# Trepenoids in Marine Heterobranch Molluscs
## 2 Monoterpenoids
### Linear Monoterpenoids  
Anaspideans and sacoglossans possess several kinds of linear monoterpenoids, either from their diet or transformed from dietary compounds. Within anaspideans, the sea hare _Aplysia kurodai_ from Japan presents halogenated and brominated monoterpenoids distributed in different body parts, such as kurodainol (1) [61] and aplysisaterpenoid B (2) (Figure 2) [62; 63]. Mediterranean specimens of _Aplysia fasciata_ contain polyhalogenated monoterpenes similar to those of _Plocamium_ red algae [64]. These compounds include the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Group (Species)/Compounds** & **Anaspidea (5)** & **Sacoglossa (11)** \\ \hline Linear Monoterpenoids & \textgreater{}10 1 & 5 \\ Monocyclic Monoterpenoids & \textgreater{}10 1 & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Number of monoterpenoids in the different Heterobranch groups in the reviewed period. In brackets: number of species with monoterpenoids.  
3_4_-erythro-7-dichloromethyl-3-methyl-3\(4\),_8_-trichloro-1_,_5 (E)_,_ 7 _(Z)_-octatriene, previously found in the Pacific_ Plocamium cartilagineum_, and_ \(3\),_4_-three-7-dibromomethyl-3-methyl-3_,\(4\),_8_-trichloro-1_,_5 (E)_,_ 7 _(E)_-octatriene_ _[_64_]__. Some of these chemicals were also present in the red algae_ Plocamium cocineum_. Moreover, the mucus secretions of the Atlantic and Mediterranean Sea hare_ Aplysia punctata _contain several_ P. cocineum _ halogenated monoterpenes too_ _[_65; 66_]__. More specifically, these are four unusual acetates of linear polyhalogenated monoterpenes and four cyclic derivatives.  
Some shelled saccoglossans are also rich in monoterpenoids, which they can obtain from their algal food and further transform into more bioactive compounds. The Mediterranean species_ Oxynoe _olivacea_,_ Lobiger serradifalci_, and_ Ascobulla _(= Cylindrobulla) fragilis feed on different parts of the green algae_ Caulerpa prolifera_, which contains the sesquiterpenoid caulerpenyne [67]. _O. olivacea_ and _A. fragilis_ are able to modify caulerpenyne, which can be found in their digestive gland, into the potent ichthyotoxic aldehydes, oxytoxin-1 (3) and -2 (4) (Figure 2), and transport them to be secreted to the mucus and mantle. _L. serradifalci_ instead contains only oxytoxin-1 in the parapodial lobes and in its defensive mucus [68]. Similarly, in the Caribbean species_ Ascobulla _ulla _(feeding on Caulerpa fastigata)_,_ Oxynoe antillarum _(feeding on Caulerpa sp.), and_ Lobiger souberveii _(feeding on Caulerpa racemosa)_, the same chemistry is found, accumulating caulerpenyne (only this molecule is found in_ L. souberveii_) and further transforming it into oxytoxins [69]. In the Indian species_ Volvatella sp. _caulerpenyne has also been reported [70]. Caulerpenyne is a highly bioactive molecule, presenting anticancer activity, cell grown inhibition, neurotoxic activity, and inhibiting lipoxygenases, among others [71; 72; 73; 74].  
The shell-less saccoglossans of the genus_ Elysia _usually also feed on Caulerpa species and related green algae [75], from which they obtain and transform their defensive compounds. Some Caribbean  
Figure 2: Structures of selected monoterpenoids from Heterobranch molluscs: (**1**) kurodainol; (**2**) aplysiaterpenoid B; (**3**) oxytoxin-1; (**4**) oxitoxin-2; (**5**) expansinol; (**6**) ascobullin A; (**7**) costatone; (**8**) apakaochotdone A; (**9**) aplysiaterpenoid A; (**10**) loliolide.  
species of _Elysia_, such as _E. subornata_ (feeding on _Caulerpa prolifera_), _E. patina_, and _E. nisbeti_ (feeding on _Caulerpa_ sp.), contain caulerpenyne and oxytocin-1 [69]. _Elysia cf. expansa_ from India also presents caulerpenyne, together with minor amounts of certain reduced derivatives, dihydrocaulerpenyne and expansinol (5) (Figure 2) [76], similar to the Mexican _Ascobullaullaulla_ reported above.