**1. Introduction**

The name mollusc (mollusk) was derived from Latin word mollus meaning soft. This term was first used by Cuvier in 1798 to describe squids, cuttlefish and animals whose shells is reduced. After the arthropods the molluscs are the most successful of the animal phyla in terms of numbers of species. Considering the vast species of molluscs and the large number of fossil species, they are the largest marine phylum, comprising about 23% of all the named marine organisms [1].

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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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 numerous classes in this phylum [2].

The marine environment is highly competitive and being able to produce fundamental compounds which have both industrial and medical applications. Based on the species number, molluscs are the second largest phylum in the marine environment. Their morphological and physiological features attract many investigators [11]. Among molluscs, gastropods have a particular role in commercial shell craft industry. A wide variety of species exists on land, fresh water and the sea. Marine gastropods form only a minor component of marine fishery resources. Many species are exported for the purposes of manufacturing ornaments, curious and various other artifacts of commercial value. Women and children collect this gastropods and bivalves from shallow estuaries for nutritional food. Shells and shell crafts of gastropods are the major economy for the local peoples in marine coasts. Marine bivalves and gastropods are also rich source of many biologically active compounds. Owing to their medicinal and industrial properties, several species are traditionally fished for food and shell [2].

Chemical Ecology of Biocompounds in Molluscs http://dx.doi.org/10.5772/intechopen.72741 233

Mollusc species could be hazardous or pests. For example, blue-ringed octopus which is often fatal, and Octopus apollyon causes inflammations which can last for over a month. Toxic cone shells could kill or cause inflammations, while some times their venoms could become important tools in neurological research. Also, some snails and slugs are serious agricultural pests

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

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

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

or dangerous vectors for transition parasitic diseases [10, 11].

different groups during four seasons of the year [13–15].

**2. Concept of chemical ecology**

components of their dietary lipids [12].

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 exclusively marine species [3, 4].

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 habitats [4, 5].

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 money [4].

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 fascioliasis in humans [3, 5].

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

The marine environment is highly competitive and being able to produce fundamental compounds which have both industrial and medical applications. Based on the species number, molluscs are the second largest phylum in the marine environment. Their morphological and physiological features attract many investigators [11]. Among molluscs, gastropods have a particular role in commercial shell craft industry. A wide variety of species exists on land, fresh water and the sea. Marine gastropods form only a minor component of marine fishery resources. Many species are exported for the purposes of manufacturing ornaments, curious and various other artifacts of commercial value. Women and children collect this gastropods and bivalves from shallow estuaries for nutritional food. Shells and shell crafts of gastropods are the major economy for the local peoples in marine coasts. Marine bivalves and gastropods are also rich source of many biologically active compounds. Owing to their medicinal and industrial properties, several species are traditionally fished for food and shell [2].

Mollusc species could be hazardous or pests. For example, blue-ringed octopus which is often fatal, and Octopus apollyon causes inflammations which can last for over a month. Toxic cone shells could kill or cause inflammations, while some times their venoms could become important tools in neurological research. Also, some snails and slugs are serious agricultural pests or dangerous vectors for transition parasitic diseases [10, 11].
