**2. Concept of chemical ecology**

Molluscs live in very different habitats and are highly diverse especially in their ecological behaviors. The phylum consist 10 taxonomic classes, which two are entirely extinct. Among the existing classes, Cephalopods such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates, and gastropods (snails and slugs) are the most

Molluscs are highly successful animal group in terms of ecology and adaptation and they are found in all habitats ranging from deepest ocean to intertidal zone, freshwater and terrestrial lands where they occupy a wide range of habitats, but the highest diversity could be found in the sea in comparison with freshwater and terrestrial habitats. Between all classes in the phylum Mollusca, the most important class is gastropoda comprising more than 80% of all living Mollusca species. The species belonging to this class occurs in marine, freshwater and terrestrial habitats. Whereas bivalves occurs both in freshwater and marine environments, but there is not any species in terrestrial habitats. In the all classes of molluscs, 6 classes are

Molluscs are consumed as a food source for humans, birds, fish, mammals and other invertebrates, and also play a key role in the recycling of nutrients, soil-generation and water filtration. They are good bio-indicators too, for environmental quality in all types of aquatic

Molluscs have very different forms among the other animal phylums. Snails, slugs and other gastropods; clams, oysters, scallops and other bivalves; squid, cuttlefish, octopus and other cephalopods; and also lesser known subgroups have interesting diversities in structure, color and size [6]. The giant squid, which had not been observed alive in its adult form recently, is one of the largest invertebrates, with 10 m (33 ft.) long and 500 kg (1100 lb.) weighed [7].

Molluscs are an important food source for humans as mentioned earlier, but there is a risk about poisoning from toxins which can accumulate in certain molluscs under specific conditions. Besides, they are a good source of many luxury goods, such as pearls, mother of pearl, Tyrian purple dye, and sea silk. Also, in ancient periods, their shells have also been used as

Mollusca are very abundant and form an important trophic level in the aquatic food chain. A large number are filter feeders and hence, are important in nutrient recycling along with the other soil invertebrates. Numerous molluscs are important food source for humans such as Clams and Snails. Some gastropods are pest and damage crops or others hosts for some disease causing parasites such as lung worm which causes schistosomiasis and liver worm for

In addition to the wide usages of molluscs in food industry, shell decorations, dyes and medicines; determination, identification and extraction of their bioactive compounds and secondary metabolites have been an important scientific field of research recently. For instance, isolated natural products from molluscs and their structural analogues are particularly well represented in the anticancer compounds in clinical trials. These compounds and their differ-

ent chemical structures could be change in each species [8–10].

numerous classes in this phylum [2].

232 Biological Resources of Water

exclusively marine species [3, 4].

habitats [4, 5].

money [4].

fascioliasis in humans [3, 5].

The different species from molluscs probably utilizes the neutral and total lipids during cold seasons in order to survive and stores them for hot seasons. The importance of stored lipids is for reproductive purposes. However, they have also been shown to provide energy during winter, when carbohydrate reserves are depleted. This indicates that the fatty acid compositions of animals, neutral lipids in particular, are dictated by their metabolic activities and components of their dietary lipids [12].

The feeding habitats and diet composition are important factors that cause changes in the levels or type of the fatty acids in the different groups of molluscs. There are different feeding habits (such as filter feeder and detritus feeder) in the different groups of this phylum. Most of the lipids and considerable amounts of C20:5ω3 and C22:6ω3 acids are provided by diatoms and dinoflagellates, respectively. For example, diet composition of bivalves which are filter feeders, consist of dinoflagellates, bacteria and particulate organic material. It is found that diatoms have high levels of C20:5ω3 acid and low C22:6ω3 acid, but dinoflagellates have high concentrations of C22:6ω3 acid. Some species of molluscs are detritus feeder, and amounts of lipids, SFAs and MUFAs of 14–18 carbons are provided by detritus. Therefore, diet composition has the important role in the variation in the level and type of the fatty acids between different groups during four seasons of the year [13–15].

The different metabolic processes play an important role in the changes of levels and type of the fatty acids, because there are significant differences in amount of consumed energy between different metabolic processes. Between all metabolic processes, reproductive cycles is the main process for consume of energy, and this process need high levels of energy (fatty acid). Therefore, there is probably an apparent relation between reproduction cycles and fatty acid profiles. In between all fatty acids same as C20:4ω6, is mostly associated with the reproductive enzymes and highest levels of this compound is consumed in spawning times and reproductive processes.

Distribution histogram of species diversity reveals multiple metabolites, with a median number

Chemical Ecology of Biocompounds in Molluscs http://dx.doi.org/10.5772/intechopen.72741 235

Search results typically show small groups of structurally related compounds (analogues), regarding that the compounds vary in different habitats for the same species. For example, 25 compounds such as terpenes, nitrogenous aliphatic compounds, macrolides and fatty acid derivatives have been isolated from the sea hare *Aplysia kurodai*. Eight novel metabolites were

Its close related species *Aplysia dactylomela*, had 58 compounds which were primarily terpenes derived probably from algal diets of these cosmopolitan grazing sea hares. The *Patinopectin yessoensis* bivalve contained second highest number of sterols and algal toxins. Hence, it could be found that dietary sources contribute significantly to the chemical diversities in molluscs. Evidence for the biogenesis of secondary metabolites mostly stems from feeding experiments, which demonstrate the incorporation of radio-labeled precursors in certain groups of heterobranch molluscs. The secondary metabolites isolated, fall into a wide range of structural

Clearly, all the secondary metabolite types are present in both gastropods and bivalves. Terpenes are dominated In Gastropods, while only three terpenes were identified in bivalves. Terpenes have been an important field of research in soft-bodied grazing gastropods, which they might gain these compounds from their diet for their own defense [5]. Sterols are dominated in bivalves partly because of their role in reproduction cycles, while they are rare in gastropods, taking into account that the large number of researches in bivalves is probably due to their importance in fisheries and aquaculture. Polyproprionates and alkaloids have been isolated from both classes,

There are an extraordinary series of unusual compounds in marine invertebrates, many of them cause interesting biological properties. For instance, opisthobranchs and pulmonates, particularly are important due to their secondary metabolites, and the ecological role and biosynthesis of these compounds could be related to their diet such as microalgae and diatoms. Opisthobranchs which are unprotected with reduced or completely absent shells, have defensive strategies using different chemicals [12]. The selected sampling stations were along different ecosystems such as Indian, Chinese, Mediterranean, Australian and Atlantic coasts of Spain, and strongly indicate that the metabolism of the opisthobranchs is influenced by geographical location, ecosystem type and habitats. The feeding ecology and habitats of all molluscs species are very selective, so feeding metabolites possessed by related species are more similar, while those de novo biosynthesized are most identical in species belonging to the same family but with different geographically habitat. Also, some recent biosynthetic

Natural products research aimed at the isolation and identification of novel secondary metabolites, has only been undertaken on a small proportion of molluscan species to date. The bioactivity of many molluscan traditional medicines is yet to be substantiated, but preliminary data available from bivalves, cephalpods and caenogastropods suggests that there is likely to

of two and a maximum of 58 compounds isolated from a single species [18, 19].

classes, with some compounds being more dominant in certain taxa [21, 22].

whereas aliphatic nitrogenous compounds are relatively uncommon in both [8].

experiments possessed had been discussed [23, 24].

be some chemical basis to their medical applications.

isolated from this species in new environment, further [20].

Growth is one of the processes which needs high levels of energy too, and the energy levels (fatty acid) change in the different stages of the growth. The growth ratio is not similar for different organs and species, and different types of organs need different fatty acids level for growth. Among different organs, sexual organs such as gonads need high levels of fatty acids for growth, and the highest levels of energy are consumed for gonad growth. Therefore, metabolic ratio and followed energy level are varied in different processes, and it could be found that metabolic ratios are key roles in fatty acid amounts and their profiles [4, 5, 14].

The decrease in the ∑PUFA level of neutral lipids of mollusks may probably due to transport of fatty acids to the reproductive organs responsible for gonad maturation. In the different species of mollusks, which the winter is reproductive time, the level of fatty acids in the winter is low, in comparison with other seasons [6, 7].

Many studies indicate that there are a positive correlation between fatty acids and temperature in the tissues of mollusks. Accumulation and increase in the amount of lipids especially PUFAs during summer and decrease in winter may be related to the adaptable regulation of the melting point of cellular lipids. Therefore, many researches are indicated that the amount of lipids in summer is higher than the winter, which returns to; (1) consume of lipid in the reproductive organs for gonad maturation and (2) the adaptable regulation of the melting point of cellular lipids. Finally, variations in the lipid levels in their tissues are related to environmental parameters (such as temperature, light and salinity), seasonal variations, feeding habitats, spawning time and reproductive processes, sexual development and growth metabolisms of molluscs [15–17].

In the total body lipids analyses of molluscs, fatty acids, phospholipids and neutral lipid fractions identified from different tissues. These fatty acids are mostly common in marine and freshwater mollusks. Also, odd-numbered fatty acids, such as C13:0, C15:0, C17:0, and C20 polyunsaturated fatty acids in body lipids of different species, were identified. As mentioned, temperature, food availability, metabolic and physiological activities can affect the lipid and fatty acids compositions of molluscs [6, 8].
