4. Lipids in Freshwater Zooplankton
79
life stages. In cladocerans, these energy reserves are highly visible within the eggs
carried in the transparent brood chamber. The eggs progress through five developmental stages (Threlkeld, 1979). In the first developmental stage, the eggs contain
a single, readily measurable, spherical lipid droplet. In freshwater Cladocera,
these lipids have been characterized as neutral lipids belonging to the TAG family
(Goulden and Henry, 1984). The product of egg number (clutch size) and mean
lipid droplet volume in these eggs represents the maternal lipid investment (MLI)
for that brood (Arts and Sprules, 1988). It has been demonstrated (for the c1adoceran Holopedium gibberum) that most of the variation in MLI among females is
due to between-lake differences and secondarily to differences between females
within a lake (Arts and Sprules, 1988).
MLI is a useful stress indicator for three reasons. First, reproduction is often a
sensitive endpoint in toxicology. Second, MLI relates to something with real
biological meaning. Because c1adocerans provide no parental care for their offspring beyond the egg stage, maternal investment in these organisms is represented exclusively by the investment they make in their eggs (MLI) and the
protection the brood chamber affords the developing embryo. Finally, because
MLI is the product of two interdependent parameters (i.e., clutch size and mean
lipid volume), it is likely to be more sensitive than either parameter on its own.
Thus, the use of MLI in standard toxicity testing is more likely to result in
statistically significant differences (i.e., greater sensitivity) between the control
and experimental treatments.
Desy (1996) demonstrated in IS-day chronic laboratory experiments that
Daphnia pulex exposed to the herbicide triallate (C IO H I6 CI 3 NOS) at 125 j..Lg . L-I
had significantly lower MLI as compared with control Daphnia. Furthermore,
greater differences between the control and the experimental treatment were
observed for MLI than for either clutch size of mean egg volume when these data
were corrected for the effects of body length (Desy, 1996, Table 6.4). MLI
decreased from 5.00 ± 0.53 SE to 4.12 ± 0.67, representing a decrease of 17.6%,
whereas clutch size decreased only slightly (and not significantly) from 10.7 ±
0.8 to 10.3 ± l.l (a decrease of 3.7%) and mean egg volume decreased 11.9%
from 4.13 ± 0.15 to 3.64 ± 0.19.
4.4.3. Visible Lipid Energy Stores
Storage lipids are associated with pigments that make them highly visible in
freshwater and marine zooplankton. This visibility allows for inexpensive microscopical techniques to measure lipid stores. Tessier and Goulden (1982) first
proposed that a visible lipid index for Daphnia would be useful in assessing food
limitation and evaluating stress in cladoceran populations. Using Tessier and
Goulden's index, Holm and Shapiro (1984) provided empirical evidence that the
cyanobacteria Aphanizomenon fios-aquae was a nutritionally inadequate food
source for Daphnia. Dauble et al. (1985) extended the value of this index by
commenting on its use as a stress indicator for bioaccumulation studies with
Daphnia magna. Arts and Evans (1991) developed a similar but more quantitative
79
life stages. In cladocerans, these energy reserves are highly visible within the eggs
carried in the transparent brood chamber. The eggs progress through five developmental stages (Threlkeld, 1979). In the first developmental stage, the eggs contain
a single, readily measurable, spherical lipid droplet. In freshwater Cladocera,
these lipids have been characterized as neutral lipids belonging to the TAG family
(Goulden and Henry, 1984). The product of egg number (clutch size) and mean
lipid droplet volume in these eggs represents the maternal lipid investment (MLI)
for that brood (Arts and Sprules, 1988). It has been demonstrated (for the c1adoceran Holopedium gibberum) that most of the variation in MLI among females is
due to between-lake differences and secondarily to differences between females
within a lake (Arts and Sprules, 1988).
MLI is a useful stress indicator for three reasons. First, reproduction is often a
sensitive endpoint in toxicology. Second, MLI relates to something with real
biological meaning. Because c1adocerans provide no parental care for their offspring beyond the egg stage, maternal investment in these organisms is represented exclusively by the investment they make in their eggs (MLI) and the
protection the brood chamber affords the developing embryo. Finally, because
MLI is the product of two interdependent parameters (i.e., clutch size and mean
lipid volume), it is likely to be more sensitive than either parameter on its own.
Thus, the use of MLI in standard toxicity testing is more likely to result in
statistically significant differences (i.e., greater sensitivity) between the control
and experimental treatments.
Desy (1996) demonstrated in IS-day chronic laboratory experiments that
Daphnia pulex exposed to the herbicide triallate (C IO H I6 CI 3 NOS) at 125 j..Lg . L-I
had significantly lower MLI as compared with control Daphnia. Furthermore,
greater differences between the control and the experimental treatment were
observed for MLI than for either clutch size of mean egg volume when these data
were corrected for the effects of body length (Desy, 1996, Table 6.4). MLI
decreased from 5.00 ± 0.53 SE to 4.12 ± 0.67, representing a decrease of 17.6%,
whereas clutch size decreased only slightly (and not significantly) from 10.7 ±
0.8 to 10.3 ± l.l (a decrease of 3.7%) and mean egg volume decreased 11.9%
from 4.13 ± 0.15 to 3.64 ± 0.19.
4.4.3. Visible Lipid Energy Stores
Storage lipids are associated with pigments that make them highly visible in
freshwater and marine zooplankton. This visibility allows for inexpensive microscopical techniques to measure lipid stores. Tessier and Goulden (1982) first
proposed that a visible lipid index for Daphnia would be useful in assessing food
limitation and evaluating stress in cladoceran populations. Using Tessier and
Goulden's index, Holm and Shapiro (1984) provided empirical evidence that the
cyanobacteria Aphanizomenon fios-aquae was a nutritionally inadequate food
source for Daphnia. Dauble et al. (1985) extended the value of this index by
commenting on its use as a stress indicator for bioaccumulation studies with
Daphnia magna. Arts and Evans (1991) developed a similar but more quantitative