11. Lipids in Marine and Freshwater Organisms
279
triacylglycerol (13%). The fatty acid compositions were rather similar and resembled the insect fatty acid compositions with virtually no 22:6003 and 20:5003 >
22:6003. The chironomidae resembled the gammaridae in lipid classes and fatty
acids, except that they had very little 20:4006. The 22:6003 is also not present in
terrestrial arthropods (Uscian and Stanley-Samuelson, 1994). Published literature
reports on freshwater fish fatty acid compositions are often either from farmed
fish or from those on experimental diets, so publications on natural food chains
are often difficult to find.
Tropical marine fish also show the same tendency to contain elevated 20:4006, a
property discussed by Ackman (1989) for a number of northern Australian fish.
This is probably because 20:4006 is available from the local photosynthetic organisms of coral reefs. Freshwater fish triacylglycerols do tend to have moderate
proportions (~5-1O%) of 18:20>-6 and 18:3003 (Table 11.3). These differences in
polyunsaturated fatty acids probably account for species-specific different aromas
among such fish (Josephson et aI., 1984). Lipoxygenases in fish skin or subdermal
fats oxidize these fatty acids (Mohri et aI., 1992, 1990). The resulting peroxides
break down to give mixtures of aldehydes. These are the basis of many distinctive
olfactory features of fish when freshly caught. Amounts of lipids or of fatty acids
of aquatic invertebrates do not seem to directly contribute to this biochemistry;
there is only an indirect influence through the different types of polyunsaturated
fatty acids (i.e., 006 or (03) contributed to the fish lipids.
In rearing some freshwater fish, notably larval rainbow trout (Oncorhynchus
mykiss), 003 fatty acids are deemed "essential." Some confusion exists as to
whether the vegetable oil 18:3003 by itself is satisfactory. This appears to be
settled by a recent feeding study (Wirth et aI., 1997). The larvae living off their
yolk sacs conserve 22:6003, primarily for new phosholipids. When fed 18:3003 for
four weeks they could utilize these to some extent without elongation, but growth
was limited compared to those larvae fed a diet including preformed 22:6003. The
22:6003 is thus the truly essential fatty acid for larval rainbow trout.
Table 11.4 attempts to show how some fatty acids are phased out of fish lipids
during a sequence of feeding steps on intbertebrates in a larval pike food chain.
The unusual 18:5003 of the invertebrates is seen to pass into the triacylglycerols
but not into the phospholipids. Instead, the latter show a drop in saturated fatty
acid totals and an increase in the 22:6003 characteristic of the fish phospholipids.
Fatty acid studies in Penaeus kerathurus have confirmed that the conversion of
20:5003 to 22:6003 follows an elongation to 24:5003 and .16 de saturation to
24:6003, followed by chain shortening to 22:6003 (Mourente, 1996). This study
confirms that a new metabolic pathway for production of 22:6003, replacing the
hypothetical .14 desaturase (Voss et aI., 1992), is valid in a marine crustacean. It
depends on the presence of peroxisomes for the chain-shortening step. These
bodies have other useful lipid functions in fish (Henderson and Tocher, 1987) but
are not well documented in invertebrates. Freshwater Macrobrachium borellii is
also sensitive to temperature fluctuations affecting mitochondrial oxidation of
fatty acids in gill and hepatopancreas (Irazu et aI., 1992). Moreover, the 20:4006
also increases in the phospholipids. Polyunsaturated fatty acids are thought to be
279
triacylglycerol (13%). The fatty acid compositions were rather similar and resembled the insect fatty acid compositions with virtually no 22:6003 and 20:5003 >
22:6003. The chironomidae resembled the gammaridae in lipid classes and fatty
acids, except that they had very little 20:4006. The 22:6003 is also not present in
terrestrial arthropods (Uscian and Stanley-Samuelson, 1994). Published literature
reports on freshwater fish fatty acid compositions are often either from farmed
fish or from those on experimental diets, so publications on natural food chains
are often difficult to find.
Tropical marine fish also show the same tendency to contain elevated 20:4006, a
property discussed by Ackman (1989) for a number of northern Australian fish.
This is probably because 20:4006 is available from the local photosynthetic organisms of coral reefs. Freshwater fish triacylglycerols do tend to have moderate
proportions (~5-1O%) of 18:20>-6 and 18:3003 (Table 11.3). These differences in
polyunsaturated fatty acids probably account for species-specific different aromas
among such fish (Josephson et aI., 1984). Lipoxygenases in fish skin or subdermal
fats oxidize these fatty acids (Mohri et aI., 1992, 1990). The resulting peroxides
break down to give mixtures of aldehydes. These are the basis of many distinctive
olfactory features of fish when freshly caught. Amounts of lipids or of fatty acids
of aquatic invertebrates do not seem to directly contribute to this biochemistry;
there is only an indirect influence through the different types of polyunsaturated
fatty acids (i.e., 006 or (03) contributed to the fish lipids.
In rearing some freshwater fish, notably larval rainbow trout (Oncorhynchus
mykiss), 003 fatty acids are deemed "essential." Some confusion exists as to
whether the vegetable oil 18:3003 by itself is satisfactory. This appears to be
settled by a recent feeding study (Wirth et aI., 1997). The larvae living off their
yolk sacs conserve 22:6003, primarily for new phosholipids. When fed 18:3003 for
four weeks they could utilize these to some extent without elongation, but growth
was limited compared to those larvae fed a diet including preformed 22:6003. The
22:6003 is thus the truly essential fatty acid for larval rainbow trout.
Table 11.4 attempts to show how some fatty acids are phased out of fish lipids
during a sequence of feeding steps on intbertebrates in a larval pike food chain.
The unusual 18:5003 of the invertebrates is seen to pass into the triacylglycerols
but not into the phospholipids. Instead, the latter show a drop in saturated fatty
acid totals and an increase in the 22:6003 characteristic of the fish phospholipids.
Fatty acid studies in Penaeus kerathurus have confirmed that the conversion of
20:5003 to 22:6003 follows an elongation to 24:5003 and .16 de saturation to
24:6003, followed by chain shortening to 22:6003 (Mourente, 1996). This study
confirms that a new metabolic pathway for production of 22:6003, replacing the
hypothetical .14 desaturase (Voss et aI., 1992), is valid in a marine crustacean. It
depends on the presence of peroxisomes for the chain-shortening step. These
bodies have other useful lipid functions in fish (Henderson and Tocher, 1987) but
are not well documented in invertebrates. Freshwater Macrobrachium borellii is
also sensitive to temperature fluctuations affecting mitochondrial oxidation of
fatty acids in gill and hepatopancreas (Irazu et aI., 1992). Moreover, the 20:4006
also increases in the phospholipids. Polyunsaturated fatty acids are thought to be