# Trepenoids in Marine Heterobranch Molluscs
## 1 Background  
Marine organisms produce a wide variety of natural products, often unique and critical for their survival and ecological performance [1; 2; 3; 4]. Among these molecules, terpenes are remarkably abundant, with about 60% of all known natural products being terpenoids [1; 2]. Trepenoids (or isoprenoids) are a large and diverse class of naturally occurring organic chemicals derived from terpenes; they are the largest class of natural products, with estimates of \(>\) 70 000 distinct compounds providing a vast pool of complexity that can interact with biological targets in a huge variety of ways [5]. They usually are multicyclic structures with oxygen-containing functional groups. Biochemical modifications of terpenes produce the related terpenoids. Trepenes are biosynthetically derived from units of isopentenyl pyrophosphate (IPP). Animals produce terpenes through the HMG-CoA reductase pathway, the mevalonate pathway, which also produces cholesterol. These reactions take place in the cytoplasm of the cells, where IPP and dimethylallyl pyrophosphate (DMAPP) condenseto produce geranyl pyrophosphate (GPP), the precursor to monoterpenes and monoterpenoids (C10), and in fact to all terpenes and terpenoids. IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase. GPP is also converted to farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP), respectively, C15 and C20 precursors to sesquiterpenes and diterpenes (as well as sesquiterpenoids and diterpenoids), and so on [6]. Biosynthesis is mediated by terpene synthases and further includes secondary transformations to produce modifications of the basic parent skeletons generating the astonishing variety of different terpenoids, usually involving oxidation, reduction, isomerization, and conjugation reactions, which provide specific functional properties to the terpenoids [7; 8]. Thus, terpenoids are usually classified according to the number of C, based on the isoprene unit (C5), that comprise the parent terpene [9; 10]. Terpenoids exhibit highly specialized biological activities ranging from nerve regeneration to blood-sugar regulation [11]. While terrestrial terpenids have been known and used by humans for centuries, marine terpenoids are still understudied. Marine terpenoids are reported in annual reviews on the isolation of new C10, C15, C20, and C30 isoprenoids and sterols, and there is a wealth of literature dealing predominantly with marine monoterpenoids, diterpenoids, sesquterpenes, sterols, and others [12]. Some studies report the presence and potential significance of terpenoids occurring in marine invertebrates, while others deal with their synthesis [11; 12]. The methodology for extracting natural compounds from organisms includes diverse techniques that are usually expensive and time-consuming [13; 14]. Some studies have recently dealt with different methodologies and their advantages, and in particular, with new extraction techniques, including selective extraction of marine natural products, some of which are useful for terpenoids [13; 14]. No studies, however, have used these new methods in heterobranchs, as far as we know [13]. Within marine organisms, molluscs are one of the most abundant and chemically rich invertebrate groups, possessing high amounts of terpenoids [3; 15; 16]. Heterobranch molluscs are widespread animals, living in all oceans, from the tropics to the poles, and from shallow waters to deep basins [3]. Most heterobranchs are soft-bodied and shell-less animals possessing a huge array of defensive strategies, from behavioral, morphological (mechanical or physical), as well as chemical strategies [15]. They comprise the commonly known sea slugs and sea hares, as well as marine pulmonates. In the past, Gastropod molluscs were divided into three subclasses: Prosobranchia, Opisthobranchia, and Pulmonata, because of the position and type of their respiratory structures.