or tricyclic molecules. Diterpenoids may include linear, monocyclic, bicyclic, tricyclic, or tetracyclic compounds. Sesterterpenoids, instead, are linear, bicyclic, or tetracyclic. Triterpenoids, tetraterpenoids, and steroids are not as abundant as the previously mentioned types. Remarkably, no terpenids have been described in this period in tylodinideans, cephalaspideans, or pteropods; and most terpenids have been found in nudibranchs, anspideans, and sacoglossans, with very few compounds in pleurobranchoideans and pulmonates. Many of these compounds found in heterobranchs are obtained from their diet, while others are biotransformed,  
Figure 12: Structures of selected steroids from Heterobranch molluscs: (**86**) diaulusterol A; (**87**) 3-epi-aplykurodinone B; (**88**) aplykurodinone-2. or _de novo_ biosynthesized by the molluscs. Bacterial origin has been suggested for several types of compounds in heterobranchs [280]. However, very few studies rigorously demonstrate this. The structural similarity and dietary origin of some natural products from microorganisms (particularly cyanobacteria) indicate that this is possible, but further studies are required to shed light on this very interesting topic. Monoterpenoids are present mostly in anaspidea and are less abundant in sacoglossa. Nudibranchs are especially rich in sesquiterpenes, which are also present in anaspidea, and in less numbers in sacoglossa and pulmonata. Detreponoids are also abundant in nudibranchs, present also in anaspidea, and scarce in pleurobranchoidea, sacoglossa, and pulmonata. Sesterterpenoids are only found in nudibranchia, while triterpenoids, carotenoids, and steroids are only reported for a few nudibranchia, pleurobranchoidea, and anaspidea. Anaspidea include many halogenated and non-halogenated terpenoids, mostly from diet. Nudibranchs possess many sesquiterpenoids and triterpenoids, mostly from diet, but also _de novo_ biosynthesized. Some compounds are present as glyceryl esters, such as verrucosins. _De novo_ biosynthesis has been demonstrated in a few cases, as well as the putative symbiotic origin of some metabolites [3; 15], probably related to the intrinsic difficulties of this kind of research. Dietary origin is much better established, with compounds originating from many diverse organisms, from different kinds of algae and seagrasses to metazoans, such as Porifera, Cnidaria, Mollusca, Bryozoa, Tunicata, and others. In some cases, however, it remains a hypothesis that should be further demonstrated. The ecological role of these compounds includes many examples of feeding deterrence against sympatric predators, mostly fish and crabs, as well as toxicity. However, many assays are still conducted with species that do not live in the same habitat as the heterobranchs, and thus the ecological significance of many compounds continues to be further examined. The low numbers of reliable field experimental data available is evident. The pharmacological potential of heterobranch terpenoids is clear from the examples reported above--from cytotoxicity to antiplasmodial activity, antituberculosis, antifouling, antifungal, antibacterial, antitumoral, and apoptosis inducers. Overall, a huge variety of terpenoid structures is found in heterobranchs, indicating that chemodiversity correlates to the biodiversity of this amazing group of molluscs.