11. Lipids in Marine and Freshwater Organisms
267
the inexact correlations between productivity and zooplankton lipid classes in
smaller freshwater lakes (Wainman et aI., 1993).
11.2.2. Sterols and Cholesterol
The sterol lipid class is about 95% cholesterol (sterols in Fig. 11.2) in higher
organisms, but cholesterol may be only about one-half of the total sterols in
molluskan filter feeders (Napolitano et aI., 1993). It is known that the American
lobster (Homarus americanus) and related crustacea have an absolute requirement
for cholesterol and do not thrive on phytosterols (Kean et aI., 1985; D' Abramo et
aI., 1984; Castell et aI., 1975). In invertebrates, sterols circulate in the blood or
hemolymph (Giese, 1966) in analogy to their role in fish (Waellert and Babin,
1994) and in mammalian food digestion and transport of lipids. Sterols also
possibly have a functional role in maintaining the fluidity of membranes. The
cholesterol content of membranes does not appear to be related to salinity (Hazel
and Williams, 1990).
Studies on the mollusk species Diplodom patagonicus (freshwater) and D.
variabilis (estuarine, subject to marine influence) of Argentina showed a much
higher proportion of cholesterol in the freshwater lake animals than in the estuarine relative (Pollero et aI., 1983). In original experiments on three other
freshwater bivalves collected in Bulgaria and in a discussion and review, other
authors (Popov et aI., 1981) found evidence of control over the proportion of
cholesterol in such animals. Pollero et al. (1983) also found the sterol composition
in freshwater bivalves to be simpler than that in marine analogues. The latter
finding may explain the apparent higher level of cholesterol in freshwater
bivalves.
11.2.3. Wax Esters and Triacylglycerols
Wax esters in near-surface copepods are accepted as important in energy transfers
rather than as structural elements (Graeve and Kattner, 1992). Another role may
be for buoyancy control (Phleger and Grigor, 1990). Wax esters are found in verydeep-water fish such as the orange roughy (Hoplostethus atlanticus) (Grigor el aI.,
1990) and also in midwater fish (e.g., the lanternfish Lampanyctodes hectoris).
These perform diurnal vertical migrations, and this and other similar species
contain moderate amounts of wax esters (de Koning and Evans, 1991). In other
species of midwater fish, Saito and Murata (1996) have reported either very low
(0.5%) or very high (87.9%) amounts of wax esters in different types of myctophid fish. Wax esters are thus more important than is generally realized by many
invertebrate specialists. The tropical reef corals use wax ester to store photosynthetic energy (Lee and Patton, 1989). The energy of wax esters is easily
assimilated by predatory fish, and after hydrolysis, fatty alcohols are converted to
fatty acids (Lie and Lambertsen, 1991; Tocher and Sargent, 1984; Sargent et aI.,
1979). In the Baltic, where the general salinity is too low for calanoid copepods
267
the inexact correlations between productivity and zooplankton lipid classes in
smaller freshwater lakes (Wainman et aI., 1993).
11.2.2. Sterols and Cholesterol
The sterol lipid class is about 95% cholesterol (sterols in Fig. 11.2) in higher
organisms, but cholesterol may be only about one-half of the total sterols in
molluskan filter feeders (Napolitano et aI., 1993). It is known that the American
lobster (Homarus americanus) and related crustacea have an absolute requirement
for cholesterol and do not thrive on phytosterols (Kean et aI., 1985; D' Abramo et
aI., 1984; Castell et aI., 1975). In invertebrates, sterols circulate in the blood or
hemolymph (Giese, 1966) in analogy to their role in fish (Waellert and Babin,
1994) and in mammalian food digestion and transport of lipids. Sterols also
possibly have a functional role in maintaining the fluidity of membranes. The
cholesterol content of membranes does not appear to be related to salinity (Hazel
and Williams, 1990).
Studies on the mollusk species Diplodom patagonicus (freshwater) and D.
variabilis (estuarine, subject to marine influence) of Argentina showed a much
higher proportion of cholesterol in the freshwater lake animals than in the estuarine relative (Pollero et aI., 1983). In original experiments on three other
freshwater bivalves collected in Bulgaria and in a discussion and review, other
authors (Popov et aI., 1981) found evidence of control over the proportion of
cholesterol in such animals. Pollero et al. (1983) also found the sterol composition
in freshwater bivalves to be simpler than that in marine analogues. The latter
finding may explain the apparent higher level of cholesterol in freshwater
bivalves.
11.2.3. Wax Esters and Triacylglycerols
Wax esters in near-surface copepods are accepted as important in energy transfers
rather than as structural elements (Graeve and Kattner, 1992). Another role may
be for buoyancy control (Phleger and Grigor, 1990). Wax esters are found in verydeep-water fish such as the orange roughy (Hoplostethus atlanticus) (Grigor el aI.,
1990) and also in midwater fish (e.g., the lanternfish Lampanyctodes hectoris).
These perform diurnal vertical migrations, and this and other similar species
contain moderate amounts of wax esters (de Koning and Evans, 1991). In other
species of midwater fish, Saito and Murata (1996) have reported either very low
(0.5%) or very high (87.9%) amounts of wax esters in different types of myctophid fish. Wax esters are thus more important than is generally realized by many
invertebrate specialists. The tropical reef corals use wax ester to store photosynthetic energy (Lee and Patton, 1989). The energy of wax esters is easily
assimilated by predatory fish, and after hydrolysis, fatty alcohols are converted to
fatty acids (Lie and Lambertsen, 1991; Tocher and Sargent, 1984; Sargent et aI.,
1979). In the Baltic, where the general salinity is too low for calanoid copepods