Individual Mortality and Population Losses
97
As mentioned in Section 4.1, two major components of Daphnia eggs,
oocytes and yolk precursors, are kept separate until the later stages of oocyte
maturation. Tessier et al. (1983) raised Daphnia to adulthood at high food
levels, and began starving different groups of animals from 0 to 60 h after
entering the third adult instar. Their experiment showed a linear relationship
between clutch size and the fraction of the instar spent at high food levels, indicating that allocation to oocyte material is irreversible. Goulden et al. (1984)
found that neonates from Daphnia magna raised in transfer culture under
food limitation had reduced levels of triacy1g1ycerol (TAG) lipids, indicating
that, contrary to oocytes, the material accumulated as energy reserve for the
eggs can be reclaimed by the mother under conditions of acute starvation.
Taken together, these pieces of information could indicate that oocyte
material and yolk precursors are allocated in proportion during the initial part
of the transfer cycle when food is ample, while some of the yolk material is used
for maternal maintenance during the starvation phase of the cycle. The reduced
TAG lipid content of neonates raised under low food conditions would then be
mainly an artifact created by the feeding regime in transfer cultures through the
implied decoupling of the allocation of oocyte material and energy reserves. If a
female is able to reclaim material allocated to yolk precursors in the starvation
part of the transfer cycle, there will be less yolk material available to the oocytes
produced in the same brood cycle. If yolk and oocytes are combined serially as
the oocytes pass through the oviducts (Zaffagnini 1987), the insufficient total
amount of yolk material might be divided so that the first eggs to enter the
oviducts receive their full share, while the rest have none.
Lynch and Ennis (1983) found that Daphnia pulex neonates, from mothers
raised at low and high food levels, showed significant differences in both
juvenile mortality and adult longevity when raised at the same food level The
observed maternal effect on juvenile survival is consistent with the proposed
decoupling of oocyte and yolk precursor accumulation in food-limited transfer cultures, while the effect on longevity is probably due to factors other than
neonatal energy reserves. Lynch and Ennis (1983) also found increased
juvenile growth rates in offspring of mothers raised at high food levels,
indicating that early juvenile growth is partly dependent on maternally
supplied resources, as also suggested by Goulden et al. (1984). If maternally
supplied resources are of such vital importance to juvenile survival as indicated by Tessier et al. 1983, it is more likely that juvenile mortality under food
limitation is an effect of maternal starvation, and therefore to a large extent a
result of the feeding regime in transfer cultures. The fact that most workers
have been careful to precondition the animals, with experiments being
started after several generations at the same food level, would tend to enforce
such a maternal effect on juvenile mortality.
Under this assumption, the clutch of a starving female should be composed of eggs with both normal and with suboptimal yolk content. A fraction of the eggs would then be expected to develop into neonates with
normal survival probability, while the other fraction would suffer a higher
97
As mentioned in Section 4.1, two major components of Daphnia eggs,
oocytes and yolk precursors, are kept separate until the later stages of oocyte
maturation. Tessier et al. (1983) raised Daphnia to adulthood at high food
levels, and began starving different groups of animals from 0 to 60 h after
entering the third adult instar. Their experiment showed a linear relationship
between clutch size and the fraction of the instar spent at high food levels, indicating that allocation to oocyte material is irreversible. Goulden et al. (1984)
found that neonates from Daphnia magna raised in transfer culture under
food limitation had reduced levels of triacy1g1ycerol (TAG) lipids, indicating
that, contrary to oocytes, the material accumulated as energy reserve for the
eggs can be reclaimed by the mother under conditions of acute starvation.
Taken together, these pieces of information could indicate that oocyte
material and yolk precursors are allocated in proportion during the initial part
of the transfer cycle when food is ample, while some of the yolk material is used
for maternal maintenance during the starvation phase of the cycle. The reduced
TAG lipid content of neonates raised under low food conditions would then be
mainly an artifact created by the feeding regime in transfer cultures through the
implied decoupling of the allocation of oocyte material and energy reserves. If a
female is able to reclaim material allocated to yolk precursors in the starvation
part of the transfer cycle, there will be less yolk material available to the oocytes
produced in the same brood cycle. If yolk and oocytes are combined serially as
the oocytes pass through the oviducts (Zaffagnini 1987), the insufficient total
amount of yolk material might be divided so that the first eggs to enter the
oviducts receive their full share, while the rest have none.
Lynch and Ennis (1983) found that Daphnia pulex neonates, from mothers
raised at low and high food levels, showed significant differences in both
juvenile mortality and adult longevity when raised at the same food level The
observed maternal effect on juvenile survival is consistent with the proposed
decoupling of oocyte and yolk precursor accumulation in food-limited transfer cultures, while the effect on longevity is probably due to factors other than
neonatal energy reserves. Lynch and Ennis (1983) also found increased
juvenile growth rates in offspring of mothers raised at high food levels,
indicating that early juvenile growth is partly dependent on maternally
supplied resources, as also suggested by Goulden et al. (1984). If maternally
supplied resources are of such vital importance to juvenile survival as indicated by Tessier et al. 1983, it is more likely that juvenile mortality under food
limitation is an effect of maternal starvation, and therefore to a large extent a
result of the feeding regime in transfer cultures. The fact that most workers
have been careful to precondition the animals, with experiments being
started after several generations at the same food level, would tend to enforce
such a maternal effect on juvenile mortality.
Under this assumption, the clutch of a starving female should be composed of eggs with both normal and with suboptimal yolk content. A fraction of the eggs would then be expected to develop into neonates with
normal survival probability, while the other fraction would suffer a higher