Marine lipids
blooded tuna are unusually rich in 22:6 (n-3). The phospholipids of
marine invertebrates often have 20:5 (n-3) and 22:6 (n-3) present in
approximately equal amounts, whereas those of marine vertebrates often
have approximately twice as much 22:6 (n-3) as 20:5 (n-3). Phosphoglycerides of neural tissues (brain and eyes) of marine vertebrates generally are particularly rich in 22:6 (n-3) with di-22:6 (n-3) species being
very abundant. A specific role for 22:6 (n-3) in fish vision has been
established. Arachidonic acid, 20:4 (n-6), which is concentrated in
phosphatidylinositol, is the main source of eicosanoids (2-series prostanoids and 4-series leukotrienes) in marine fish, although 20:5 (n-3)
can also generate eicosanoids (3-series prostanoids and 5-series leukotrienes). Because marine fish cannot convert C18 PUFAs to their C20
and C22 end products, 20:4 (n-6), 20:5 (n-3) and 22:6 (n-3) are dietary
essential fatty acids in marine fish. Pathologies can be readily induced in
fish, particularly larval fish, by elevating dietary ratios of 20:4 (n-6) and
20:5 (n-3).
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