Characteristics of Daphnia Biology
65
The Fertility Cycle of Parthenogenetic Reproduction. Under normal conditions, parthenogenic eggs are released from the oviducts into the brood
chamber following the maternal molt, while the new female carapace is still
soft and elastic (Zaffagnini 1987). Most cladoceran genera, including
Daphnia, do not secrete any nutritional fluid into the brood pouch, so that
embryonic development is exclusively at the expense of resources deposited with the egg in the form of yolk and lipid droplets (Green 1956). Much
recent work on Daphnia energetics has been focused on triacylglycerol
lipids (e.g., Goulden et a1. 1984), although it seems certain that Daphnia
storage products also include many other macromolecules like glycogen
and lipoproteins (Zaffagnini 1987; Elendt 1989).
Crustaceans are unique in the sense that yolk material can be synthesized
outside the ovaries (extraovarian vitellogenesis; Meusy 1980), and combined with the rest of the egg at a late stage in oogenesis. Ultrastructural
studies by Zaffagnini (1987) indicate that the yolk of parthenogenetic eggs
in Daphnia is entirely of extraovarian origin, probably synthesized by the
fat cells surrounding the gut. According to Goulden et al. (1984), the fate of
the storage products intended for yolk synthesis seems to be undetermined
until the later stages of egg development; under acute starvation they might
be metabolized by the adult herself instead of being transferred to the eggs.
The fraction of the maternally supplied yolk that remains after embryonic
development is used to support growth in the early juvenile instars (Tessier
and Goulden 1982).
Individual Life History Events. Daphnia egg development is phased to the
duration of the intermolt interval (Banta 1939), so that the neonates swim
away from the brood pouch shortly before the next molt. The intermolt
duration seems to be mainly determined by the temperature and largely
unaffected by other environmental factors (Bottrell 1975; Lynch 1989).
Unlike many other crustaceans, Daphnia development into adulthood is
direct without any metamorphosis; neonates are in a way miniature
immature adults.
It seems likely that maturation in cladocerans is determined by reaching
a critical body size (Lynch 1980a, 1989; Perrin 1989), while the age at first
maternity can be prolonged severalfold under low food conditions. In
Daphnia and several other cladocerans, body growth is continued after
reaching the size of maturity (Lynch 1980a). Changes in carapace size are
limited to the first few minutes after molting (Green 1956), after which the
carapace length stays constant throughout the rest of the instar while the
body mass will be continuously changing. Parthenogenetic reproduction in
Daphnia is iteroparous, with a female being able to produce up to 25
clutches in her lifetime under favorable conditions.
65
The Fertility Cycle of Parthenogenetic Reproduction. Under normal conditions, parthenogenic eggs are released from the oviducts into the brood
chamber following the maternal molt, while the new female carapace is still
soft and elastic (Zaffagnini 1987). Most cladoceran genera, including
Daphnia, do not secrete any nutritional fluid into the brood pouch, so that
embryonic development is exclusively at the expense of resources deposited with the egg in the form of yolk and lipid droplets (Green 1956). Much
recent work on Daphnia energetics has been focused on triacylglycerol
lipids (e.g., Goulden et a1. 1984), although it seems certain that Daphnia
storage products also include many other macromolecules like glycogen
and lipoproteins (Zaffagnini 1987; Elendt 1989).
Crustaceans are unique in the sense that yolk material can be synthesized
outside the ovaries (extraovarian vitellogenesis; Meusy 1980), and combined with the rest of the egg at a late stage in oogenesis. Ultrastructural
studies by Zaffagnini (1987) indicate that the yolk of parthenogenetic eggs
in Daphnia is entirely of extraovarian origin, probably synthesized by the
fat cells surrounding the gut. According to Goulden et al. (1984), the fate of
the storage products intended for yolk synthesis seems to be undetermined
until the later stages of egg development; under acute starvation they might
be metabolized by the adult herself instead of being transferred to the eggs.
The fraction of the maternally supplied yolk that remains after embryonic
development is used to support growth in the early juvenile instars (Tessier
and Goulden 1982).
Individual Life History Events. Daphnia egg development is phased to the
duration of the intermolt interval (Banta 1939), so that the neonates swim
away from the brood pouch shortly before the next molt. The intermolt
duration seems to be mainly determined by the temperature and largely
unaffected by other environmental factors (Bottrell 1975; Lynch 1989).
Unlike many other crustaceans, Daphnia development into adulthood is
direct without any metamorphosis; neonates are in a way miniature
immature adults.
It seems likely that maturation in cladocerans is determined by reaching
a critical body size (Lynch 1980a, 1989; Perrin 1989), while the age at first
maternity can be prolonged severalfold under low food conditions. In
Daphnia and several other cladocerans, body growth is continued after
reaching the size of maturity (Lynch 1980a). Changes in carapace size are
limited to the first few minutes after molting (Green 1956), after which the
carapace length stays constant throughout the rest of the instar while the
body mass will be continuously changing. Parthenogenetic reproduction in
Daphnia is iteroparous, with a female being able to produce up to 25
clutches in her lifetime under favorable conditions.