14 December 2011 Caulerpa species viz. C. racemosa, C. scalpelliformis and C. veravelensis with higher enzymatic and non-enzymatic antioxidant properties can be utilized as a source of natural antioxidant compounds in aquafeed. Nutraceutical Value of Seaweeds There are numerous reports of compounds derived from macroalgae with a broad range of biological activities, such as antibacterial, antivirals, antitumorals, anticoagulant and antifouling. In fish, it has been shown that these substances obtained from seaweeds, mainly polysaccharides, can modify the immune response and have been shown to have antibacterial activity in vitro against Grampositive and Gram-negative bacteria. The algal extracts of Enteropmorpha compressa, Cladophorosis zoolingeri, Padina gymnospora, Sargassum wightii and Gracilaria corticata are active against gram positive and gram negative bacteria (Kandhasamy and Arunachalam 2008). Carrageenan, a polysaccharide abundant in certain red seaweeds, induces an increase in macrophage phagocytic activity and in resistance against bacterial infections of common carp Cyprinus carpio via an intraperitoneal injection. Sodium alginate extracted from the brown algae, Undaria pinnatifida and Macrocystis pyritera enhances the non-specific defence system of common carp and its resistance against Edwardsiella tarda. Ergosan, an algal extract containing alginic acid are found to increase the non-specific defence response of Channa striatus, Onchorynchus mykiss, Dicentrarchus labrax and Epinephelus coioides which were injected intraperitoneally with κ-carrageenan and sodium alginate had elevated non-specific immune parameter responses and resistance against Vibrio alginolyticus. Fucoidans from Seaweed Fucoidans, polysaccharides containing substantial percentages of L-fucose and sulfate ester groups, are constituents of brown algae. Biological activities of fucoidans occur mainly because of their high degree of sulfation, although these activities possibly depend on fine structural peculiarities and molecular weight (Eluvakkal et al. 2010). Most biological studies have been done on commercial preparations of fucoidan from Fucus vesiculosus. Fucoidan from Cladosiphon okamuranus is reported to control WSSV in Penaeus japonicus. Fucoidan can also be used to enhance growth and prevent or reduce mortalities in P. monodon postlarvae due to V. harveyi infection. In general, sulfated polysaccharides (fucoidan and dextran sulfate) inhibit the adsorption of enveloped viruses to host cells. Variations in the viral envelope of glycoprotein may result in different in susceptibility to the fucoidan. Although the fucoidan can be used as the immunostimulant to control shrimp disease in culture, it may not be effective against all diseases. Therefore, the timing, dosages and methods of administration as well as the side effects need to be investigated. Conclusion and Future Directions The limitations over the global demand for animal protein and fat sources in terms of fish meal and oil and, also, the possible environmental impact has led to a search for more potential protein and lipid sources for aquafeed preparation. The seaweeds are abundant in nature. The possibilities of seaweed mariculture in many parts of the world have made these diversified microalgae available for incorporation in fish feed. Apart from being used as rich protein and lipid sources, these seaweeds are also rich in essential minerals, vitamins, antioxidants. The use of seaweed proteins with good amino acid profiles in feed for fish seems to be a promising way for the utilization of this marine resource. Besides, a as source of proteins, they can be used as excellent binders in formulated feeds, inasmuch as a majority of them contain phycocolloids, such as agar, algin and carrageenan. Research on various seaweeds has shown it to be a potent antimicrobial agent against many pathogens. Steps, such as biotechnological treatment by enzymatic degradation of algal fibres could be attempted to improve protein digestibility and to increase the nutritional value of these proteins. The future research focus should be targeted towards finding out the different compounds in seaweed and its interference in host immune system. Notes 1Aquaculture Division, 2Division of Fish Nutrition, Biochemistry and Physiology Central Institute of Fisheries Education, Mumbai – 400061, India *Corresponding author: gengang@gmail.com, Phone: +918108305598 References Chiu, S. T., R. T. Shai, J. P. Shu, C.H. Liu and W. Cheng. 2008. 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