5. Lipid Dietary Dependencies in Zooplankton
93
instar begins after an exoskeleton molt and, in daphniids, is coincident with the
release of new ly formed eggs into the brood pouch. These eggs develop during the
instar and are released just before the molt at the end of the instar. For the
determination of the amount of lipids accumulated during an instar, newly formed
eggs were removed from part of a cohort of clonal adult females at the beginning
of the instar and the adults then analyzed for total lipids and lipid classes. A
second set of adult females of the same cohort and age was cultured until the
beginning of the next instar and was sacrificed without removal of the new eggs,
and their lipid content was analyzed.
The results indicated that in adults at the beginning of an instar (with eggs
removed), lipid contents ranged from 10-15% of dry mass of the body. At the
beginning of the following instar, adult Cladocera (with fresh eggs) contained up
to 35-40% of their dry mass as lipids. Total body mass of the animals increased,
as did protein and carbohydrate content, but on a relative basis, the biochemical
composition of these adults was most altered by the substantial increases in their
lipid composition.
To explain this, we must assume that animals (l) preferentially assimilate
lipids, (2) preferentially metabolize assimilated carbohydrate or protein while
storing lipids, or (3) biosynthesize lipids from carbohydrate and possibly protein.
The second set of experiments measured fatty acid synthesis rates in daphniids.
In a series of tracer experiments using 14C-acetate or 3H-water, Goulden and Place
(1990) measured incorporation rates of the isotopes into extractable fatty acids
and compared synthesis rates with fatty acid accumulation rates in two daphniid
species. They found that given either high food levels or low food levels, the
daphniids (Daphnia pulicaria and D. magna) synthesized <2% of their accumulated fatty acids. So, >98% of the fatty acids was derived directly from the diet.
Most of the incorporated isotope appeared in the fatty acids of phospholipids, and
the authors suggested that most of the incorporation resulted from chain elongation of dietary acids.
This low rate of synthesis relative to the amount of lipid accumulated indicates
that zooplankton are quite dependent on the total lipid and the quality of the lipid
(fatty acid composition) in their food.
Because total lipids may represent a small fraction of the biochemical composition of exponential growth-phase algae under laboratory conditions (Cowgill et
aI., 1984; Lee et ai., 1971), lipids or their constituent fatty acids may represent a
primary cause of food limitation in daphniids.
In this chapter, we focus primarily on whether lipids may limit reproduction in
daphniid Cladocera in two north-temperate freshwater lakes. Although the focus
is on lipids, protein was also studied. We do not emphasize the protein results in
this chapter, although the data are included and results briefly described.
Our hypotheses of food limitation to be tested in this study are the following:
• Total food hypothesis: Food limitation is due simply to the total quantity of
food; specific dietary components always occur in adequate ratios so that none
acts as a limiting nutrient.
93
instar begins after an exoskeleton molt and, in daphniids, is coincident with the
release of new ly formed eggs into the brood pouch. These eggs develop during the
instar and are released just before the molt at the end of the instar. For the
determination of the amount of lipids accumulated during an instar, newly formed
eggs were removed from part of a cohort of clonal adult females at the beginning
of the instar and the adults then analyzed for total lipids and lipid classes. A
second set of adult females of the same cohort and age was cultured until the
beginning of the next instar and was sacrificed without removal of the new eggs,
and their lipid content was analyzed.
The results indicated that in adults at the beginning of an instar (with eggs
removed), lipid contents ranged from 10-15% of dry mass of the body. At the
beginning of the following instar, adult Cladocera (with fresh eggs) contained up
to 35-40% of their dry mass as lipids. Total body mass of the animals increased,
as did protein and carbohydrate content, but on a relative basis, the biochemical
composition of these adults was most altered by the substantial increases in their
lipid composition.
To explain this, we must assume that animals (l) preferentially assimilate
lipids, (2) preferentially metabolize assimilated carbohydrate or protein while
storing lipids, or (3) biosynthesize lipids from carbohydrate and possibly protein.
The second set of experiments measured fatty acid synthesis rates in daphniids.
In a series of tracer experiments using 14C-acetate or 3H-water, Goulden and Place
(1990) measured incorporation rates of the isotopes into extractable fatty acids
and compared synthesis rates with fatty acid accumulation rates in two daphniid
species. They found that given either high food levels or low food levels, the
daphniids (Daphnia pulicaria and D. magna) synthesized <2% of their accumulated fatty acids. So, >98% of the fatty acids was derived directly from the diet.
Most of the incorporated isotope appeared in the fatty acids of phospholipids, and
the authors suggested that most of the incorporation resulted from chain elongation of dietary acids.
This low rate of synthesis relative to the amount of lipid accumulated indicates
that zooplankton are quite dependent on the total lipid and the quality of the lipid
(fatty acid composition) in their food.
Because total lipids may represent a small fraction of the biochemical composition of exponential growth-phase algae under laboratory conditions (Cowgill et
aI., 1984; Lee et ai., 1971), lipids or their constituent fatty acids may represent a
primary cause of food limitation in daphniids.
In this chapter, we focus primarily on whether lipids may limit reproduction in
daphniid Cladocera in two north-temperate freshwater lakes. Although the focus
is on lipids, protein was also studied. We do not emphasize the protein results in
this chapter, although the data are included and results briefly described.
Our hypotheses of food limitation to be tested in this study are the following:
• Total food hypothesis: Food limitation is due simply to the total quantity of
food; specific dietary components always occur in adequate ratios so that none
acts as a limiting nutrient.