268
R.G. Ackman
rich in wax esters, including 22: I fatty alcohol, the fatty acids of the Baltic herring
in consequence have very little 22: 1 fatty acid (Linko et a!., 1985). This is one of
the main fatty acids in marine fish triacylglycerols and results from oxidation of
dietary wax ester alcohols. In one copepod sample, this 22: I alcohol comprised
>40% of the fatty alcohols (Ackman et aI., 1974). This leads to wax ester chain
lengths of up to C 44 , as the 22: I alcohol can, in most samples, be complemented
by a range of fatty acids up to and including the important fatty acid 22:6m3
(Kattner et a!. 1990). Albers et a!. (1996) point out that in omnivorous and
carnivorous species, wax esters rich in 14:0 or 16:0 alcohol may be more
important.
The presence of wax esters in freshwater zooplankton was not confirmed until
1989. Cavaletto et a!. (1989) then showed that in the total lipids of two calanoid
copepods, Limnocalanus macrurus and Senecella calanoides, from Lake Michigan 57 -80% of wax esters were stored in a sac, with up to 17.5% triacylglycerols
also present in this energy reserve. The coexisting calanoid Diaptomus sicilis had
a substantial lipid component (36.3% of dry weight), but only 1.1 % was made up
of wax or sterol esters. The common thin-layer chromatography (TLC) technology for separating total lipids on silica gel-coated plates or by Iatroscan TLC-FID
on silica gel Chromarods does not usually separate these two lipid classes, so
minor amounts of wax esters may have been overlooked by other researchers.
Gas-liquid chromatography (GLC) of neutral lipids does not eliminate such problems, although hydrogenation of the neutral lipids can help resolution and quantitation of several lipid classes by GLC (Yang et aI., 1996) and of lipids generally
by TLC-FID (Shantha and Ackman, 1990). The farmed (Sri Lanka) marine
decapod crustacean Penaeus monodon had more steryl esters than free cholesterol
(28 and 4.1 f.Lg . g-I lipid, respectively) in the hepatopancreas, whereas in gill,
muscle, and ovary there was more free sterol than steryl ester (Young et a!., 1992).
In the latter tissues, the cholesterol in free form would be fitted into phospholipidrich membranes. In the hepatopancreas storage organ, steryl esters would remain
liquid, probably dissolved in triacylglycerols in adipocytes, whereas free cholesterol (m.p. 148°C) has a marked tendency to crystallize under all favorable
conditions. The fatty acids of the cholesteryl esters were undistinguished except
for 20:4m6 being equal to or exceeding 20:5m3. The proportion of 20:4m6 in fatty
acids is, however, higher in lipids of freshwater animals than in those of marine
life (see below) and also usually higher in animals of tropical marine waters than
in those of higher latitudes.
Polychaetes are a known food resource for in-shore fish and also for migrating
shore-feeding birds (Napolitano et a!., 1992). Most do not have much storage
lipid, and yet lipid droplets are observed in the Pacific species Sabella pavonina.
Koechlin et a!. (1981) demonstrated that these droplets in the nephridial epithelia
were roughly half free cholesterol and half cholesterol esters. The popularity of
colorimetric assays for free cholesterol could have missed this occurrence of
steryl esters, which these authors illustrated clearly by TLC and have confirmed
by mass spectrometry.
R.G. Ackman
rich in wax esters, including 22: I fatty alcohol, the fatty acids of the Baltic herring
in consequence have very little 22: 1 fatty acid (Linko et a!., 1985). This is one of
the main fatty acids in marine fish triacylglycerols and results from oxidation of
dietary wax ester alcohols. In one copepod sample, this 22: I alcohol comprised
>40% of the fatty alcohols (Ackman et aI., 1974). This leads to wax ester chain
lengths of up to C 44 , as the 22: I alcohol can, in most samples, be complemented
by a range of fatty acids up to and including the important fatty acid 22:6m3
(Kattner et a!. 1990). Albers et a!. (1996) point out that in omnivorous and
carnivorous species, wax esters rich in 14:0 or 16:0 alcohol may be more
important.
The presence of wax esters in freshwater zooplankton was not confirmed until
1989. Cavaletto et a!. (1989) then showed that in the total lipids of two calanoid
copepods, Limnocalanus macrurus and Senecella calanoides, from Lake Michigan 57 -80% of wax esters were stored in a sac, with up to 17.5% triacylglycerols
also present in this energy reserve. The coexisting calanoid Diaptomus sicilis had
a substantial lipid component (36.3% of dry weight), but only 1.1 % was made up
of wax or sterol esters. The common thin-layer chromatography (TLC) technology for separating total lipids on silica gel-coated plates or by Iatroscan TLC-FID
on silica gel Chromarods does not usually separate these two lipid classes, so
minor amounts of wax esters may have been overlooked by other researchers.
Gas-liquid chromatography (GLC) of neutral lipids does not eliminate such problems, although hydrogenation of the neutral lipids can help resolution and quantitation of several lipid classes by GLC (Yang et aI., 1996) and of lipids generally
by TLC-FID (Shantha and Ackman, 1990). The farmed (Sri Lanka) marine
decapod crustacean Penaeus monodon had more steryl esters than free cholesterol
(28 and 4.1 f.Lg . g-I lipid, respectively) in the hepatopancreas, whereas in gill,
muscle, and ovary there was more free sterol than steryl ester (Young et a!., 1992).
In the latter tissues, the cholesterol in free form would be fitted into phospholipidrich membranes. In the hepatopancreas storage organ, steryl esters would remain
liquid, probably dissolved in triacylglycerols in adipocytes, whereas free cholesterol (m.p. 148°C) has a marked tendency to crystallize under all favorable
conditions. The fatty acids of the cholesteryl esters were undistinguished except
for 20:4m6 being equal to or exceeding 20:5m3. The proportion of 20:4m6 in fatty
acids is, however, higher in lipids of freshwater animals than in those of marine
life (see below) and also usually higher in animals of tropical marine waters than
in those of higher latitudes.
Polychaetes are a known food resource for in-shore fish and also for migrating
shore-feeding birds (Napolitano et a!., 1992). Most do not have much storage
lipid, and yet lipid droplets are observed in the Pacific species Sabella pavonina.
Koechlin et a!. (1981) demonstrated that these droplets in the nephridial epithelia
were roughly half free cholesterol and half cholesterol esters. The popularity of
colorimetric assays for free cholesterol could have missed this occurrence of
steryl esters, which these authors illustrated clearly by TLC and have confirmed
by mass spectrometry.