66 SEPTEMBER 2012 and mustard oil cake, which are otherwise potentially attractive protein sources in fish feeds. Heat treatment by wet pressure cooking reduces the glucosinolate content of rapeseed meal from 40 to 26 µmol/g. Saponins are steroid or triterpenoid glycosides found in many of the potential, alternate plant-derived feed ingredients for fish, such as various legume seeds (18-41 mg/kg) and defatted roasted soybean flour (67 mg/kg). Saponins increase the digestibility of carbohydraterich foods by detergent-like activity that reduces viscosity, preventing the normal obstructing action of such foods in the intestine. When added to water, saponins are highly toxic to fish, with the detergent action of saponins causing damage to the respiratory epithelium of gills and inhibition of active transport of nutrients. Simultaneous consumption of saponin and tannin results in the loss of individual toxicity because the formation of tannin-saponin complexes inactivates the separate biological activity of both tannins and saponins. Tannins are secondary compounds of various chemical structures widely occurring in plants and are divided into hydrolysable and condensed forms. Anti-nutritional effects include interference with digestion by binding to proteins or minerals. Tannins also reduce the absorption of vitamin B12. Feeds containing tannins, including rapeseed meal and pea seed meal, are tolerated by different fish species. Tannins interact with other anti-nutrients. Methods for the removal of condensed tannins include dehulling seeds to remove the tannin-rich outer layer, autoclaving, or treatment with alkali. Lectins or phytohaemagglutinins are found in many legume seeds and can bind reversibly to carbohydrate moieties of complex glycol-conjugates present on membranes. Although lectins are proteins, they are at least partially resistant to proteolytic degradation in the intestine. Common biological effects include disruption of small intestinal metabolism and morphological damage to villi. Soybean lectins, known as soybean agglutinin, are able to bind extensively to brush border surfaces, particularly in the distal part of the small intestine. Lectins can be removed by aqueous heat treatment (100 oC for 10 min) or autoclaving. Phytoestrogens. Non-steroidal estrogenic substances are widely distributed in potential plant-derived feedstuffs. Estrogenic activity is present in soybeans, cottonseed, linseed and safflower. Chemically plant estrogens are mostly isoflavones that occur in the form of glycosides. These compounds act by binding directly to estrogen receptors or converting to compounds that have estrogenic effects, such as equol. Alkaloids are secondary metabolites found in plants. True alkaloids are basic, contain nitrogen in the heterocyclic ring and are derived from amino acid precursors. Alkaloid-containing grain legumes, such as lupins (Lupinus albinus), are otherwise ideally suited as a feedstuff in aquafeeds because of high digestible protein content (30-50 percent). Aqueous extraction removes alkaloids from some materials. Antigenic compounds in some legume seeds and cereals elicit antigenic effects in animals. These compounds are capable of inducing intestinal mucosal lesions, abnormalities in the villi, specific and non-specific immune responses, and abnormal movement of digesta through the gut. Soybean protein contains glycinin and beta conglycinin that act as allergens in several animals. Gossypols are polyphenols contained in pigment glands of plants in the genus Gossypium and in certain other members of the order Malvales. Commercially available cottonseed meal has gossypol levels ranging from <0.01 percent in glandless cottonseed meals to an average of about 1.3 percent in glanded varieties, levels that may cause toxicity. Pre-pressed and solventextracted glandless cottonseed meal can support good growth even at high inclusion levels (about 50 percent) in most cultured fishes if supplemented with deficient amino acids, such as cystine, lysine, and methionine. Miscellaneous anti-nutrients. Cyanogens are compounds found at high concentrations in a number of pulses, root crops (cassava), and some oilseeds (linseed). Cyanogens, when hydrolyzed, produce toxic products (e.g., hydrogen cyanide and probably other carbonyl compounds) that suppress respiration and cause cardiac arrest. Mimosine is an unusual amino acid present in Leucaena leucocephala, comprising about 3-5 percent of total protein dry weight. Leucaena leaf meal is poorly digested by tilapia. Therefore, it is questionable whether sufficient mimosine enters the body to cause physiological effects. Cyclopropenoic fatty acids, such as sterculic acid and malvalic acid present in cottonseed oil and meal, are known to cause abnormalities in the reproductive processes and alterations in the lipid metabolism in mammals. In rainbow trout, they interfere with long chain fatty acid metabolism and with stearic and palmitic dehydrogenation (Roehm et al. 1970). Cyclopropenoic fatty acids and other toxins, such as aflatoxins, are suspected carcinogens in fish. Canavanine is a thermoresistant free amino acid present in legume species and is an antagonist of arginine. Jack bean (Canavalia ensiformis) meal treated for removal of canavanine is as effective as fishmeal as a feed ingredient for tilapia (Martinez-Palacios et al. 1988). Plants are widely available and reasonably priced, encouraging the use of plant protein sources in aquafeeds. A wide variety of plant protein ingredients are promising candidates to replace fishmeal as a protein source in aquafeeds. These include oilseeds (soybean, jatropha, cottonseed and canola), legumes (lupin and pea), cereal grains (corn and wheat), and protein concentrate (soy, canola and lupin).
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