# Trepenoids in Marine Heterobranch Molluscs
## 1 Background
However, this is not considered valid anymore and currently Heterobranchia include the old \"opisthobranch\" gastropods as well as their marine pulmonate relatives (Figure 1) [17; 18; 19; 20]. Heterobranchia comprises more than 8400 species [21], from which less than 300 have been chemically studied [1; 2; 3; 15; 22]. This is an evolutionarily old group of animals--from the Paleozoic era--therefore, a serendipitous morphological and ecological diversification can be observed, with many relevant adaptations in feeding modes, reproduction, and defensive strategies, among others [18]. Heterobranchs possess a wide variety of bioactive molecules, which protect them against potential predators and competitors, enhancing their ecological performance, being involved in reproduction, development, growth, or feeding, and affecting species distribution, community structure, and biodiversity [3; 23]. Their chemical defenses may include natural products directly obtained from their prey, transformed from diet, or _de novo_ biosynthesized. Among these chemicals, terpenoids are widely present. Heterobranchs currently comprise eight major taxa, namely, Nudibranchia, Pleurobranchoidea, Tylodiniodea, Cephalaspidea, Anaspidea, Pteropoda, Sacoglossa, and Pulmonata [17; 18; 19; 20] (Figure 1). Nudibranchs, or sea slugs, include four major taxa (Doridacea, Dendronotida, Euarminida, and Aeolidida), and comprise the most diverse group within Heterobranchia, with a huge variety of biological and chemical defensive strategies. Doridacea generally feed upon sponges, bryozoa, tunicates, or other \"opisthobranchs\". They possess mainly terpenoids, either from their diet or _de novo_ biosynthesized, but also alkaloids and other compounds [3; 15]. Dendronotids predate either on cridarians or certain small animals (crustaceans and turbellarians). Euarminida may prey upon octocorals or bryozoans. Aeolidida are mainly cridarian-feeders usually protected by nematocystsfrom their prey (relocated into their dorsal cerata) but they also contain relevant chemicals [15]. Pleurobranchoidea include ascidian predators and scavengers. They often exude sulfuric acid at very high concentrations [24; 25; 26]. Tylodinidea, or false limpets, feed upon sponges. Cephalaspideans, or head-shielded slugs and snails, present mainly polykeptides and polyacetates, some of which are _de novo_ biosynthesized, biomodified, or accumulated from their prey. Most of them are algal feeders, but some of them are active predators of other \"opisthobranchs\" (including other cephalosporateans), as well as annelid worms and sponges [15]. Anaspideans, or sea hares or aplysiomorphs, produce ink and other secretions and are herbivorous. They tend to accumulate all sorts of compounds from their diet. Pteropods include pelagic heterobranchs feeding on phytoplankton or preying upon other pteropods. Sacoglossans are usually cryptic and are very specialized herbivores, feeding upon different algal groups. They possess many sesquiterpenoids and triterpenoids (often acyclic), either from their diet or _de novo_ biosynthesized, in addition to alkaloids and other compounds [3; 15; 27]. Marine pulmonates live at the intertidal and possess a wide array of propionates and terpenoids. Overall, heterobranchs display a huge diversity of biological and ecological strategies based on their chemical defenses. The presence of symbiotic microorganisms, which could in fact be producing some of these natural compounds, is unknown for this group [3]. The natural products of heterobranchs, however, are pivotal for their ecological specialization, as said above, although many aspects of their chemical ecology, as in other marine organisms, remain currently unknown [3; 28; 29; 23]. As already mentioned, heterobranchs may feed upon a wide range of other organisms, such as algae (Chlorophyta, Orthophyta, Rhodophyta (green, red, and brown algae, respectively)), sea grasses, Porifera, Cnidaria, Annelida, Bryozoa, Chordata (tunicates), other Mollusca, and others.