**4. Different types of natural compounds in molluscs**

Amino acids are classified into essential amino acids (EAA) (cannot synthesized by humans) and non-essential amino acids (NEAA). In addition to oils and other hydrocarbon derivatives in the marine environment, the hydrocarbons synthesized by organisms occur normally in this environment. Aliphatic hydrocarbons are the principal group, and can occur in several species of marine as well as terrestrial plants and animals.

There are different type of fatty acids such as ∑SFA, ∑MUFA, and ∑PUFA in the whole body of molluscs. There could be changes or variations in their levels of different groups in the different seasons. These differences might be based on temperature, feeding habitats, or metabolic demands [24–26].

The triacylglycerol compounds store SFAs for energy purposes in different processes in body and they also may be interim PUFAs reservoir, which could be transferred to the structural lipids or directed to specific metabolic routes for function of different organs. In contrast, phospholipid compounds fractions of mollusks show considerably less seasonal variations to maintain structural exactness of the cell as compared to the store of saturated fatty acids to be used as a source of energy and store of PUFAs required for phospholipid synthesis to multiple membrane structures or to be integrated in several metabolic processes [25, 26].

Molluscs are sources of many important and different natural compounds such as amino acids, fatty acids, lipids, terpenes and steroids. Different types of fatty acids such as lipids, ∑SFA, ∑MUFA, and ∑PUFA, omega 6 to omega 3 and other compounds are produced by different classes of molluscs specially polyplacophora, gastropoda, cephalopoda and bivalves. Four classes are important, that they could produce about 600 natural compounds. The level of natural compounds between different species, organ and sexes are different, and many of biotic and abiotic factors can cause variations in those levels. Also, the process and metabolism are different for all compounds. Finally, amino acids, lipids and fatty acids, terpenes and steroids are important natural compounds that they could be produced [27–29].
