# Trepenoids in Marine Heterobranch Molluscs
## 1 Background
Therefore, they are relevant in marine ecosystems because they occupy many different ecological niches and display a wide array of trophic relationships with organisms from many different taxonomic groups, comprising macroalgal or plant herbivory, carnivore prey-predator relationships, and occasionally cannibalism. They are also able to use leptochemistry, incorporating chemicals from their diet (cleptochemicals), often called leptochemodefenses when used for their own defensive means [15; 30; 31; 32]. Marine organisms are a still underexplored source of unique natural compounds produced or accumulated both by micro- and macroorganisms with pharmacologically interesting properties to be used as drugs [1; 2; 23; 33; 34; 35], displaying specific biological activities and unique skeletons, rarely found in non-marine organisms [36; 37; 38; 39; 40]. The pharmacological potential of terpenoids from marine organisms has been reviewed in the past [12]. For heterobranchs, their pharmacological potential in drug discovery is remarkable. Promising heterobranch compounds include several aplyronines and dolastatins from the anaspidean _Dolabella auricularia_ and bursatellanins from _Bursatella leachii_, kahalalides from the sacoglossan _Elysia rufescens_, jorumycin from the d  
Figure 1: Schematic representation of the phylogeny of Heterobranchchia, adapted and simplified from [3; 19; 20]. In brackets: number of terpenoids reviewed here. ulapualides from _Hexabranchus sanguineus_, and kabiramide from _Hexabranchus_ sp, some of them being examples of highly effective compounds against tumors in clinical trials [39; 41; 42; 43]. A detailed overview of the natural products involved in chemical defense in Heterobranchs was published recently, adding to previous reviews on heterobranch molluses [3; 15]. The review included a detailed discussion on the different kinds of compounds found in different groups, as well as on the origin and anatomical allocation of these chemical defenses, their biosynthesis, biogeography, and evolutionary patterns. These aspects will not be evaluated here. In addition, previous reviews covered the literature on \"opisthobranch\" chemistry and chemical ecology in particular geographical areas, in selected groups of compounds, or for certain heterobranch groups [27; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53]. Other wide reviews have also been published regarding molluses, general marine chemical ecology, or marine chemistry [54; 55; 56; 57; 58; 59; 60; 23; 28; 33; 50; 54; 59; 70; 29; 28; 33; 50; 56; 57; 23; 28; 35; 58; 59; 60]. [PERSON] and collaborators [1; 2] have been regularly providing accurate reports of new marine natural products described, with many terpenoids being found every year. Therefore, a specific review on terpenoids in heterobranch molluses seemed timely and necessary. The aim of this review is to present an overview of the main types of terpenoids found in heterobranch molluses and the groups in which are they found, even if their bioactivity has not yet been described. There are more than 330 molecules fully referenced in the original papers, and the intention here is to show examples of each type to illustrate their variety, for which the molecular structures have been reported. This review does not cover data regarding synthesis of the compounds. The described compounds are included here according to the type of terpene, even though some carbons may have been added or lost; compounds already reported in previous reviews will not be repeated here, except for relevant new information about them. Thus, this review mostly includes selected data published over the last 25 years [3; 15]. It is remarkable that no terpenoids have been described in this period in tylodinoideans, cephalaspideans, or pteropods (Figure 1). Most terpenoids have been found in nudibranchs, anaspideans and sacoglossans, and very few in pleurobranchoideans and pulmonates, as reported below.