**3. Chemical ecology of natural compounds in molluscs**

Marine molluscs have become the focus of many chemical studies aimed at isolating and identifying novel natural products and secondary metabolites. As scant information is available on the chemistry of terrestrial and freshwater species, this review focuses on marines. Considering the chemical redundancy between species, at least 977 distinct compounds have been isolated from about 251 species in the annual reviews of marine natural products [6], which indicates different chemical diversities and related compounds derived from their biochemical pathways. These compounds could be isolated from a single species merely, or from the same family or genus [9]. Distribution histogram of species diversity reveals multiple metabolites, with a median number of two and a maximum of 58 compounds isolated from a single species [18, 19].

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

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

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 win-

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

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

Marine molluscs have become the focus of many chemical studies aimed at isolating and identifying novel natural products and secondary metabolites. As scant information is available on the chemistry of terrestrial and freshwater species, this review focuses on marines. Considering the chemical redundancy between species, at least 977 distinct compounds have been isolated from about 251 species in the annual reviews of marine natural products [6], which indicates different chemical diversities and related compounds derived from their biochemical pathways. These compounds could be isolated from a single species merely, or from the same family or genus [9].

that metabolic ratios are key roles in fatty acid amounts and their profiles [4, 5, 14].

ter is low, in comparison with other seasons [6, 7].

metabolisms of molluscs [15–17].

fatty acids compositions of molluscs [6, 8].

**3. Chemical ecology of natural compounds in molluscs**

reproductive processes.

234 Biological Resources of Water

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

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

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

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 experiments possessed had been discussed [23, 24].

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 be some chemical basis to their medical applications.

All compounds which are produced by molluscs are varied because of environmental factors such as temperature, salinity and seasonally variations. Therefore, changes in environmental factors could cause variations in the chemical components. Therefore differences in chemical components need different conditions for production, for example fatty acids and amino acids are related to specific temperature and salinity. In conclusion, environmental factor changes in different seasons could be caused in decrease or increase level of compounds. Also, other biological factors such as food availability, metabolic and physiological activities can affect the compounds such as lipid and fatty acids composition of molluscs [13–15].

They are very important in the physiology and reproductive processes of marine animals and reflect the special biochemical and ecological conditions of the marine environment. Lipids also provide energy for growth during conditions of limited food supply, when carbohydrate

Chemical Ecology of Biocompounds in Molluscs http://dx.doi.org/10.5772/intechopen.72741 237

The lipid composition can be affected by external (exogenous) factors, such as fluctuations in the environmental conditions and qualitative and/or quantitative changes in food availability,

Accumulation and depletion of stored reserves in molluscs depends mainly on the stage of gonad development, environmental factors affecting metabolic activities and on the quantity and nutritional value of the food supply. Usually, the glycogen compound is the major energy source in species, while lipids are considered as the nutritive store source of the gonad organs. A high correlation between the gonad lipid content and the phase of the reproductive process cycle has been established in different species of bivalves and also prosobranch species.

Seasonal variations in lipid and fatty acid compositions have been reported for several marine molluscs and are generally related to the growth process and the maturation cycle: in the summer season and in the high temperature when the growth process takes place, receptacles of lipid compounds are build up and stored, and these are later consume for gametogenesis in the maturation cycle (often autumn or winter), normally are decreased during spawning process. However, the majority of these publications have focused on the class bivalve class, probably because of their major commercial importance and influence on the public health of people. Studies about biochemical compounds and their chemical structures, particularly

Limpets are herbivore grazers which remove large quantities of unicellular microbes, algal germ lings and detritus, apparently unselectively, during feeding excursions around the home scar. As a consequence, there are considerable variations in their diets. There is a large amount of literature detailing about fatty acid compositions of a large number of species of marine algae. Availability and quality of algal lipids are very important in the nutrition, growth and development of aquatic animals such as marine fish larvae, shrimps and mol-

Molluscs phylum are of important aquatic invertebrates that the levels of the chemical compounds such as fatty acid components are higher in their tissues in comparison with other animals. They exhibit a range of lipid and fatty acid components in both freshwater and marine species and therefore fatty acid contents in mollusks are studied in many habitats, because of their importance in human's life. Among the marine invertebrates, the molluscs are the potential source of bioactive substances. The bioactive compounds isolated from the gastropods are considered to have a role in the chemical defense of the animals against their predators. Molluscs in the oceans are common sight and are virtually untapped resource for the discov-

Marine molluscs are excellent sources of nutritionally important compounds, such as fatty acids, amino acids and sterols. Fatty acids are essential for life, due to their key role as a good source of energy, membrane constituents, as well as metabolic and signaling mediators. In recent years, poly unsaturated fatty acids (PUFAs) have been recognized as a good remedy

levels (the main energetic reserve in molluscs) are low.

or by internal (endogenous) factors such as sexual maturation [28–30].

fatty acid compounds in prosobranch gastropods, are strait [30].

luscs [28, 31, 32].

ery of novel compounds [27, 29].
