Comparative studies with species of the same order have shown that these tropical freshwater species contain highly variable levels of n-3 and n-6 PUFA in their muscle tissues. However, it is important to distinguish between wild and farmed fish since the
latter tend to be fed diets which as well as containing higher lipid levels than found in natural food also have a high n-6 FA content derived from plant seed oils and meals. The predominance of C16:0, C18:1n-9, C18:2n-6, C20:4n-6, C22:6n-3 in the muscle
lipids of Thai catfish (C. macrocephalus) was also shown in a study by Shirai et al. (2002). However, in contrast to our findings, these cultured fish contained poor levels (<1% of total FA) of C22:6n-3, C20:5n-3 and C20:4n-6. In the same study, wild Japanese catfish (S. asotus) contained moderate levels of the above fatty acids. Idoniboye-Obu et al. (1993) found small quantities of C22:6n-3 and C20:4n-6 in three species of cultured clariid fish. In agreement, Rahman et al. (1995) analysing the FA composition of several
freshwater fish including species of snakehead, catfish and carps, found very low contents of C20:4n-6, C20:5n-3 and C22:6n-3 in the majority of them. Dutta et al. (1985) showed that there was a seasonal influence on the levels of C20:5n-3 and C22:6n-3 in
Indian snakehead (C. punctatus), which were higher in the colder period. Very high levels of C20:4n-6 have also been found in various snakehead species: C. argus (Choi et al., 1985), C. striatus and C. melanosoma (Jais et al., 1995), C. gachua, C. punctatus and C. marulius (Sen et al., 1976). However, the levels of C20:5n-3 and C22:6n-3 in these species were very low (Choi et al., 1985; Jais et al., 1995). The FA composition in fish is known to vary considerably, both within and between species (Ackman, 1989; Henderson and Tocher, 1987). This depends on whether the species is freshwater
or marine, while a wide range of factors have been reported to affect the FA variability, including water temperature, seasonality, reproductive stage, wild or reared in captivity among others (Ackman, 1989). In general, the lipids of freshwater fish species are composed by higher proportions of saturated fat and C18 PUFA and
lower levels of C20 and C22 PUFA than the lipids of marine fish species (Sargent et al., 2002). In addition, the FA composition of freshwater fish is characterized by higher levels of n-6 PUFA, especially C18:2n-6 and C20:4n-6, and their ratio of n-3/n-6 PUFA
is lower than that of marine fish, ranging from 0.5 to 4.0 (Henderson and Tocher, 1987) with the lowest values generally being found in tropical species. This reflects the FA composition of their dietary lipids. Freshwater micro-algae, such as blue green
algae, are rich in C18:2n-6 but PUFA of chain lengths greater than C18 are not present in large amounts (Ahlgren et al., 1992). Green leaves of terrestrial plants that constitute a significant input to freshwater ecosystems are generally rich in C18:3n-3, while C20:5n-3 is contained in some primitive mosses, liverworts and ferns (Kenyon, 1972). Insects and insect larvae that can form a large part of the diet of many freshwater fish are rich in C18:2n-6 and C18:3n-3 with smaller amounts of C20:4n-6 and C20:5n-3, and some C22:6n-3 (Sargent et al., 2002). Good sources of C22:6n-3 in freshwater ecosystems are certain flagellates, such as Cryptomonas, Rhodomonas and Peridinium (Ahlgren et al., 1992). Thus, freshwater fish species tend to contain higher proportions of C18 PUFA and lower levels of C20 and C22 PUFA in their lipids compared to marine fish species where the food chain is rich in the latter fatty acids (Henderson and Tocher, 1987). Given that the flora and fauna of the tropics are incredibly diverse, it is not surprising that there were marked differences in the FA profiles of the aquatic animals studied here with those of the literature. The lipids of the aquatic animals, especially of snakehead, coming from the rice fields in Northeast Thailand were characterized by relatively high levels of C22:6n-3 and C20:4n-6 concurrently
with substantial contents of C18:2n-6, C18:3n-3 and C20:5n-3. The levels of C22:6n-3 were comparable to those of marine fish species (Table 4), but such high levels of C20:4n-6 are not commonly seen in either marine or freshwater ecosystems. Such elevated levels of C20:4n-6 have been reported in some Australian and Malaysian marine fish species (Dunstan et al., 1988) and in wild Nile tilapia (Oreochromis niloticus) from Thailand (Karapanagiotidis et al., 2006). The high levels of C22:6n-3 could have originated directly from the feeding on food rich in these nutrients. Flagellated algae are known to contain substantial levels of C22:6n-3, while certain filamentous fungus, such as Mortierella alliacea, is known as a rich source of C20:4n-6 (Aki et al., 2001). The fatty acid profiles of micro-algae and zooplankton from tropical freshwater environments are poorly known and further research is required to give information on this area. We should also bear in mind that snakehead, in which the levels of C22:6n-3 and C20:4n- 6 were found to be the highest, is a piscivore species so that the high concentrations of these FA may have occurred up the food chain. Alternatively, these high levels of C22:6n-3 and C20:4n-6 could be the result of chain elongations and desaturations of their precursors, viz. C18:3n-3 and C18:2n-6, respectively. It is known
that freshwater fish species have greater in vivo abilities to convert C18 PUFA to their C20 and C22 analogues, by an alternating succession of desaturation and elongation (reviewed by Turchini et al., 2009). Perhaps these tropical freshwater animal species have greater capacities for these bioconversions than marine fish. In this respect, more information is needed on how or whether organisms in the tropical freshwater food chains can transform the shorter chain n-3 and n-6 PUFA present in phytoplankton into longer
chained PUFA of higher nutritional value. However, studies so far have shown that such bioconversions by fish cannot achieve the tissue levels of C20 and C22 PUFA found in fish fed diets containing these PUFA (Bell and Dick, 2004; Karapanagiotidis et al., 2007).
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